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9.1 Introduction

Vigna species play a particularly important role in diets of people in Asia. In East Asia, azuki bean has a cultural significance as it is mixed with glutinous rice (red rice) on days of celebration (Lumpkin and McClary 1994). In the hills of Southeast Asia, rice bean is a source of protein for poor people. Mung bean is used in a variety of forms including noodles, sprouts, flour, and whole seeds and is an ingredient of many Asian dishes. In South Asia, several different Vigna species form an essential component of a vegetarian’s diet. Several of the wild Vigna species are harvested as an occasional food. In addition, cowpea introduced to Asia long ago from Africa is used for both its seeds and, in the form of yard-long bean, its pods as a vegetable.

The Asian Vigna generally refers to the Vigna species in the subgenus Ceratotropis (Tomooka et al. 2002a). However, there are Vigna species in other subgenera that occur in Asia; some of them are domesticated such as V. unguiculata and V. vexillata. In this chapter, we will discuss all the Vigna crops of Asia (Table 9.1). Despite the obvious importance of Vigna in Asia, where most of the world’s poor and undernourished live, it is only recently that there has been increased attention to their understudied genetic resources and application of biotechnology tools to them. Surprisingly, a recent paper on “orphan grain legumes” does not mention the Asian Vigna (Varshney et al. 2009), so perhaps the Asian Vigna should be called the “forgotten grain legumes.” The international agricultural research community of the Consultative Group on International Agriculture Research (CGIAR) system has not conducted research on the Vigna crops of Asia, although an affiliate organization, the World Vegetable Center (AVRDC), does conduct research on mung bean. Despite this, there have been two recent genetic resource monographs, one on the Asian and the other on the African Vigna (Maxted et al. 2004; Tomooka et al. 2002a). Laboratories in different countries have conducted various genomic and genetic studies on the Vigna crops of Asia, but this research community has, unfortunately, not benefitted from a globally coordinated effort. A summary of the current status of Vigna genome maps (updated from Kaga et al. 2005) and molecular mapping of agronomically important traits is presented (Tables 9.2 and 9.3). In this chapter, we will discuss first the recent progress that has been made in understanding the Vigna crops of Asia and their wild progenitor species. Then the other wild Vigna species of Asia that may have a role to play in future crop improvement are discussed.

Table 9.1 Vigna species in Asia
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Table 9.2 Genome linkage maps for Vigna species
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Table 9.3 Molecular mapping of agronomically important traits in Vigna
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9.2 The Vigna Crops of Asia and Their Wild Progenitor Species

9.2.1 Mung Bean [Vigna radiata (L.) Wilczek]

Wild mung bean is very widely distributed from West Africa to Northern Australia. Within its range, germplasm from Australia and the island of New Guinea represents a distinct gene pool (Sangiri et al. 2007). Based on the diversity of domesticated mung bean and archaeobotanical records, mung bean seems to have been domesticated in India (Sangiri et al. 2007; Fuller and Harvey 2006). Recently, a comprehensive genome map of mung bean was published (Isemura et al. 2012). In common with many crops, domestication traits in mung bean are controlled by a few major genes including some minor genes. However, compared to other crops, including domesticated Vigna, domestication-related traits in mung bean were more dispersed across the genome. Several genes, such as hilum and seed color, are found at a similar location in mung bean and other Asian Vigna species. Several QTLs for 100-seed weight in mung bean were located in a similar genomic position to other Vigna species, such as QTLs on linkage groups 1 and 2 that were at a similar location to those on the azuki bean and rice bean genome maps (Fig. 9.1). However, the QTL with the largest effect for 100-seed weight in mung bean was found on linkage group 8 and is specific to mung bean. Further studies of the differences among QTLs and their effect in relation to seed size in mung bean and other domesticated Vigna species will help to explain the marked differences for seed size among domesticated Vigna. Both the mitochondrial and chloroplast genomes of mung bean have been sequenced (Alverson et al. 2011; Tangphatsornruang et al. 2010). The mung bean mitochondrial genome sequence was the first for a legume and is of unexceptional size. The genome is unusually depauperate in repetitive DNA compared with the mitochondrial genomes of other species (Alverson et al. 2011). The chloroplast genome includes a pair of inverted repeats (IRs). In addition, compared to other plant chloroplast genome sequences, V. radiata has two distinct rearrangements of 50 and 78 kb (Tangphatsornruang et al. 2010).

Fig. 9.1
figure 1

Comparison of QTLs for three domestication-related traits among four Vigna crop species

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Among domesticated pulses, mung bean has one of the smallest seed sizes, and seeds are usually green. It has a short growth duration and can readily be intercropped with cereals. Powdery mildew is a major disease of mung bean and breeding for resistance to this disease is a priority. However, breeding progress has been slow because there appear to be many races of the disease, and these races have not yet been described. Several sources of resistance or partial resistance to the disease have been identified. Results of different studies have identified sources of a major resistance QTL on both linkage groups 9 and 6 (Chankaew et al. 2013; Kasettranan et al. 2010). Other resistance genes have been found on linkage group 4 (Chaitieng et al. 2002; Chankaew et al. 2013). Chaitieng et al. (2002) analyzed a mapping population using mung bean accession VC1210A as the resistant parent and found a QTL for field resistance to Thai races of powdery mildew that explained 65 % of the variation to powdery mildew resistance. Mung bean yellow mosaic viruses belong to the geminivirus group (Sunitha et al. 2013) and are highly destructive diseases of mung bean. Breeders have focused on finding resistance to MYMV (mung bean yellow mosaic virus) that is mainly found in south and west South Asia and MYMIV (mung bean yellow mosaic Indian virus) that is prevalent in central and northern South Asia. They are also a threat to a wide range of other legumes such as black gram and soybean.

There are a number of reports analyzing resistance to MYMV in different germplasm and both recessive and dominant genes. The resistant variety SML-668 has two recessive genes for resistance. Sudha et al. (2013) reported that the resistance of mung bean variety ‘KMG189’ is controlled by a single recessive gene. Two studies using different sources of MYMIV resistance, one using the wild mung bean (V. radiata var. sublobata) and the other a breeding line from Pakistan, have found a common major resistance QTL (variously named MYMIV’9_25, qMYMIV1, qMYMIV4) (Chen et al. 2013; Kitsanachandee et al. 2013). This locus was detected in different locations, years, sources of resistance, and scoring systems. The locus has been associated with specific markers; hence, these can be used in marker-assisted selection. Other QTLs have also been detected for MYMIV resistance, but these have not been consistently found in different tests.

Overcoming yellow mosaic virus may require various approaches, both genetic and agronomic, because this virus undergoes genetic recombination in its host whitefly (Bemisia tabaci) which is an efficient vector. Two iron deficiency chlorosis QTL (qIDC) for resistance to iron deficiency have been identified in a cross between susceptible ‘Kamphaeng Saen 2’ and a resistant line from Pakistan (NM10-12) (Prathet et al. 2012). The major QTL on linkage group 3 was the same as the dominant gene (IR) reported by Srinives et al. (2010) in a cross between ‘Kamphaeng Saen 1’ and the same resistant Pakistan line. Resistance to bruchid has been reported in mung bean cultivars (Somta P et al. 2006, 2008; Somta C et al. 2008); however, mung bean breeders are interested in new sources of resistance to this important pest from other Asian Vigna species such as V. umbellata and V. nepalensis (Pandiyan et al. 2010; Somta P et al. 2008; Somta C et al. 2008).

9.2.2 Azuki Bean [Vigna angularis (Willd.) Ohwi and Ohashi]

Wild azuki bean (Vigna angularis var. nipponensis) is widely distributed in the ecological and cultural zone associated with broad-leaved evergreen forests (Isemura et al. 2011) (Fig. 9.2). Analysis of the present-day diversity of azuki beans and abundance of wild azuki beans suggests that Japan is the center of diversity of azuki beans (Yamaguchi 1992; Xu et al. 2000a, b, 2008). The earliest archaeobotanical remains of domesticated azuki bean have been found in the central regions of Japan, Shiga, and Fukui prefectures, dated at 5–6000 BP (Tomooka 2007). These dates are earlier than for archaeobotanical remains in China and Korea (Crawford 2005), suggesting azuki bean was domesticated in Japan. Two paper analyses domestication-related traits by the same method and with similar markers, but involving different parents (Isemura et al. 2007; Kaga et al. 2008). One paper results from a cross between V. angularis and its closely related species, V. nepalensis (female). The other mapping population was derived from wild and cultivated (female) azuki bean parents from Japan. While some results were similar, such as single QTL for pod dehiscence, about 60 % of the QTLs were considered to be different. This highlights the extent of genetic diversity that is available for Vigna breeders among the close relatives of Vigna crops. The useful diversity in species closely related to azuki bean was also shown when resistance to azuki brown stem rot was analyzed (Kondo and Tomooka 2012). Eight disease responses were observed, of which four were newly detected and 28 accessions (of 4 species) among 252 accessions from 26 Asian Vigna species were considered as potential sources of multiple resistance to azuki brown stem rot.

Fig. 9.2
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Distribution of the “Broad-leaved Evergreen Forest Culture (Laurel Forest Culture),” center of wild azuki bean diversity and sites of the archaeological remains where putative azuki bean seeds were found (after Isemura et al. (2011))

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The chloroplast and mitochondrial genomes of azuki bean have been sequenced (Naito et al. 2013). The results were very similar to those for the chloroplast and mitochondrial genomes of mung bean (Alverson et al. 2011; Tangphatsornruang et al. 2010). The results have confirmed the relative stability of the chloroplast genome compared to the mitochondrial genome as has been shown in other species including mung bean. Since Vigna species in Asia are relatively recently evolved, the mitochondrial genome may be more suitable for understanding their evolutionary dynamics than the chloroplast genome (Naito et al. 2013).

9.2.3 Rice Bean [Vigna umbellata (Thunb.) Ohwi and Ohashi]

Wild rice bean (V. umbellata) is not taxonomically distinguished from its domesticated form. This reflects the lack of prominent differentiation between the two forms and possibly the recent domestication of rice bean (Tomooka et al. 2002a). The wild form is distributed in tropical monsoon climate areas from Nepal to East Timor but with most diversity centered on Southeast Asia (Isemura et al. 2011; Tian et al. 2013; Tomooka 2009). Rice bean is cultivated on small scale across a wide area of South, Southeast, and East Asia. It is most common in highlands, sometimes cultivated in the slash-and-burn agriculture of northeast India (Arora et al. 1980), Myanmar, Laos, Thailand, Vietnam, and southern China. It is occasionally grown in Japan, Korea, Indonesia, and East Timor. Rice bean is of particular interest, because it has the highest level of bruchid resistance among the species of Vigna subgenus Ceratotropis (Kashiwaba et al. 2002; Tomooka et al. 2000). Three chemicals, based on the structure of the flavonoid naringenin, were found to be associated with bruchid resistance in rice bean (Tomooka et al. 2006). There have been many efforts to introduce bruchid resistance from rice bean to other domesticated Vigna species. Somta et al. (2006) used a wild species V. nakashimae to develop rice bean linkage map and identify a source of resistance to bruchids. In India, direct crossing between mung bean and rice bean resulted in viable hybrids, and this may enable introduction of bruchid resistance to mung bean varieties (Pandiyan et al. 2010). In addition, resistance to mung bean yellow mosaic virus in rice bean has been analyzed in a cross with mung bean, and it was found to be conferred by a single recessive gene (Sudha et al. 2012, 2013). In Thailand, scientists have attempted to transfer useful traits from rice bean to mung bean by developing tetraploid interspecific hybrids from sterile F1 hybrids (Chaisan et al. 2013). The fertile tetraploid produced artificially may have a potential for improving V. reflexo-pilosa var. glabra (V. glabrescens); the only domesticated Asian Vigna that is tetraploid. Rice bean is a highly prolific seed producer which may reflect its similarity to the wild form. In legumes, domestication has not always resulted in higher seed yield on plant basis (Kaga et al. 2008). A genome map developed for V. umbellata revealed many differences in the location of QTLs for domestication-related traits compared with V. angularis. Major QTLs for domestication-related traits in rice bean were reported to be on linkage group 4, whereas for azuki bean, they were mainly on linkage group 9 (Isemura et al. 2010). Among the QTLs detected in rice bean (69) and azuki bean (76) for domestication-related traits, only 15 were considered to be common. Of these, 15 QTLs were for seed size-related traits such as 100-seed weight, seed length, width, and thickness (Isemura et al. 2011). The numerous species-specific QTLs between these closely related species suggest that they can provide novel genes for breeding.

9.2.4 Black Gram (Vigna mungo (L.) Hepper)

Black gram is thought to have been domesticated in Gujarat and northern Peninsular India (Fuller 2007). Recently, the close relatives of black gram that still grow in northern Peninsular India have been studied and a new nomenclature has been published; Vigna silvestris is the presumed progenitor of black gram, and V. sahyadriana is a species closely related to black gram and other species in India of the section Ceratotropis, V. hainiana, V. radiata, and V. subramaniana (Aitawade et al. 2012). The largest production area for black gram is India. Black gram shows complete resistance to azuki bean weevil (Callosobruchus chinensis) but is susceptible to cowpea weevil (C. maculatus). In contrast, the wild progenitor of black gram shows complete resistance to both azuki bean weevil and cowpea weevil (Tomooka et al. 2000; Souframanien et al. 2010). Genome maps for black gram have been developed and compared to V. angularis (Chaitieng et al. 2006; Gupta et al. 2008). There have been a number of studies of a gigantism mutant of black gram that produces seeds, pods, and vegetative parts much larger than the parent from which it was derived, e.g., parent has 100-seed weight of 4.8 g compared to the mutant’s 7.9 g (Tomooka et al. 2010). The mutant gene is located on linkage group 8 at a site different from seed size (100-seed weight) QTLs in other Vigna species. The potential of transferring this gene to other species is being explored to produce super-domesticated legumes (Vaughan et al. 2007).

Resistance to mung bean yellow mosaic India virus (MYMIV) has been found in an accession of black gram, and this resistance gene has been mapped using SSR markers (Gupta et al. 2013). An SSR marker closely linked to the resistant locus was found that can be used for marker-assisted selection.

9.2.5 Moth Bean (Vigna aconitifolia (Jacq.) Maréchal)

Moth bean is thought to have been domesticated in South Asia where its wild conspecific progenitor is reported to be widely distributed (Arora and Nayar 1984). The outstanding characteristic of this species is its drought and heat tolerance. Consequently, it is grown in arid and semiarid zones of northwest South Asia (Jain and Mehra 1980). Among 15 Vigna species, moth bean showed the highest heat tolerance, surviving conditions of 36 °C for 12 days followed by 40 °C for 11 days; all other Vigna species tested died at 40 °C (Tomooka et al. 2001).

9.2.6 Creole Bean (Vigna reflexo-pilosa Hayata var. glabra (Vigna glabrescens))

Vigna reflexo-pilosa var. glabra is a rare domesticated species, and very few accessions are to be found in the world’s germplasm collections (Tomooka et al. 2002a). It seems, based on herbarium specimens and direct collection, to be mainly grown in Vietnam although there are reports of it being cultivated in Angola, Mauritius, West Bengal, and the Philippines. Little is known about the agronomic aspects of this crop. On the other hand, the wild progenitor, var. reflexo-pilosa, is widely distributed across Southeast Asia into the Pacific (Tomooka et al. 2002a). There have been a number of studies to determine the origins of the two genomes that constitute this allotetraploid (Egawa and Tomooka 1994). The most recent and comprehensive survey confirms some earlier reports that the likely donor species are V. trinervia and V. hirtella (Chankaew et al. 2014b).

9.2.7 Cowpea Complex

Evidence suggests that cowpea (V. unguiculata) was domesticated in West Africa (Ng and Maréchal 1985). Early archaeological remains of domesticated cowpea in Ghana, West Africa, have been dated at between 3360 and 3840 BP (D’Andrea et al. 2007). It is presumed that sometime not long after domestication, the crop came to Asia by trading networks that developed from about 4,000 years ago (Fuller 2003; Fuller et al. 2011). In Africa, cowpea was selected for its seeds (V. unguiculata subsp. unguiculata cv.-gr. Unguiculata), but sometime after it arrived in Asia, divergent selection resulted in a new form of cowpea with long tender pods that could be eaten raw or cooked—yard-long bean (V. unguiculata subsp. unguiculata cv.-gr. Sesquipedalis). In some parts of Asia, such as Sri Lanka, cowpea is being planted instead of other Vigna species because of its higher yield and suitability to double cropping due to its short life cycle. Yard-long bean has the longest pod of any domesticated legume, and the pod can reach up to 90 cm long (35.4 in.) which is quite close to being a yard (36 in.) (http://www.gene.affrc.go.jp/databases-plant_images_detail_en.php?plno=5420610049). However, this is well short of the pod length of some wild legumes with Cassia fistula reported as having pods up to 238 cm (Jayasuriya 2012).

Three recent studies have analyzed the important agronomic traits related to the domestication of yard-long bean (Kongjaimun et al. 2012a, b, 2013). QTLs for domestication-related traits show co-localization on many linkage groups, but linkage groups 3, 7, 8, and 11 appear to be most important. Genomic dissection of pod length revealed 6 and 7 QTLs for this trait on different linkage groups in two populations, F2 and BC1F1, respectively. The QTL with the main effect was on linkage group 7, explaining 30.5 % of the variation. A QTL with a large effect for pod length was also found on linkage group 7 in azuki bean. Linkage group 7 in yard-long bean appears to be important for a range of other domestication-related traits such as size of seed, stem, and leaf. This suggests that further study of genes for trait gigantism in yard-long bean should focus on linkage group 7.

9.3 Wild Vigna in Asia

9.3.1 Vigna aridicola N. Tomooka and Maxted

This species is recently found in the dry zone of Sri Lanka (Tomooka et al. 2002c). It is closely related to the wild form of V. aconitifolia. V. aridicola has been little studied, and there is an urgent need to understand the adaptive mechanism of this species and V. aconitifolia to harsh dry habitats.

9.3.2 Vigna dalzelliana (Kuntze) Verdc.

This species is poorly understood and previously was confused with V. minima (Tomooka et al. 2006). Recent germplasm collecting has found this species in Sri Lanka and India. There have also been collections of Vigna from southern Myanmar that appear to be V. dalzelliana (Tomooka et al. 2006).

9.3.3 Vigna exilis Tateishi & Maxted

Vigna exilis is only found growing in limestone outcrops (Tomooka et al. 2011b). It has been little studied but its distinctive habitat suggests that it may have useful genes for adaptation to specific high pH and dry soil conditions.

9.3.4 Vigna grandiflora (Prain) Tateishi & Maxted

This species should join V. khandalensis on the IUCN Red List of Threatened Species, since it is known currently from only a few rather vulnerable populations in Thailand.

9.3.5 Vigna hirtella Ridley and V. tenuicaulis N. Tomooka and Maxted

Based on AFLP analysis, V. hirtella appears to be a variable species with two eco-types, one of low altitude and another of high altitude (Seehalak et al. 2006; Tomooka et al. 2002b). V. hirtella grows sympatrically with a number of other species in section Angulares of the subgenus Ceratotropis in highland Southeast Asia. A natural hybrid between V. hirtella and V. minima has been found at one site (author’s unpublished data). It is probable that this variable species may naturally hybridize with other species where it grows. Vigna tenuicaulis is relatively recently described using an accession collected in northern Thailand and is closely related to V. hirtella to which it is sometimes confused (Tomooka et al. 2002c). It has a low level of trypsin inhibitor activity that might be a useful characteristic to transfer to Vigna crops (Konarev et al. 2002).

9.3.6 Vigna khandalensis (Santapau) Raghavan & Wadhwa

V. khandalensis is poorly known; not even its chromosome number has been reported. It is considered “near threatened” in the IUCN Red List of Threatened Species (http://www.iucnredlist.org/details/19892969/0). Among the wild Asian Vigna, this species is an erect herb rather than a climbing vine. It is only found in India, mainly in the Western Ghats. It is reported that the seeds are sometimes used as a famine food (Babu et al. 1985).

9.3.7 Vigna minima (Roxb.) Ohwi & Ohashi, V. nakashimae (Ohwi) Ohwi and Ohashi, V. riukiuensis (Ohwi) Ohwi & Ohashi

Vigna minima is widely distributed in variable habitats across East Asia (China), Southeast Asia, and as far east as Papua New Guinea (Tomooka et al. 2002a). The closely related species V. nakashimae and V. riukiuensis have more restricted distributions. These three species are genetically closely related and are called Vigna minima complex (Yoon et al. 2000). V. nakashimae is widely distributed on the Korean peninsula and the Goto islands, Nagasaki, Japan (Yoon et al. 2007; Tomooka et al. 2013). Some populations of V. nakashimae from the Goto islands, living on a hill exposed to strong sea winds, are revealed to have a high level of salt tolerance. V. riukiuensis is found in the southern Okinawa Islands in Japan. It grows on a cliff near the sea and shows a high level of salt tolerance. V. minima is found in both shaded forest floor and open habitats such as around paddy field, suggesting a high level of diversity. Since it has been little studied, it may well consist of several varieties or subspecies. Recently, studies have shown that some accessions of V. minima and V. nakashimae have a high level of resistance to all races of soybean cyst nematode found in Japan (Kushida et al. 2013). These resistant sources are being used in azuki breeding since soybean cyst nematode is becoming an increasingly problematic pest on legumes in Hokkaido, Japan. V. nakashimae has been used to develop an interspecific linkage map with V. umbellata (Somta et al. 2006).

9.3.8 Vigna nepalensis Tateishi & Maxted

V. nepalensis is closely related to V. angularis with which it makes fertile hybrids. V. nepalensis was used as the female parent in a mapping population to analyze domestication-related traits in azuki bean (Isemura et al. 2007). It has a restricted distribution in Nepal and its adjacent localities in India and Bhutan, and it was found growing between the altitude of 350 and 1,650 m (Tomooka et al. 2002a). QTLs for bruchid resistance have been reported from V. nepalensis, and some of these appear to be new sources of resistance including one QTL that is unrelated to seed size (Somta P et al. 2008).

9.3.9 Vigna stipulacea Kuntze

This species is characterized by extremely long peduncles 22–30 cm long that result in the flower and pod rising conspicuously above canopy. This characteristic is probably why this species has been harvested, and some harvested forms have been semidomesticated having increased vegetative organs and weak seed dormancy (Tomooka et al. 2011a). In Tamil Nadu, India, semidomesticated V. stipulacea is grown before or after rice in paddy fields and is considered good forage for cattle and green manure. Despite the labor required to harvest it because of shattering pods, this species is also sometimes used as a human food. Based on comments by farmers, V. stipulacea is resistant to many insects and diseases such as stinkbug and powdery mildew, grows faster (early flowering and maturing) than mung bean and black gram, and has high palatability (Tomooka et al. 2006, 2011b). Seeds are sold in a local seed shop in Tamil Nadu (Tomooka et al. 2011b). Given these positive attributes, the species warrants particular study of its agronomic potential and potential for furnishing new and useful genes to legume breeding.

9.3.10 Vigna subramaniana (Babu ex Raizada) M. Sharma and Vigna hainiana Babu, Gopinathan, and Sharma

V. subramaniana has the lowest level of trypsin inhibitor activity among the subgenus Ceratotropis species studied and absence of chymotrypsin inhibitor activity (Konarev et al. 2002). Taxonomic confusion surrounding this species and V. hainiana has been discussed by Tomooka et al. (2006). There remains a need to more thoroughly understand relationships in the mung bean complex to which these two species belong. There is now sufficient germplasm in various gene banks to permit these studies to be conducted.

9.3.11 Vigna trinervia (Heyne ex Wall.) Tateishi & Maxted

V. trinervia consists of two varieties. Vigna trinervia var. trinervia is widely distributed in South and Southeast Asia and also in the Indian Ocean Islands and Tanzania. Vigna trinervia var. bourneae (Vigna bourneae) is found only in southern India and is distinguished by being covered with white villose hairs. This species is an important cover crop in between plantations of coconut and rubber.

9.3.12 Vigna trilobata (L.) Verdc. and V. indica T.M. Dixit, K.V. Bhat & S.R. Yadav

Vigna trilobata characteristically is found in sandy soils; hence, it is found along the beaches in Sri Lanka. However, it is more commonly found inland where sandy soils predominate such as Tamil Nadu, India, and Sagaing, Myanmar. This species is characterized by its very long taproot and so is found along the coastline of the dry zone, whereas V. marina with shallower roots is found predominately along the coast of the wet zone in Sri Lanka (Liyanage 2013, personal communication to the authors). Previously, variation in the V. trilobata gene pool resulted in the species being divided into two subspecies, subsp. trilobata and subsp. pusilla. However, recently, subsp. pusilla has been raised to the rank of a species called V. indica, differing from V. trilobata in various characteristics of which cylindrical seeds with truncated ends are most obvious. The two subspecies in India are found in the western, central, and northern regions for V. indica and southern and eastern India for V. trilobata (Dixit et al. 2011). V. trilobata is occasionally harvested as seeds or young pods in India (Tomooka et al. 2011a). It has been successfully hybridized with mung bean (female parent) (Pandiyan et al. 2012).

9.3.13 Vigna vexillata (L.) A. Rich. (Tuber Cowpea)

V. vexillata has a pantropical distribution and, like many legumes, produces tubers. In Bali and Timor, Indonesia, this species is cultivated. Since Bali’s cultivated accession seeds increased their size remarkably and lost strong dormancy and pods are indehiscent, it may be considered as fully domesticated. In Bali and Timor, V. vexillata is used for its tubers and seeds as well as forage with estimated yields of 18–30 t ha−1 and 0.8–1.2 t ha−1 for tubers and seeds, respectively. Root protein content is ca. 15 % which is about 2.5 times higher than that of yam (6 %), 3 times higher than that of potato (5 %) and sweet potato (5 %), and 5 times higher than that of cassava (3 %) (Karuniawan et al. 2006). It is also reported to be cultivated in parts of India and harvested by aboriginals in Australia. Based on the genetic study of agronomic traits by hybridization, it was concluded that the wild African and Australian accessions could be used along with var. macrosperma (putative cultivated form) for breeding improved varieties of V. vexillata for forage, cover crop, and vegetable uses (Damayanti et al. 2010a). They also reported a high level of cross incompatibility between the wild and cultivated form in Bali (Damayanti et al. 2010b).

9.3.14 Vigna hosei (Craib) Backer (V. parkeri Backer)

V. hosei is widely distributed in Asia, Africa, and both North and South America. It is a useful legume in intensively grazed pastures and forms a good ground cover in lightly shaded areas. (http://www.tropicalforages.info/key/Forages/Media/Html/Vigna_parkeri.htm).

9.3.15 Vigna luteola (Jacq.) Benth.

V. luteola is native to Africa, Asia, and Australasia but has been used as a short-season and highly palatable pasture or green manure in Europe and the New World. It is particularly useful in wet or waterlogged conditions. It has a wide range of adaptation to soil types, light conditions, and temperatures. An Australian accession, CPI 60428, has been reported to have some jassid insect resistance in humid, subtropical Australia (http://www.tropicalforages.info/key/Forages/Media/Html/Vigna_luteola.htm). It has been used to develop a linkage map to analyze salt tolerance in V. marina (Chankaew et al. 2014a).

9.3.16 Vigna marina (Burm.) Merr.

This species consists of two subspecies that have different geographic distributions: subspecies marina is found in Asia, Australia, the coast of Indian Ocean, and countries of Africa, whereas subspecies oblonga is found on the coastal areas of the tropical Atlantic countries of Africa. The species is reportedly used by some African farmers as a cover crop and green manure and has a potential as a sand-binding agent (Padulosi and Ng 1993). It also produces edible tubers that are eaten by aboriginals in Australia, while in the Maldives seeds are harvested and eaten (Padulosi and Ng 1993). The seeds are used as a coffee substitute in Gabon (Burkill 1995). Diversity analysis has shown that V. marina subsp. oblonga is more closely related to V. luteola than subsp. marina (Sonnante et al. 1997). As a consequence, recent studies to identify genes for salt tolerance in Vigna marina have concentrated on an interspecific mapping population between subsp. oblonga and V. luteola (Chankaew et al. 2014a). The F2:3 population was evaluated for salt tolerance under hydroponic conditions at the seedling and developmental stages. Segregation analysis indicated that salt tolerance in V. marina is controlled by a few genes. Multiple-interval mapping (MIM) consistently identified one major QTL for each trait—Saltol1.1, Saltol1.2, and Saltol1.3 for the percentage of surviving plants in salt water at the seedling stage and leaf wilt and recovery score during vegetative stage, respectively. All of the QTLs detected were located on LG1. These QTLs explained 50.7 %, 41.4 %, and 20.0 % of variation in the evaluated trait, respectively. All of the detected salt-tolerant QTLs, alleles from V. marina subsp. oblonga, increased salt tolerance. The flanking markers of each QTL can facilitate transferring of the salt-tolerant allele from V. marina subsp. oblonga into related Vigna crops. Unlike other Vigna species, Sinorhizobium spp. not Bradyrhizobium spp. forms nodules on V. marina roots (Akatsu, personal communication). The isolated Sinorhizobium strains showed an extremely high level of salt tolerance. The isolates could grow even in a nutrient solution with 5 % NaCl (Tomooka et al. 2011b).

9.3.17 Vigna adenantha (G.F. Meyer) Maréchal, Mascherpa & Stainier

V. adenantha has a pantropical distribution and is occasionally cultivated (Brink and Belay 2006). The green pods and ripe seeds are used as an emergency food. In India, its tuberous roots are eaten in times of food scarcity. It grows in humid swampy locations, along seashores and rivers. The seed has a large cavity between the cotyledons that enables it to float, and distribution patterns suggest it can be dispersed by sea (Brink and Belay 2006).

9.3.18 Interspecific Hybridization

Cross compatibility studies have been reviewed by Tomooka et al. (2002a). Generally, there is no barrier to gene flow between domesticated forms and their closest relatives. Species in the same section of the subgenus Ceratotropis can usually cross with little difficulty. Natural interspecific hybrids have been found between V. hirtella and V. minima in northern Thailand (author’s unpublished observations). Recently, Pandiyan et al. (2010) reported a number of cross-sectional and cross subgenus hybrids. Among these hybrids, the cross between V. radiata and V. umbellata is particularly significant as V. umbellata possesses a high level of resistance to bruchid beetles, one of the most serious pests of Vigna.

9.4 Domestication

The Vigna of Asia offer an unusually broad spectrum of species in various stages of domestication. Plant parts used by humans vary from seeds, green pods, to swollen roots. Several are useful forage species, such as V. stipulacea, V. luteola, and V. marina. Among them, V. marina may be de-domesticated since it has large seeds with non-dehiscent pods. Also in many parts of southern Japan and Southeast Asia, nonnative populations of escaped (de-domesticated or feral) cowpea are common with dehiscent pods (Bervillé et al. 2005).

The Vigna of Asia exhibit extremes in certain agronomic characteristics; among domesticated legumes, the longest pod is found in yard-long bean (V. unguiculata subsp. unguiculata cv.-gr. Sesquipedalis), and among the smallest seeds for a domesticated pulse used for its seeds are mung bean (V. radiata), black gram (V. mungo), and moth bean (V. aconitifolia).

9.5 Ecological Adaptation

The Vigna of Asia are adapted to a range of ecological conditions, among them harsh environments such as arid conditions (V. aconitifolia), sandy and saline soils (V. marina and V. trilobata), alkaline limestone rock soils (V. exilis), exposed windy cliff top environments (V. riukiuensis), waterlogged riverside (V. luteola), and shady forests (V. minima).

9.6 Genomic Studies

Currently, a large sequencing project is under way that aims to sequence 16 Vigna species of Asia (Naito et al. 2013). This information coupled with complete sequence information for the chloroplast and mitochondria of azuki bean and mung bean will provide a foundation of genome information resources for Vigna improvement in the future. Although the Vigna of Asia have been “forgotten” by much of the international agricultural research community, considerable progress has been made by national programs. However, the effort devoted to research on Asian Vigna does not do justice to their importance and potential contribution to agriculture in the future. The very wide ecological adaptation of Vigna in Asia, highlighted in this chapter, suggests that Vigna have much to offer other crops. Gigantism exhibited by the pods of yard-long bean and very high levels of salt tolerance in Vigna marina are two outstanding examples.