Salt stress is becoming a serious problem in food production field. To find sources of salt tolerance, we screened 74 accessions of adzuki bean (Vigna angularis) and 145 accessions of cross-compatible wild relatives (seven species). We performed the primary screening in soil culture and the secondary screening in hydroponic culture, and identified JP205833 of V. riukiuensis (strain 'Tojinbaka') and JP107879 of V. nakashimae (strain 'Ukushima') as the valuable source of salt tolerance. We found these two strains had different salt tolerance mechanism, where 'Ukushima' prevented Na ? accumulation in leaves by filtering Na ? in roots and stems, while 'Tojinbaka' accumulated Na ? throughout the whole plant body. We also found 'Tojinbaka' and 'Ukushima' could retain photosynthesis even under salt stress. In addition, 'Ukushima' and especially 'Tojinbaka' showed even better growth in a salt-damaged field in Fukushima, Japan where soybean cultivar 'Tachinagaha' could not survive. Since both salt tolerant strains are cross-compatible with adzuki bean, our results will facilitate developing salt tolerant cultivar by introducing two different mechanisms of salt tolerance.

Vigna vexillata (L.) A. Rich. (tuber cowpea) is an underutilized crop for consuming its tuber and mature seeds. Wild form of V. vexillata is a pan-tropical perennial herbaceous plant which has been used by local people as a food. Wild V. vexillata has also been considered as useful gene(s) source for V. unguiculata (cowpea), since it was reported to have various resistance gene(s) for insects and diseases of cowpea. To exploit the potential of V. vexillata, an SSR-based linkage map of V. vexillata was developed. A total of 874 SSR markers successfully amplified single DNA fragment in V. vexillata among 1,336 SSR markers developed from Vigna angularis (azuki bean), V. unguiculata and Phaseolus vulgaris (common bean). An F2 population of 300 plants derived from a cross between salt resistant (V1) and susceptible (V5) accessions was used for mapping. A genetic linkage map was constructed using 82 polymorphic SSR markers loci, which could be assigned to 11 linkage groups spanning 511.5 cM in length with a mean distance of 7.2 cM between adjacent markers. To develop higher density molecular linkage map and to confirm SSR markers position in a linkage map, RAD markers were developed and a combined SSR and RAD markers linkage map of V. vexillata was constructed. A total of 559 (84 SSR and 475 RAD) markers loci could be assigned to 11 linkage groups spanning 973.9 cM in length with a mean distance of 1.8 cM between adjacent markers. Linkage and genetic position of all SSR markers in an SSR linkage map were confirmed. When an SSR genetic linkage map of V. vexillata was compared with those of V. radiata and V. unguiculata, it was suggested that the structure of V. vexillata chromosome was considerably differentiated. This map is the first SSR and RAD marker-based V. vexillata linkage map which can be used for the mapping of useful traits.

A rapid quantification method for leaf sodium (Na + ) and potassium (K + ) concentrations was developed using a compact ion meter. Leaf ion concentrations were evaluated for species of Vigna vexillata (L.) A. Rich. after two weeks of treatment with 0-200 mM of sodium chloride. To compare the ion extraction efficiency, extraction solutions of distilled water and ammonium acetate were tested. The ion concentrations of extracts obtained by both solutions were measured using an ion meter, and the values were validated using ion chromatography. For both extraction solutions, the ion meter values were highly correlated with those of the ion chromatograph. However, correlations between ion meter and chromatograph values were largely different for Na + and K + . The rapid quantification of ion concentrations using an ion meter developed in this study was successfully utilized for evaluating differences in leaf Na + concentrations, K + concentrations, and K + /Na + ratio in species of V. vexillata.

Wild relatives of crops have the potential to improve food crops, especially in terms of improving abiotic stress tolerance. Two closely related wild species of the traditional East Asian legume crops, Azuki bean (Vigna angularis), V. riukiuensis “Tojinbaka” and V. nakashimae “Ukushima” were shown to have much higher levels of salt tolerance than azuki beans. To identify the genomic regions responsible for salt tolerance in “Tojinbaka” and “Ukushima”, three interspecific hybrids were developed: (A) azuki bean cultivar “Kyoto Dainagon” × “Tojinbaka”, (B) “Kyoto Dainagon” × “Ukushima” and (C) “Ukushima” × “Tojinbaka”. Linkage maps were developed using SSR or restriction-site-associated DNA markers. There were three QTLs for “percentage of wilt leaves” in populations A, B and C, while populations A and B had three QTLs and population C had two QTLs for “days to wilt”. In population C, four QTLs were detected for Na+ concentration in the primary leaf. Among the F2 individuals in population C, 24% showed higher salt tolerance than both wild parents, suggesting that the salt tolerance of azuki beans can be further improved by combining the QTL alleles of the two wild relatives. The marker information would facilitate the transfer of salt tolerance alleles from “Tojinbaka” and “Ukushima” to azuki beans.