Planta (2019) 250:891–909 https://doi.org/10.1007/s00425-019-03184-5 REVIEW Ancient orphan legume horse gram: a potential food and forage crop of future J. P. Aditya1 · Anuradha Bhartiya1 · Rakesh K. Chahota2 · Dinesh Joshi1 · Nirmal Chandra1 · Lakshmi Kant1 · Arunava Pattanayak1 Received: 31 August 2018 / Accepted: 9 May 2019 / Published online: 21 May 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Main conclusion Tailoring crops to withstand rising temperature and declining precipitation may be unrewarding, therefore the potential of alternative undervalued crops such as horse gram can be explored for safeguarding food and nutritional security with health benefits in the era of climate change. Abstract Horse gram [Macrotyloma uniflorum (Lam.) Verdc] under the family Fabaceae, has been cultivated for food, folklore medicine and fodder mainly by rural and tribal communities since prehistoric times in Asian and African countries. This valuable ancient legume not only offers diversification and resilience to agro-ecosystems but it also possesses high adaptation in risk-prone traditional farming systems in marginal environments of semi-arid and arid regions. Being a nutrient dense legume with remedial health-promoting effects due to the presence of various bioactive compounds, it is suitable for the development of functional food as well as for addressing micronutrient deficiencies among poor rural communities. Despite its enormous potential and a growing awareness about the utility of this underutilized crop for future climate adaptation and human well-being, this legume continues to be seriously neglected and labelled as “food of the poors”. India is the major producer of horse gram and presently, cultivation of horse gram remains confined to small-scale farming systems as an inter- or mixed crop. This crop is alienated from mainstream agriculture and relegated to a status of “underutilized” due to its limited competitiveness as compared to other commercial crops. Besides a scanty basic research on this crop, no attention has been paid to the factors like improvement of plant type, yield improvement, processing, value addition to suit consumer needs and reduction of anti-nutritional factors, which restricted the diffusion of this crop outside its niche area. The present review therefore is an attempt to compile the meagre information available on crop history, evolution, genetic enhancement, nutritional and health benefits to make the crop competitive and revitalize horse gram cultivation. Keywords Breeding · Domestication · Genetic and genomic resources · Macrotyloma · Nutritional value · Nutraceutical properties * Anuradha Bhartiya Anuradha.Bhartiya@icar.gov.in Lakshmi Kant Lakshmi.Kant@icar.gov.in J. P. Aditya Jay.Aditya@icar.gov.in Arunava Pattanayak apat.icarneh@gmail.com Rakesh K. Chahota rkchahota@hillagric.ac.in 1 ICAR-Vivekananda Parvatiya Krishi Anusandhan Sansthan, Almora, Uttarakhand 263601, India Dinesh Joshi Dinesh.Joshi@icar.gov.in 2 Department of Agricultural Biotechnology, CSK Himachal Pradesh Agriculture University, Palampur, India Nirmal Chandra Nirmal.Chandra@icar.gov.in 13 Vol.:(0123456789) 892 Introduction There are more than 50,000 known edible plants in the world (Cheng et al. 2015). Despite the existence of huge crop diversity, presently crop production is mainly focused on three crops—maize (Zea mays), wheat (Triticum aestivum) and rice (Oryza sativa) which are supplying 60% of the world’s food energy intake (FAO 1999). The tapering diversity in crop species contributing to the world’s food supplies has been considered a potential threat to food security (Khoury et al. 2014) and in the present era, this dependency on few crops for ensuring food and nutritional security has been challenged with a greater need for diversification of rural cropping systems through nonconventional underutilized crops as potential future crops (Mabhaudhi et al. 2017). There are many underutilized edible crops existing in the world which have been used for food, fibre, fodder, oil or medicinal values for centuries and often form a vital part of the culture and diets of the people growing them (Padulosi et al. 2006) as well as diversify the human diet, combat malnutrition and increase food production levels thus, enabling more sustainable and resilient agro-horti food systems (Baldermann et al. 2016). In traditional subsistence farming systems, these underutilized crops have a significant role in alleviating hunger in rural households particularly, during challenging situations like dry seasons, drought and famine (Mayes et al. 2012; Bhartiya et al. 2015). Presently, the changing climate, shrinking arable acreages and sluggish annual yield enhancement rate of major crops have also raised concerns towards addressing food insecurity and undernourishment for the burgeoning population (Chivenge et al. 2015). Therefore, to sustain food security and address the issues of malnutrition among poor sections of the world, it is imperative to diversify global food sources and cropping systems by explicitly reorienting orphan or non-conventional underutilized crops to mainstream agriculture as possible future crops (Mabhaudhi et al. 2017). Horse gram [Macrotyloma uniflorum (Lam.) Verdc] is an orphan and/or underutilized crop largely grown locally by communities as cultural heritage in special niches in developing countries (Chahota et al. 2013; Uma et al. 2013; Bhartiya et al. 2015; Cullis and Kunert 2017; Ellis 2016; Mall 2017; Fuller and Murphy 2018). Although this orphan legume serves as an important component of diets of resource-poor masses to provide balanced nutrition, feed for animals in drought-prone regions along with their potential health and environmental benefits, it has been typically overlooked in terms of resources for their promotion as well as not being traded internationally due to the limited economic importance in the global market (Bhartiya et al. 2015; Cullis and Kunert 2017). Among 13 Planta (2019) 250:891–909 various orphan crops, horse gram is an important legume with the potential to be developed into a commercial crop (Cullis and Kunert 2017). However, its cultural and agricultural importance is undervalued and little attention has been paid by the scientific community and industry either because it is not considered economically important or due to its large and complex genomes like other orphan crops (Deodhar 2016; Kamei et al. 2016). Among underutilized crops, horse gram has special significance as a legume of indemnity under harsher and more dynamic environmental conditions. It belongs to the genus Macrotyloma, tribe Phaseoleae, sub-family Faboideae of the family Fabaceae (Ranasinghe and Ediriweera 2017). Horse gram has been an important legume since the beginning of agriculture and has earned its common name ‘horse gram’ for its use as fodder to horses and cattle for centuries (Fuller and Murphy 2018). Horse gram confides itself in temperate and subtropical regions of the world encompassing African countries (Sudan, Zaire, Angola, Zimbabwe, Mozambique, Ethiopia, Kenya, Namibia, Somalia, South Africa, Tanzania), Asian countries (Bhutan, China, India, Nepal, Pakistan, Philippines, Sri Lanka and Taiwan) and its cropping zones are also found in Australia (Blumenthal and Staplesz 1993; Durga 2012; Krishna, 2013). It is widely used as a food and fodder crop among low-income groups in developing countries and serves as a co-staple food as well as a key vegetable protein source for millions of rural inhabitants in the Indian subcontinent (Kadam et al. 1985). In India, horse gram is a minor legume grown annually on nearly 3.26 lakh ha (Directorate of Economics and Statistics (DES), 2016–2017) of land and constitutes approx. 1–2% of total pulses area mainly in states such as Karnataka, Andhra Pradesh, Tamil Nadu, Odisha, Maharashtra, Chhattisgarh, Bihar, Jharkhand and MP (Singh 2013). A sharp declining trend in acreages, production and productivity levels have been observed from almost the past 2 decades (Fig. 1) in horse gram as compared to other major pulses grown in India due to social disdain, changing lifestyle and lack of policies to mainstream traditional crops. This legume is nutrient dense and could be useful in diversifying diets and alleviating micronutrient deficiencies among poor rural communities (Prasad and Singh 2015). Beyond its use as food, plant residues and straw of horse gram are commonly used as nutritious fodder for livestock, fuel supplement and green manure in Australia and Southeast Asia (Brink, 2006; Bhardwaj et al. 2013). In addition, horse gram is a hardy legume having appreciable tolerance against various abiotic factors such as drought, heat, salinity and heavy metal stresses (Sharma et al. 2015a) with indomitable pest resistance due to antimicrobial properties (Kawsar et al. 2008). Horse gram is mainly grown as an intercrop or mixed crop with various cereals like sorghum, pearl millet, finger millet, maize and little millet in all horse Planta (2019) 250:891–909 893 Fig. 1 The cultivation and productivity of pulses in India from 2000/01 to 2016/17 (a area, b yield). (Source: Directorate of Economics and Statistics, India https://eands.dacnet.nic.in) gram-growing regions (Krishna 2010) without any adverse impact on yield and availability of fodder (Witcombe et al. 2008), besides minimizing undesirable environmental impacts through reduced fertilizer and pesticide requirements (Lithourgidis et al. 2011). In subsistence farming systems, Panch Dhani (i.e., mixed cropping of five crops, viz., horse gram, cow pea, Indian bean, niger and castor) in Karnataka (Kumar 2006) and Barah Anaaja (i.e., a traditional system mixed cropping in which seeds of twelve food grains, viz., black soybean, amaranth, kidney beans, cowpea, horse gram, buck wheat, sesame, Perilla, maize, green gram, black gram, etc., are mixed with finger millet) in Uttarakhand are unique examples of traditional mixed cropping involving horse gram to combat drought conditions as well as to conserve crop diversity (Zhardhari 2001). Owing to high nutritional quality and its adaptability to harsh environmental conditions, horse gram is emerging as a potential future food legume that could contribute to global food and nutrition security (Morris 2008). This review provides an overview of ancient orphan legume horse gram on its nutritional, nutraceutical potential, recent advances in genetic improvement and its prospects as a food and forage crop to contribute for the solution of future food and nutritional security in climate change era. Horse gram: super food legume of nutritional importance Ancient orphan legume horse gram has been recognised as a potential food source for the future by US National Academy of Sciences (Shukla et al. 2006). It has high nutritional value comparable to other commonly grown pulses (Table 1). As a nutrient-dense legume, horse gram provides food and nutritional security to many low-income communities of developing countries. It serves as nutritious food, green leafy vegetable, sprouts, feed and fodder (Bhartiya et al. 2015) owing to high protein (18–29%) content, carbohydrate (57.2%), minerals (3.2%) comprising calcium (287 mg/100 g), iron (8.4 mg/100 g) and phosphorus (311 mg/100 g), crude fibre (5.3%) and vitamins like thiamine (0.42 mg), riboflavin (0.2 mg), niacin (1.5 mg) and vitamin C (1.0 mg/100 g) (Gopalan et al. 2006). The crude fibre, calcium, iron, manganese and molybdenum content of horse gram are higher than chick pea and pigeon pea which are largely consumed by majority of the Indian population (Longvah et al. 2017; Sadawarte et al. 2018). The seeds and sprouts of horse gram are excellent examples of ‘functional food’, as it has a role in lowering the risk of various diseases and exerting health-promoting effects in addition to their nutritive value (Ramesh et al. 2011). It is also consumed as a leafy vegetable in some parts of India and its leaves possess relatively high mineral content (4.50%) as compared to other common vegetables (1.5–2.4%) (Mandle et al. 2012) besides anthocyanins which are potent antioxidants and have antiinflammatory properties (Morris 2008). Hay prepared from horse gram also contains 16.2% protein and 1.8% fat (Haq 2011). Protein content of horse gram (21.73%) is higher compared to other commercial pulses like chick pea (Cicer arietinum) (18.77%), kidney bean (Phaseolus vulgaris) (19.91%), 13 894 Planta (2019) 250:891–909 Table 1 Nutritional composition of horse gram in comparison to other major pulses (per 100 g edible portion) Composition Bengal gram Moisture (g) 8.56 ± 0.37 Protein (g) 18.77 ± 0.42 Minerals (g) 2.78 ± 0.13 Fat (g) 5.11 ± 0.11 Total dietary fibre (g) 25.22 ± 0.39 Carbohydrate (g) 39.56 ± 0.16 Energy (kcal) 1201 ± 9 Minerals and trace elements Iron (mg) 6.78 ± 0.75 Magnesium (mg) 160 ± 17.5 Manganese (mg) 2.17 ± 0.33 Molybdenum (mg) 0.064 ± 0.031 Phosphorus (mg) 267 ± 21.9 Potassium (mg) 935 ± 37.9 Sodium (mg) 26.56 ± 10.12 Calcium (mg) 150 ± 18.3 Zinc (mg) 3.37 ± 0.26 Vitamins Thiamine (mg) 0.37 ± 0.40 Riboflavin (mg) 0.24 ± 0.011 Niacin (mg) 2.10 ± 0.06 Total folic acid (µg) 233 ± 12.9 Vitamin E (mg) 1.72 ± 0.07 Red gram Black gram Green gram Lentil Dry peas Kidney bean Horse gram 9.30 ± 0.45 20.27 ± 0.72 3.53 ± 0.03 1.38 ± 0.08 22.84 ± 0.43 42.48 ± 0.77 1146 ± 10 8.70 ± 0.33 21.97 ± 0.63 3.35 ± 0.03 1.58 ± 0.06 20.41 ± 0.06 43.99 ± 0.76 1219 ± 5 9.95 ± 0.42 22.53 ± 0.43 3.22 ± 0.04 1.14 ± 0.17 17.04 ± 0.38 46.13 ± 0.64 1229 ± 10 9.20 ± 0.77 22.49 ± 0.58 2.39 ± 0.35 0.64 ± 0.02 16.82 ± 1.30 48.47 ± 1.12 1251 ± 23 9.33 ± 0.61 20.43 ± 0.79 2.41 ± 0.09 1.89 ± 0.06 17.01 ± 0.63 48.93 ± 0.45 1269 ± 13 9.87 ± 0.30 19.91 ± 1.44 3.28 ± 0.21 1.77 ± 0.04 16.57 ± 0.63 48.61 ± 0.65 1252 ± 14 9.28 ± 0.57 21.73 ± 0.29 3.24 ± 0.11 0.62 ± 0.04 7.88 ± 0.02 57.24 ± 0.50 1379 ± 9 5.37 ± 1.36 5.97 ± 0.56 155 ± 23.1 190 ± 19.1 1.34 ± 0.29 1.83 ± 0.34 0.087 ± 0.054 0.99 ± 0.053 312 ± 38.3 345 ± 36.5 1303 ± 103 1093 ± 24.5 19.03 ± 0.11 26.80 ± 3.77 139 ± 11.8 86.18 ± 8.99 2.99 ± 0.25 3.05 ± 0.24 0.74 ± 0.028 0.15 ± 0.015 2.42 ± 0.18 229 ± 19.0 0.80 ± 0.06 4.89 ± 0.46 7.57 ± 0.67 5.09 ± 0.45 6.13 ± 0.77 8.76 ± 1.16 198 ± 39.2 101 ± 13.9 123 ± 8.1 173 ± 9.7 152 ± 18.1 1.05 ± 0.08 1.55 ± 0.26 1.08 ± 0.09 1.24 ± 0.11 3.13 ± 0.41 0.174 ± 0.058 0.076 ± 0.057 0.113 ± 0.084 0.054 ± 0.035 0.124 ± 0.056 353 ± 33.6 274 ± 27.4 334 ± 18.3 409 ± 32.4 296 ± 22.5 1177 ± 74.3 756 ± 63.6 922 ± 67.4 1324 ± 195 1065 ± 42.4 12.48 ± 0.07 11.20 ± 0.08 23.40 ± 0.07 10.45 ± 0.05 12.14 ± 0.17 92.43 ± 10.68 76.13 ± 7.23 75.11 ± 13.93 126 ± 8.1 269 ± 34.9 2.67 ± 0.13 3.60 ± 0.23 3.10 ± 0.14 2.69 ± 0.34 2.71 ± 0.21 0.32 ± 0.024 0.11 ± 0.08 1.85 ± 0.13 134 ± 14.2 0.23 ± 0.02 0.45 ± 0.027 0.27 ± 0.011 2.16 ± 0.13 145 ± 5.4 0.33 ± 0.02 0.40 ± 0.073 0.22 ± 0.026 2.54 ± 0.12 132 ± 6.7 0.19 ± 0.02 0.56 ± 0.049 0.16 ± 0.013 2.69 ± 0.15 110 ± 9.3 0.32 ± 0.02 0.30 ± 0.020 0.19 ± 0.018 2.42 ± 0.15 316 ± 20.1 0.23 ± 0.01 0.32 ± 0.002 0.24 ± 0.033 1.82 ± 0.26 163 ± 5.3 0.27 ± 0.02 Source: Indian Food Composition Tables, National Institute of Nutrition, Indian Council of Medical Research (Longvah et al. 2017) pigeon pea (Cajanus cajan) (20.27%) and dry peas (Pisum sativum) (20.43%) but comparable to black gram (Vigna mungo) (21.97%), lentil (Lens culinaris) (22.49%) and green gram (Vigna radiata) (22.53%) (Longvah et al. 2017). The amino acid profiles of horse gram are similar to other grain legumes, whereas it contains high lysine content, an essential amino acid as compared to chick pea, pigeon pea and black gram along with other major amino acids such as arginine, histidine, valine and leucine (Prasad and Singh 2015). Deficiency of essential sulphur-containing amino acids, such as methionine and tryptophan may be overcome through cereal-based diet (Bhartiya et al. 2015). Horse gram proteins exhibit free radical scavenging capacities which can be used as a food supplement, natural antioxidant and useful as therapeutics for health benefits in humans (Petchiammal and Hopper 2014). Bioactive peptides of horse gram protein possess antimicrobial activity, antioxidant activity, anticarcinogenic activity, hypocholesterolemic effect, reduced serum triglycerides, increased lean muscle mass, protection against pathogens, regulation of blood glucose levels, and satiety effects (Prasad and Singh 2015). Horse gram seeds are rich in low glycemic carbohydrates, resistant starch, oligosaccharides and dietary fibre than cow pea (Vigna unguiculata) and green gram (Vigna radiata) (Herath et al. 2018). 13 Carbohydrate of horse gram seed have about 36% starch comprising digestible (85%), resistant (14.47%) and resistant starch associated to insoluble dietary fibres (3.38%) (Bravo et al. 1999). Insoluble dietary fibres of horse gram seeds have positive effects on intestine and colon physiology, besides other homoeostatic and therapeutic functions in human nutrition (Kawale et al. 2005; Sreerama et al. 2012b). Resistant starch is considered as prebiotic among the new generation of dietary fibres, whereas non-digestible carbohydrates of horse gram are helpful in the dietary management of diabetes (Samanta et al. 2011). Moreover, horse gram is highly suitable for human consumption, because 72% of the fatty acids are polyunsaturated comprising linoleic acid and α-linolenic acid (Mishra and Pathan 2011), which impart a beneficial effect on the functional development of the brain and nervous system (Ryan et al. 2007; Morris et al. 2013). Phytosterol esters of horse gram lipids possess antiulcer and healing effects on acute gastric ulceration produced by alcohol (Jayraj et al. 2000; Berger et al. 2004). It also contains a considerable quantity of water and liposoluble vitamins such as thiamin, riboflavin, niacin and tocopherols (Bolbhat and Dhumal 2012; Longvah et al. 2017). Despite the well-known nutritional and health-promoting effects, horse gram has more anti-nutritional factors like trypsin inhibitor activity Planta (2019) 250:891–909 (9246 TIU/g), phytic acid (10.2 mg/g), polyphenols (14.3 mgGA/g) and oligosaccharides (26.8 mg/g) than other commonly consumed pulses which have restricted its utilization as human food (Sreerama et al. 2012b). However, some of the commonly known non-nutritive compounds like phytic acid, phenols, tannins are presently being considered as bioactive substances having antioxidant, anticarcinogenic and hypoglycaemic activities therefore, as per consumer preference, retaining or removal of these compounds could be facilitated (Bhatt and Karim 2009). Conventional processing methods such as dehulling, germination, cooking and roasting have enhanced acceptability and nutritional quality in addition to optimal utilization of horse gram as human food (Kadam et al. 1985). Horse gram has been linked to reduced risk of various diseases and its daily consumption may impart immense nutritional and health benefits due to presence of both nutritive and non-nutritive bioactive substances (Prasad and Singh 2015). Moreover, it has the potential for utilization as nutraceutical, food and forage for malnourished and drought-prone areas of the world (Morris 2008). So far, this legume has not received much attention by researchers and food industries and meagre attempts have been made for its utilization as nutraceutical and/or functional food, thus it needs to be explored as a source of nutraceutical in food industries (Bhartiya et al. 2015). 895 Nutraceutical properties of horse gram Recent research has revealed that most of the bioactive compounds in legumes possess antioxidant properties which play a role in the prevention of some cancers, heart ailments, osteoporosis and other degenerative diseases (Maphosa and Jideani 2017). In spite of the scanty literary evidences on the role of horse gram for disease prevention, its relation with imparting health benefits is fairly extensive. This legume is a treasure house of recuperative properties due to the richness of proteins, dietary fibres, macro- and micro-nutrients and beneficial phytochemicals essential for maintaining human health (Fig. 2). In the recent years, there has been increased awareness about functional foods and nutraceuticals possessing various bioactive substances which have antioxidant activity, anticarcinogenic, antiinflammatory, antineurodegenerative, antidiabetic, antiviral, skin photoprotective, antiallergic, antiplatelet, antiaging, cytoprotective and DNA-protective properties (Prakash and Sharma 2014). In the traditional system of medicine, horse gram was used for treating haemorrhoids, tumours, bronchitis, cardiopathy, nephrolithiasis, urolithiasis, splenomegaly, strangury, ophthalmopathy, verminosis, inflammation and liver problems (Suriyamoorthy et al. 2014). However, ingredients imparting nutraceutical properties from horse gram still need to be isolated and characterized. Horse gram possesses various Fig. 2 Therapeutic importance of horse gram imparted by various nutrients and bioactive compounds 13 896 nutraceutical properties which are beneficial in maintaining human health and these are described below. Antidiabetic properties Diabetes is a major public health concern across the globe and mainly of type I (insulin-dependent diabetes) and type II (progressive impairment of insulin secretion by pancreatic ß-cells and by a relative decreased sensitivity of target tissues to the action of this hormone) (Singhal et al. 2014). Unprocessed raw horse gram seeds not only possess antihyperglycemic properties but also have qualities which reduce insulin resistance (Tiwari et al. 2013). Horse gram is beneficial for the dietary management of diabetes as it cause lower glucose release into the blood stream due to high non-digestible carbohydrate content (Bhartiya et al. 2015). Dietary calcium and magnesium consumption was also suggested to reduce the type II diabetes risk (Kumar et al. 2016), thus, plentiful quantity of these minerals in horse gram could be helpful in reducing the risk of type II diabetes. The presence of phytic acid also exhibits protection against diabetes mellitus (Kumar et al. 2010) by inhibiting enzymatic digestion of starch (Pugalenthi et al. 2005). Further, reduction in starch-induced postprandial glycemic excursion by virtue of potent intestinal α-glucosidase inhibitory activity as well as lowering insulin resistance by inhibiting protein-tyrosine phosphatase 1β was found in unprocessed raw horse gram (Tiwari et al. 2013). Alpha-amylase inhibitor isolated from the seeds of horse gram was shown to have an antidiabetic effect with minimum pathological changes in streptozotocinnicotinamide-induced diabetic mice (Ranasinghe and Ediriweera 2017). Phenolic extracts from horse gram flour also showed inhibition of enzymes associated with hyperglycemia (Sreerama et al. 2012a). Thus, functional ingredients of horse gram may be effectively utilized for curing diabetes and related complications. Antioxidative and anticarcinogenic properties Several phytochemicals act as dietary antioxidants to protect chronic diseases caused due to oxidative damage of cellular molecules. In recent years, with the advent of nutraceutical concept and increased health consciousness among masses, prevention of diseases in natural ways through healthy foods has gained huge attention. Richness of bioactive compounds like polyphenols, proteins and its peptides, phytic acid, flavonoids, saponins, isoflavones and lignans causing strong antioxidant and anticarcinogenic activity suggest the use of horse gram as an alternate functional food (Prasad and Singh 2015). Horse gram seeds, particularly the seed coat contain a high amount of polyphenols mainly phenolic acids (3,4-dihydroxy benzoic acid, vanillic acid, caffeic acid, p-coumaric acid, ferulic acid, chlorogenic acid, syringic 13 Planta (2019) 250:891–909 acid and sinapic acid), flavanoids (quercetin, kaempferol and myricetin) and tannins which have been reported to exhibit antioxidant activity (Panda et al. 2015; Singh et al. 2017a, b). In the extracts of horse gram plant, it was found that the concentration of p-coumaric acid and p-hydroxy benzoic acid was the most abundant (Kawsar et al. 2008). Horse gram also has a high concentration of 2,2-diphenyl1-picrylhydrazyl (DPPH), 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), radical scavenging capacity and ferric-reducing antioxidant power (FRAP) than other legumes (Bhardwaj and Yadav 2015). In vitro models such as reducing power assay, DPPH assay, total phenolic assay and total antioxidant assays showed higher antioxidant abilities in horse gram sprouts than its seeds (Ramesh et al. 2011). A proteinaceous protease inhibitors ‘Hayanin’ isolated from horse gram seed coats displayed significant antioxidant activity against different reactive oxygen species and effectively prevented hydroxyl and superoxide radicals (Lingaiah and Srinivas 2016). Horse gram extract also improved the concentration of antioxidant enzymes such as superoxide dismutase, catalase and glutathione in rabbits with high-fat diet-induced oxidative stress (Panda and Suresh 2015). Methanolic extract of horse gram significantly lowered the level of thiobarbituric acid reactive substances (TBARS) and enhanced the level of reduced glutathione in high-fat diet rabbits (Singhal et al. 2014). Horse gram acts as an antiinflammatory and antioxidant agent and its supplementation significantly increased the activities of antioxidant enzymes superoxide dismutase, catalase, glutathione peroxidase without altering the activities of the inflammatory mediators such as cyclooxygenase, lipoxygenase, myeloperoxidase, nitric oxide synthase, monocyte chemo attractant protein-1 (MCP-1), tumour necrosis factor-alpha (TNF-α), and interleukin-1-beta (IL-1β) (Rajagopal et al. 2017). Phytochemicals and antioxidants are two particular nutraceutical components that impart extensive anticarcinogenic effect by acting as terminators of free radical and singlet oxygen species (Kumar et al. 2016). Presence of such bioactive substances in horse gram imparts protection from different types of cancers prevalent in human population. Moderate to high antioxidant and anticancer activity against human osteosarcoma cell line (MG 63) was observed by seed extracts of horse gram (Chakraborty and Abraham 2016). Horse gram sprouts are also known to be a rich source of isoflavones like genistein and daidzein which are known to inhibit the proliferation of breast cancer cells (MCF-7) (Sukanya and Gayathri 2014). In in- vivo and in- vitro studies, horse gram trypsin inhibitor was also found to be associated with suppression of carcinogenesis (Bhardwaj and Yadav 2015). Horse gram has high levels of antioxidant and radical scavenging activities as well as protective role in carcinogenesis therefore, research attempts for isolation and utilization of potential bioactive compounds are essential for the best Planta (2019) 250:891–909 applicability of this legume as nutraceutical and functional food ingredient. Antihyperlipidemic properties Globally, cardiovascular disease has emerged as a ubiquitous cause of morbidity and leading contributor to mortality (Panda et al. 2015). Metabolic risk factors like insulin resistance, hypertension, cholesterol abnormalities, central obesity and an increased risk for blood clotting generally intensify the risk cardiovascular diseases (Saely et al. 2005). Horse gram seed extract provides protection against hyperlipidemia and cardiac abnormalities through its phenolic phytoconstituents particularly ferulic acid (Panda et al. 2015). Methanolic extract of horse gram to high-fat diet rats showed normalization of levels of these lipids in the plasma and tissues (Singhal et al. 2014) and anti-hypercholesterolemic effect of horse gram extract resulted in significant decrease of total cholesterol, triglycerides, low-density and very low-density lipoprotein, serum glutamate oxaloacetate transaminase and serum glutamate pyruvate transaminase levels with significant increase in high-density lipoprotein (Kumar et al. 2013). The globulin fraction from horse gram was also found to have hypolipidemic action in rats fed with high-fat high-cholesterol diet (Bhardwaj and Yadav 2015). Thus, investigations suggest the utilization of horse gram in treating hyperlipidemia and cardiac abnormalities. Anticalcifying properties Kidney stone is one of the oldest known diseases of urinary tract and the third most common disorder among urinary diseases (Nirumand et al. 2018). It is solid concretion or crystal aggregation of dietary minerals (Upadhyay et al. 2015) generally formed when urine becomes super-saturated with insoluble compounds containing calcium, oxalate (CaOx) and phosphate (CaP) and disruption of balance between solubility precipitation of salts in the urinary tract and in the kidneys (Han et al. 2015). The Indian herbal medicine system Ayurveda acclaimed horse gram to have litholytic property and it is popularly used as folklore medicine for dissolving urinary and kidney stones among rural and tribal masses (Garimella et al. 2001). Clinical studies have also confirmed the presence of water-soluble, heat-stable, polar, non-tannin, non-protein crystallization inhibitors in seed extract of horse gram (Bhartiya et al. 2015). Reduction in stone size with reduced recurrence of calcium oxalate stones has been observed by leaf and seed extract of horse gram (Singh et al. 2010; Khare et al. 2017). Hydro-alcoholic extract of horse gram seeds has been found to be preventive against ethylene glycol-induced nephrolithiasis due to the antioxidant activity of the active compounds of the plant (Saha and Verma 2015). Various phytochemicals like 897 phenolic compounds, flavonoids and steroids isolated from the aqueous fraction of horse gram seeds were found to be effective in dissolution of experimentally prepared kidney stones (Atodariya et al. 2013). The role of phytochemicals in the management of stone formation in the urinary tract include diuretic, antispasmodic and antioxidant activity of the active compounds of the plant as well as an inhibitory effect on crystallization, nucleation and aggregation of crystals (Nirumand et al. 2018). Perusal of various research studies revealed a great scope for the isolation of functional active ingredients from horse gram and development of drug formulations for curing kidney stones efficiently in a costeffective manner. Miscellaneous health properties Existing health reports also have evidence that phytochemicals present in seed extract of horse gram possess significant antimicrobial activities against some human fungal and bacterial strains. Among them, tannins and phenolic compounds elicited broad spectrum activity against human bacterial pathogens such as Escherichia coli, Klebsiella pneumonia, Pseudomonas argentinensis, Pseudomonas sp., Bacillus subtilis, Vibrio harveyi, Salmonella paratyphi, Pseudomonas aeruginosa and Vibrio mimicus through cell membrane lysis, inhibition of protein synthesis, proteolytic enzymes and microbial adhesions (Parvathiraj et al. 2015), whereas flavonoids have the potential to be used in treating various skin diseases (Suriyavathana et al. 2018). Fresh and dried sprouts of horse gram also revealed a wide spectrum of potential phytopharmaceuticals possessing antibacterial activity against several human pathogens (Abiraami and Gowrie 2017). Search for new and alternative antimicrobial substances from plant sources is important for therapeutic and protective uses to avoid the problem of drug resistance in human pathogens against commonly used antibiotics (Marimuthu et al. 2016). Potent dietary protective factor for peptic ulcer is also present in horse gram (Jones 1987). Phospholipids, sterol esters and sterols present in horse gram lipid are found to have gastroprotective properties as well as promote the healing of gastric ulceration (Tovey 2015). Phenolic acid (p-coumaric acid) in horse gram also exhibited antiulcer activity by attenuating the ulcer elevated levels of malondialdehyde and restored the ulcer depleted levels of reduced glutathione and the antioxidant enzymes superoxide dismutase, catalase, glutathione peroxidise and glutathione reductase (Panda and Suresh 2015), therefore, horse gram extracts could play an important role in protection against ulceration. Alcohol extracts of horse gram seeds exhibited potent anthelmintic activity comparable to standard piperazine citrate against Pheretima posthuma (Sree et al. 2014) suggesting its use as dietary food for eradicating worms (Singh et al. 2017a, b). Horse gram is known for various 13 898 health benefits, still this legume is unexplored for many undiscovered phytochemicals and innate health-promoting aspects. Great possibilities exist for isolation, chemo-profiling, pharmacology, biological evaluation, toxicological consequences of functional active ingredients from horse gram and development of drug formulations and functional foods for curing various diseases. History, origin and domestication The genus Macrotyloma consist of 25 wild species (Panda et al. 1985) having diploid chromosome numbers 2n = 2 × = 20, 22, 24 (Bhardwaj et al. 2013) with possible genome size of 400 mbps (Hirakawa et al. 2017). The name Macrotyloma was derived from the Greek words macros (large), tylos (knob) and loma (margin) in reference to knobby structures on the pods (Kaldate et al. 2017). There is no knowledge about the progenitors of horse gram and the species relationship has also not been properly understood (Kumar 2006; Krishna 2013). However, the chromosomal evolution of Macrotyloma uniflorum has been known to progress in two directions, 1 group with M. uniflorum, M. baumannii and M. axillare having 20 small chromosomes, whereas the other group with M. glabrescens, M. lignosus and M. argentines has 22 large chromosomes (Kumar 2006). India is considered to be the centre of origin of cultivated species of horse gram (Sharma et al. 2015a), however, the presence of wild or naturalized horse gram has been recorded both in Africa (Central, East and Southern Africa) and India (Verdcourt 1982; Brink 2006) therefore, Southwest India (Arora and Chandel 1972) and Africa (Dana 1976) are considered as gene-rich centres of horse gram (Mehra and Magoon 1974). The African continent possesses all the species of genus Macrotyloma (Dikshit et al. 2013) and Savannah woodlands are well documented as the favoured habitat for wild African populations of horse gram which may be a suggestive evidence of preferred ecology for the wild forms (Fuller and Harvey 2006). In India, two species Macrotyloma uniflorum and Macrotyloma ciliatum are found (Panda et al. 1985) and as per South Asian archaeobotany, horse gram appears to have been widely cultivated across the Gangetic belt during 2500–2000 BC (Fuller and Harvey 2006). Horse gram has been an important crop since the beginning of agriculture in many parts of South Asia and the most widely recovered pulse crop in prehistoric or early historic sites in India (Murphy and Fuller 2017). This legume has been well represented by archaeological finds across India and has been extensively reported from the Northwest in Haryana state, the Western part of Gujarat (the Saurashtra peninsula) and the South Deccan (Karnataka). These regions are also considered as plausible foci of early cultivation or domestication of horse gram (Fuller and Murphy 2018). The regional origins of horse gram are 13 Planta (2019) 250:891–909 obscure, however, as per the evidence of remnant wild populations, two separate domestication regions, Northwestern and Peninsula regions of India have been identified for the wild progenitors of horse gram (Fuller and Murphy 2018). The plains and hills of low altitude extending Southwards in the Western Ghats in Southwest India are considered as the primary centre of origin and due to counter-migration of human beings from Southern Indian plains and hills, this legume probably diffused to Northern and Western parts of the Indian subcontinent (Krishna 2013; Bhartiya et al. 2015). Evolution of cultivated horse gram from wild progenitors has not been well explored and wild species of Macrotyloma from African woods and savannahs are unlikely to have ever contributed to the domesticated gene pool, since this species is not cultivated in Africa (Fuller and Harvey 2006). Similarly, evidences from grains and cotyledons from Neolithic settlements selected all over the Indo-Gangetic belt seems not to have contributed to the evolution of domesticated Southern Indian horse gram (Krishna 2013). During the course of evolution, modifications from wild to domesticated form (i.e., domestication syndrome) due to selection pressure has occurred, but domestication syndrome has also not been extensively studied in horse gram. The domestication syndrome for pulses mainly includes increase in seed size, reduced pod shattering and loss in germination inhibition (Fuller 2007) and evidences show that in Southern Indian horse gram, reduction of seed dormancy occurred between 2000 and 1200 BC, whereas increase in seed size may have been delayed by 2000–4000 years after domestication (Murphy and Fuller 2017). Among evolutionary rates of these two domestication traits, the non-shattering trait evolved faster than grain size with a higher selection coefficient, whereas grain size increase resulted from selection for increased seedling vigour of larger grains (Purugganan and Fuller 2011). Ecology and botanical description Horse gram is an annual legume which can be grown in sub-humid to semi-arid climates, generally in drought and heat stress-prone areas where other crops do not thrive and invariably fail to grow (Krishna 2010; Ramya et al. 2013). Under natural precipitation, this legume is fast growing, suitable for both grains and fodder and its drought tolerance as well as adaptability to marginal environmental conditions make this legume an important crop for famine and dry areas (Ramteke et al. 2016). It requires an optimum temperature of 20–30 °C for normal growth and development with annual rainfall of 300–600 mm and its wider adaptability allows it to grow at 40 °C temperature (Krishna 2010; Mehra and Upadhyaya 2013) and up to an altitude of 1800 m above mean sea level (Haq 2011). Horse gram grow well on soils with low nitrogen and soil organic matter with a pH range Planta (2019) 250:891–909 of 5–7.5 in inceptisols, alfisols and coastal sandy soils of East Africa and also in oxisols in Central Africa whereas, in inceptisols, alfisols, laterites and sandy soils as cover crop in Asia (Krishna 2013). This hardy legume can withstand poor climatic conditions and edaphic factors, but it cannot withstand water logging and it is completely intolerant to frost (Bhartiya et al. 2015). The horse gram plant is a low-growing, slender, suberect annual or perennial herb with slightly twining downy stems and branches and grows up to 30–60 cm in height. It is variable in flowering time, plant growth habit and growth pattern, flower colour, stem colour, plant height and other yield-related traits as well as in the altitudinal variations observed for agro-morphological, nutritional and antioxidant properties (Bhartiya et al. 2017). It is a relatively short duration crop which easily fits into crop sequences that involves two main crops (Krishna 2010) and completes the life cycle from seed to seed in 120–180 days (Cook et al. 2005) depending upon genotype and its growth environment. However, it may take longer time at higher altitudes 899 under Northern hill ecosystem of India. In India, it is sown in both Rabi and Kharif seasons but yield levels are generally more in Rabi season due to fewer incidences of diseases and pest insects (Bhartiya et al. 2014). Leaves are trifoliate with entire leaflets, 2.5–5.0 cm in length, pilose, ovate with minute stipules that are 1 cm in length. Horsegram flowers in cluster on short axillary peduncles, bisexual, hypogynous, zygomorphic, complete, pentamerous, calyx downy, papilionaceous, petals five and standard longer than wings, keel obtuse, stamens diadelphous with filaments alternatively short and long, anthers dithecous, introse, uniform, dorsified. Gynaecium with superior monocarpellary ovary, unilocular with 5–7 marginal placentation (Kumar 2006). Pod linear, 3–5 cm long, curved, beaked, downy, and dehiscent, with 5–7 small, rhomboid shaped, 3–6 mm long, flattened, light red, brown, black coloured or mottled seeds (Fig. 3). The four M. uniflorum varieties, viz., M. uniflorum var. uniflorum (pods 6–8 mm wide, native of India and widely cultivated in the tropics as cover and forage crop), M. uniflorum var. stenocarpum (pods 4–5.5 mm wide, pubescent leaflets, wild Fig. 3 a Field view of horse gram crop at ICAR-VPKAS, Almora, Uttarakhand, India. b Mature plant, c Different types of seed coat colours in horse gram 13 900 plant in Acacia bushland and thicket in Africa and India), M. uniflorum var. verrucosum (pods 4–5.5 mm wide, pubescent leaflets, wild plant in grassland in Kenya, Tanzania and Mozambique) and M. uniflorum var. benadirianum (pods 4–5.5 mm wide, leaflets densely velvety, wild plant in sand dunes of Somalia and Kenya) can easily be distinguished based on pod characteristics (Jansen 1989). Utilization of horse gram gene pool and genetic diversity: where we stand? In genus Macrotyloma, all the species, viz., M. africanum, M. dewildemanianum, M. ellipticum, M. fimbriatum, M. stenophyllum, M. stipulosum, M. oliganthum, M. geocarpa, M. bieense, M. brevicaule, M. coddii, M. daltonii, M. decipiens, M. densiflorum, M. hockii, M. prostratum, M. schweinfurthii, M. maranguense, M. kasaiense, M. rupestre and M. tenuiflorum are distributed in Africa, whereas M. axillare occurs both in Australia and Africa, M. ciliatum occurs in Asia and Africa and M. uniflorum in Asia, Australia and Africa (Dikshit et al. 2013; http://apps.kew.org/). Efforts to conserve the horse gram germplasm at the global level are lacking to a great extent and only a few organizations namely, Germplasm Resources Information Network (GRIN) of the US Department of Agriculture (USDA) conserved 35 accessions, Kenya Agricultural Research Institute (KARI), Kikuyu, Kenya has 21 accessions and the Australian Tropical Crops and Forages Genetic Resources Centre, Biloela, Queensland have conserved only 38 accessions of horse gram (Brink 2006; Chahota et al. 2013). Diversity in horse gram is eroding sharply due to changing socio-cultural and economic dimensions of the farming community resulting in considerable reduction in acreage of this crop in India (Bhartiya et al. 2017). However, substantial efforts have been made to conserve the diversity of this valuable food legume by Indian institutes, but the information regarding farm conservation is scanty. The efforts to collect and conserve the horse gram germplasm started in 1970s with the inception of a collaborative project between Indian Council of Agricultural Research and USDA on food security in Haiti under Public Law 480 and since then 1627 accessions of horse gram have been collected and maintained at different satellite stations of NBPGR from almost all the horse gram-growing areas of India (Chahota et al. 2013). Phenotypic characterization of the horse gram germplasm from different growing regions has indicated high diversity among accessions (Gupta et al. 2010; Sahoo et al. 2014; Bhartiya et al. 2017). Promising accessions were identified for various traits, viz., early high yielding (PLKU 20, PLKU 49A, IC 16976, IC 18679 and BC 18037), long podded (NIC 11387, IC 94591, IC 120825,120837, IC 120838, 120844 and NIC 11095) and high pod number (IC 94592, IC 110286, IC 120837) (Patel et al. 1997) through germplasm 13 Planta (2019) 250:891–909 evaluation studies. A hill gene pool of horse gram is being maintained at ICAR-VPKAS, Almora and breeders over the years have developed cultivars like VL Gahat 1, VL Gahat 8, VL Gahat 10, VL Gahat 15 and VL Gahat 19 utilizing the highly diverse accessions. Further, meagre information is available on the utilization of wild horse gram species in breeding programmes, however, M. axillare is known to be resistant to yellow mosaic disease (Kumar 2006), whereas a wild horse gram species M. sar-gharwalensis (accession is registered with NBPGR vide number IC 212722) was identified from Uttarakhand hills that contains about two times higher protein levels (38.37 ± 1.03%) than other commonly grown horse gram genotypes (Yadav et al. 2004). Similarly, three Macrotyloma species namely M. axillare, M. daltonii and M. africanum from Australia have potential as forage plants (Kumar 2006). There are very few reports of genetic diversity and molecular marker development on this legume. Genetic diversity and population structure using RAPD and ISSR markers also exhibited a high level of genetic diversity, strong genetic structuring and low level of natural admixture in horse gram accessions from different geographical origins in India (Sharma et al. 2015a). A high level of diversity was found among 360 Indian horse gram accessions from different geographical origins and two gene pools namely Himalayan origin and Southern India were revealed through SSR primers and 24 morphological traits (Chahota et al. 2017). The diversity among horse gram landraces from diverse geographic origins using isozymes and PCR-based (RAPD, ISSR and SSR) markers was also examined and accessions originating from Himachal Pradesh showed maximum diversity with resistant reaction against powdery mildew and anthracnose (Rahar et al. 2007) suggesting their potential to be utilized in breeding programmes. Utilization of available horse gram genetic resources for crop improvement falls far short of the desired characterization and evaluation of germplasm for important agronomic traits that are a crucial component driving the utilization of germplasm by crop breeders. Horse gram improvement: breeding progress so far Horse gram is considered as a low-grade pulse of poor tribal masses has not attracted much research efforts like other major pulses and very limited work has been carried out for its improvement. Despite the presence of many significant properties in the species, the area and production could not be increased due to its poor plant architecture with many wild characteristics such as indeterminate twining growth habit, thermo and photosensitivity, late flowering, asynchronous maturity and poor harvest index resulting in unsuitability of this crop for modern farming system (Henry et al. 2006). Further, little genetic information for yield and other desirable traits has restricted its genetic Planta (2019) 250:891–909 improvement and posed a hurdle in systematic breeding of this legume. In India, horse gram breeding is being carried out since the crop is included in the Indian Council of Agricultural Research (ICAR) coordinated, All India Networking Project (AINP) on arid legumes and more than 22 improved cultivars have been released for different horse gram-growing regions of the country (Singh 2013). For the genetic improvement of the crop, conventional breeding methods like pure line selection and hybridization followed by pedigree method of selection has remained the major breeding strategies. Horse gram is autogamous in nature with a high degree of self-pollination consequently, has a narrow genetic base and artificial hybridization is tedious due to tiny flower buds, early hours of flowering, short flowering duration and poor pod setting (≤ 2%). Therefore, mutations were induced to generate a broad spectrum of variability for plant architecture, yield, nutritional traits, photo- and thermo-insensitivity and earliness (90–110 days), whereas resistance to yellow mosaic virus and suitability to sole cropping under high population density are other important parameters for increasing horse gram production (Dikshit et al. 2013). Three doses (150, 250 and 350 Gy) of gamma radiation exhibited desirable mutants with semi-dwarf, determinate, photo-insensitive, very early flowering with early and synchronous maturity traits in M2 generation (Chahota et al. 2013). To improve the horse gram both as grain and forage crop, three highyielding mutants CRIDA-18R (with high protein content of 29.6% and non-shattering), CRHG-4 and CRHG19 were released as variety by Central Variety Release Committee (CVRC) for South India, whereas CRHG-6 and CRHG-8 have been registered by Plant Germplasm Registration Committee of ICAR with registration number INGR 11017 and INGR 11018, respectively (Salini et al. 2014). Gamma radiation (100–600 Gy) and EMS (0.2–0.6%) separately and in combination also produced viable mutants for plant height, primary branches, pods per plant, seeds per pod, pod length, 1000 grain weight and yield per plant (Bolbhat and Dhumal 2009). Attempts were also made to induce polyploidy in horse gram by colchicine treatment, but results were not encouraging as reduced seed setting was caused may be due to zygotic sterility in tetraploids (Sen and Vidyabhushan 1960). Wild progenitors can provide valuable genetic resources for the improvement of cultivated species, but most of the wild Macrotyloma species remained unexplored for desirable traits except wild species M. axillare which is known to posses desirable yield traits like high number of pods per plant as well as resistance to yellow mosaic disease, tolerance to cold and drought (Kumar 2006; Morris 2008; Dikshit et al. 2013) and M. sar-gharwalensis with high protein (38.37 ± 1.03%) and lipid (10.85 ± 0.16%) content (Yadav et al. 2004) suggests its use in selective breeding 901 for superior nutritive values. These wild species can provide valuable genetic resources for the improvement of cultivated horse gram. Horse gram is a twining and lodging prone crop and therefore, widely grown as an intercrop along with maize, grain amaranth, oilseeds (groundnuts, niger and sesame), millets (finger millet and sorghum), grasses (marvel grass, kidney bean) and trees (A. nilotica, A. indica, A. procera) in different Indian states (Kumar 2007). Altering plant architecture can enable this crop for sole cropping and the resultant reduction in fodder yield may be compensated by developing genotypes with higher number of branches as well as grain yield components. In horse gram traits, viz., 100 seed weight, number of pods per plant, numbers of pod clusters per plant, number of primary branches, pod length, number of seeds per pod and plant height are positively correlated with grain yield. Thus, developing determinate genotypes with these yield components should be the principal approach for enhancing grain yield (Alle et al. 2016). Improvement in grain yield remains the most important breeding objective in horse gram but a wide gap in yield levels exists between potential yield and yields realized at farmer’s fields, because most of the horse gram-growing areas are still under local cultivars and land races with low grain yield. Improved technology can enhance the productivity by 20–25% over the existing practice (Sharma et al. 2016). Seed yield of horse gram usually varies from 0.13–1.2 tonnes/ha in India to 1.1–2.2 tonnes/ha in Australia, whereas green forage yield varies from 5–14 tonnes/ha in India to 4.4 tonnes/ ha in Australia (Haq 2011). Improved varieties coupled with modern agronomic practices can bring about significant improvement in horse gram productivity and bridge the production gap. Horse gram is also serve as an excellent forage and selection of variety which could give high grain as well as high forage yield is the wise option for horse gram-growing regions of India. Various diseases and insect-pests insects are also a cause of yield penalty in horse gram and among them, anthracnose caused by Colletotrichum species is a major yield constraint in horse gram causing 65% reduction in seed germination and crop stand (Udayasankar et al. 2015). Affected plants show dark brown circular spots, particularly on pods. Leaves, petioles and stems are also affected. Spots are usually depressed with dark centres and bright red-yellow or orange margins and these spots later become brown. When the infection is severe, the affected parts may wither off (Majumdar 2011). Cultivars, viz., AK- 21, PHG-9, CODB 2 and IC 470275 for C. dematium and HPKC-39, HPKC- 57 and HPKC-33 against C. truncatum are sources of resistance for this disease (Udayasankar et al. 2017). Among viral diseases, yellow mosaic virus is one of the serious constraints in peninsular India with 50–100% incidence causing substantial loss in grain yield in both summer and early 13 902 rainy season crops and genotypes, viz., AK-38, HG-GP, DPI-2278, Paiyur-1 and Paiyur-2 were identified as highly resistant under field conditions (Prema and Rangaswamy 2017). Young leaves first show yellow patches and in due course the yellow discoloration increases in new growth. The infected plants mature late, bear very few flowers and have small and distorted pods with shrivelled and reduced size of seeds (Kumar 2007). Other important diseases of horse gram are Cercospora leaf spot, Fusarium wilt, rust (Uromyces appendicularis), Pellicularia root rot, bacterial leaf spot (Xanthomonas phaseoli) and Ascochyta blight (Purushothaman et al. 2006). At the early stages of the crop growth, grasshoppers, leaf-eating caterpillars and aphids infest the crop and cause considerable damage, whereas other important insect-pests of horse gram are pod borer (Etiella zinckenella), hairy caterpillar (Azazia rubicans), Bihar hairy caterpillar (Spilosoma obliqua), aphid (Aphis craccivora) and grasshoppers (Majumdar 2011). Genomic resources and molecular breeding: where are we now? Genomic resources in horse gram lagged considerably behind major pulses. The work to develop genomic resources started with the cross-transferability of SSR makers in horse gram from the well-characterized legume species, viz., Medicago truncatula, Cajanus cajan, Arachis hypogea, Trifolium spp., Cicer arietinum and Pisum sativum and it was observed that the frequency of red clover SSRs amplification in horse gram is much higher compared to other warm season crops. To further enrich the genomic resources, SSR and ILP markers were developed from expressed sequence tag (EST) sequences and transcriptome data available in public domain and the potential of these newly developed markers were also assessed for their transferability across different legume species, viz., Macrotyloma axillare, M. gharwalensis, Trifolium pratense, Phaseolus vulgaris, Vigna umbellata, V. radiata, Cicer arietinum, Pisum sativum, Lens culinaris, Vigna mungo, Glycine max and Vigna unguiculata. The cross-transferability that ranged from 25.5% (G. max) to 68.0% (V. umbelleta) revealed the extent of syntenic relationships across different legumes (Sharma et al. 2015b). A sufficient number of genic SSRs from transcriptome sequence data from two horse gram lines M-191 and M-249 and SSRs were designated as M. uniflorum micro-satellite (MUMS) and these SSR-containing sequences covered 16.25% of the total transcriptome. Sharma et al. (2015a) validated 245 primer pairs in 20 horse gram accessions and of these, 27 (13 non-specific and 14 with no amplification) primers failed to produce specific amplicons, whereas 218 amplified specified products. Given the estimated ~ 400 Mb size of the horse gram genome, the SSR density was 58 per Mb in the DNA sequence of horse gram (Chahota et al. 2017) lower 13 Planta (2019) 250:891–909 than those reported for other plant species, viz., Arabidopsis (370 SSRs/Mb), rice (529 SSRs/Mb), poplar (508 SSRs/ Mb) and grapevine (506 SSRs/Mb) (Liu et al. 2013). In all, 2458 additional SSR primer pairs were designed and 117 SSRs were characterized in 48 diverse lines of horse gram and the di-nucleotide and tri-nucleotide accounted for 47% of all of the SSR identified and the remaining 53% consisted of tetra, penta and hexa-nucleotide repeats SSRs (Chahota et al. 2017). It has been noted that the SSR in different locations within the gene might play different functional roles in organism development, adaptation, survival and evolution were never ending. The results from these studies indicated that tri-nucleotide SSRs are useful for M. uniflorum for different genetic studies. The markers developed here could be useful for linkage and QTL mapping involving interspecific mapping population. As a first step towards characterization of genes that contribute to combating abiotic stresses, 1050 ESTs were isolated and sequenced (Reddy et al. 2008). Considering horse gram to be a wonderful source for drought tolerance, transcriptome analysis was also conducted for understanding the genetic basis of responses to drought tolerance in drought-sensitive (M-191) and -tolerant (M-249) horse gram genotypes and validation of the obtained unigenes against already known drought-responsive ESTs of horse gram suggested their prominent role under drought stress conditions (Bhardwaj et al. 2013). Overexpression of NAC transcriptional factor (MuNAC4) from horse gram known for imparting tolerance to plants against abiotic stresses such as drought and salinity conferred enhanced drought tolerance in groundnut transgenic plants (Pandurangaiah et al. 2014) and also a horse gram heat shock protein (MuHSP70) provided tolerance to heat, cold, drought, salinity and oxidative stress in transgenic Arabidopsis (Masand and Yadav 2016). Genomic resources in many resource-poor orphan legumes in addition to comparative genomic analysis can be utilized in the identification of the QTLs of important agromorphological characters and subsequently, introgressed in the locally adapted genotypes of horse gram through markerassisted selection. The availability of genome sequence of warm season orphan legume pigeonpea will be extremely valuable for comparative genomics, genome mapping, marker development and molecular breeding and availability of such cross-species genetic information may be important for horse gram that have little or no genomic resources available (Chahota et al. 2013). The information of genetic transformation in horse gram is also lacking, however, the reproducible in vitro tissue culture-based direct shoot and root regeneration from shoot tip and cotyledonary node explants (Tejavathi et al. 2010) and regeneration of plants from immature cotyledon (Mohamed et al. 2005) and from leaf segments (Mohamed et al. 2004) are known in horse gram. Planta (2019) 250:891–909 Uses: as traditional food, forage or more? Horse gram is a multipurpose legume with versatile end uses as food, forage and green manure. In rural areas, horse gram seeds, sprouts and whole meal are used by large populations (Kadam et al. 1985). In the Indian Himalayan region, horse gram is a traditional pulse and generally its whole grains are used to prepare gravies and consumed with rice. Horse gram is part of various ethnic recipes in Uttarakhand and normally it is soaked prior to cooking and prolonged cooking on mild flame is practiced in most of the local recipes which make the preparation digestible (Bhartiya et al. 2014). In Southern India, it is used in various tasty preparations such as curry and papad (Kumar 2007). In some parts of India, leaves of horse gram are also used as a nutritious vegetable (Mandle et al. 2012) and a wide spectrum of antibacterial, antiinflammatory and antioxidant activities were revealed by horse gram sprouts (Abiraami and Gowrie 2017). Horse gram is highly suitable for commercial foods and processed horse gram flour could be suitably used in the preparation of various food products (Thirukkumar and Sindumathi 2014). Functional properties, viz., water absorption capacity, emulsion activity and emulsion stability of horse gram flour are superior to chickpea flour which suggest its usefulness in the preparation of bakery products, soups and snacks and composite flours as partial substitutes of chickpea flour in snacks, confectionery and other traditional food products (Sreerama et al. 2012b). However, the non-availability of readymade processed food products has restricted the usage and acceptability by consumers, despite its nutritional superiority. Therefore, there is a need of value-added ready-touse products and functional foods to offer variety, convenience and quality to consumers as well as for revamping of horse gram cultivation. Horse gram is also used as feed and nutritious forage (Morris 2008). It picks up good foliar growth in a short time and thus produces huge palatable foliage for livestock. In Northern Australia, non-shattering pods are used as stand over dry season livestock feed and as forage crop in many tropical countries, especially in Australia, Southeast Asia and Africa (Blumenthal and Staplesz 1993). The biomass of the crop, i.e., stem residues, leaves and pod walls serve as high nutritional quality forage (Brink 2006). Green hulms of horse gram contain approximately 30–40% of plant nutrients and are mostly fed to animals (Krishna 2010). It takes around 6 weeks after sowing to be used as forage, whereas only 4–6 months are needed for seed crop (Brink 2006). The green forage yield levels vary from 5–14 tonnes/ha in India, whereas up to 4.4 tonnes/ha in Australia (Haq 2011). Horse gram does not receive high levels of nutrient inputs therefore loss via soil erosion, leaching, surface flow may influence forage productivity (Krishna 2010). Horse gram seed meal has a better promise as food supplement and satisfactory 903 in supporting growth and maintenance in animal feeding (Sreelekshmi and Avita 2011). Plant residues, straw as well as whole grains (usually given after boiling) of horse gram are generally utilized as cattle feed (Kadam et al. 1985; Reddy et al. 2008) and chick/grower ration without any deleterious effect (Ravindran and Sundar 2009). Hay containing 70% M. uniflorum was found effective also for sheep to prosper best than 70% Stylosanthes hamata, Crotalaria juncea and Vigna unguiculata (Murthy and Prasad 2005). Moreover, it has the potential for further utilization as food and forage for malnourished and drought-prone areas of the world (Morris 2008). Horse gram is also grown as preparatory crop in newly reclaimed lands in many parts of South India to improve soil fertility (Kumar 2006), as a cover crop for soil and water conservation in semi-arid regions as well as was found to be useful in integrated fertility management in dry land agriculture (Reddy et al. 2008). Therefore, it has great significance in sustainable agriculture (Anitha et al. 2006) as well as dry land agriculture. Way ahead: scope and prospects The genetic improvement of horse gram has remained sluggish compared to other pulses particularly, with respect to plant architectural traits, quality, adaptation as well as resistance to various biotic and abiotic stresses due to paucity of information on genetics of various agro-morphological traits as well as of valuable genetic resources. The genetic base of the crop can be broadened by exploiting the wild relative gene pool, but still the wild species of this legume have not been evaluated and characterized systematically except M. axillare and M. sargharwalensis to some extent and most of them remained unexplored for their valuable traits. Interspecific hybridization between M. uniflorum and M. axillare may offer a promising avenue for mutual genetic improvement of the two species, however, the fertility barriers between the distantly related M. uniflorum and M. sar-gharwalensis need to be analysed and modern biotechnological tools can be employed to overcome them. There is also a need to systematically analyse the available genetic diversity of cultivated horse gram to identity sources of agronomically important traits such as higher yield, disease resistance, nutritional quality of the grain and others. Genomic information can significantly enhance the breeding efforts towards targeted improvement of horse gram, but molecular breeding efforts are lagging well behind in horse gram than other commercial pulses such as pigeon pea and chick pea. Horse gram has been remained a crop of regional importance without any share in national and international trade and this legume is losing its importance as a food crop like other minor crops in competition with major 13 904 Fig. 4 Roadmap to mainstream underutilized legume horse gram 13 Planta (2019) 250:891–909 Planta (2019) 250:891–909 pulses such as pigeon pea, chickpea, lentil, black gram and green gram in India which is a major horse gramproducing country in the world. The decline in the area and production resulted due to social disdain, changing lifestyle and lack of policies to mainstream traditional crops. However, because of its short life span, the ability to grow under harsh climatic conditions and on a wide range of less fertile soils, it has maintained its existence and may continue to remain a useful crop for dry regions in Asia and Africa. Therefore, considering the potential of horse gram to combat drought and heat stress, there is scope for research focusing on its suitability to future climate adaptation. Horse gram is nutrient dense legume, but the presence of anti-nutritional factors and lack of value addition are deterrents for its dissemination outside of traditional growing areas. The prospects for horse gram in India may gain importance by mainstreaming research on issues related to production and processing value addition. Further, the promotion of underutilised crops should be a part of future agriculture development policies of the country for eradicating malnutrition through improving dietary diversity of masses for eradicating malnutrition should be a part of the future agriculture development programmes in the country. Underutilized crops could be transformed into key players in resolving challenges of food and nutrition security under water-scarce conditions as well as in addressing the poverty and unemployment of growers through a multi-pronged approach comprising conventional research, the use of innovation platforms, policy, markets, advocacy, knowledge management and the involvement of various stakeholders (Mabhaudhi et al. 2017). Therefore, to relocate this legume to have an equal repute as other commodity crops in national and global markets, there is a need to come up with a road map to promote this underutilized legume through innovative research, development and innovations related to various aspects from crop production to consumption (Fig. 4). Collective efforts for public awareness on nutritional value and synergistic government policies supporting cultivation, value addition and marketing of this orphan legume could revamp its cultivation to contribute in addressing emerging issues of food and nutrition security and to save this valuable legume for the coming generations. Author contribution statement AP conceived the idea of the review and the manuscript was drafted by JPA and AB. RKC contributed genomic resources and molecular breeding section and DC improved the manuscript by making necessary additions. 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