Application of molecular markers in breeding rust resistant South African dry bean cultivars

Abstract

Common bean (Phaseolus vulgaris L.) is a widely cultivated legume crop and also an important component in the human diet as it is a great source of protein. It has a wide phenotypic variability especially for traits such as seed types (size and colour) and plant growth habit. Plant breeding has been used to improve traits of importance in common beans and this includes improving disease resistance in market type cultivars. The aim of this study was to phenotypically screen selected varieties for reaction to rust and to validate the use of the sequence characterized amplified regions (SCAR) molecular markers SK14, Ur4-SA1079/800, SI19, and SAE19 for rust resistant genes Ur-3, Ur-4, Ur-5 and Ur-11 in the common bean breeding populations of interest. International germplasm varieties Mexico 235, CAL 143, Flor de Mayo, Cornell 49242, G 21212, G 5828, BelDakMi-RMR-22, Mexico 309, Nep-2, Mexico 54 and BAT 332 showed complete resistance to South African rust races in field evaluations at different localities. Popular South African cultivars including red speckled sugar beans and small seeded types were mostly susceptible to rust disease. Cultivars Mkuzi, Kamiesberg, Tygerberg, Teebus-RR1, Teebus-RCR2, PAN 123 and OPS-KW1 were resistant to rust. Large seeded cultivars Sederberg, Werna and RS 7 were moderately resistant. International rust resistant genotypes Nep-2, BelDakMi-RMR-23 and Mexico 309 and the susceptible South African cultivars Bonus and RS6 were used as the parents for in crosses. The crosses were then screened for resistance in a greenhouse against rust races 1, 3, 5 and 11 following standard artificial inoculation procedures. Following hybridization, the parents, F1 and F2 plants characteristics were observed of an individual that showed the interaction of the genotype with the environment (phenotyped). The first trifoliate leaves of the F1:2 populations were harvested for DNA extraction and molecular analyses of the targeted resistance genes. The SCAR markers were tested for presence in the parents and linkage in the segregating populations. In the RS6 x BelDakMi-RMR-23 population, gene Ur-11 could be followed with marker SAE19 (64% linkage), but Ur-3 (SK14) and Ur-4 (SA1079/800) marker selection cannot be used in subsequent generations. Close linkage (91.5%) of the SK14 marker to the Ur-3 gene was observed in the Bonus x Nep-2 population. Marker SI19 will be applicable for marker selection of the Ur-5 gene (80% linkage) present in the RS6 x Mexico 309 population. These crosses will now be used to stack the resistance genes in one or more cultivars utilizing the validated markers for selection of putative resistant progeny.