Identification of quantitative trait loci involved in the response of common bean to Pseudomonas syringae pv. phaseolicola

Halo blight, caused by Pseudomonas syringae pv. phaseolicola (Burkn.) Downs (Psp), is an important disease in common bean (Phaseolus vulgaris L.). This study investigated the genetic control of the resistance to two local isolates of Psp (ITA-812 and ITA-684) in a recombinant inbred line (RIL) population derived from the cross between the bean genotypes Xana and Cornell 49242. The cultivar Cornell 49242 exhibited moderate resistance to these isolates, whereas cultivar Xana was susceptible. The RIL population showed a continuous variation in response to the two isolates. Analysis revealed four significant quantitative trait loci (QTLs): Psp4(812XC) and Psp6.1(812XC) located on linkage groups Pv04 and Pv06 (for the response to isolate ITA-812), and Psp6.1(684XC) and Psp6.2(684XC) located on Pv06 (for the response to isolate ITA-684). The QTLs Psp6.1(812XC) and Psp6.1(684XC) were located in the same genetic region (Psp6.1), close to the Psp6.2 region in which the QTL Psp6.2(684XC) was mapped. A genetic dissection was undertaken to verify the consistency of these three QTLs located on the end of Pv06. Four sets of RILs were established according to the genotypes (Xana and Cornell 49242) of the underlying markers for the regions Psp6.1 and Psp6.2. Re-evaluation of these sets of lines revealed significant differences relative only to isolate ITA-684. The set of lines with the Cornell genotype in both regions was significantly more resistant than the other three sets of lines. This suggested that both regions were necessary to detect a significant effect in the response to isolate ITA-684. In the physical positions corresponding to these two genetic regions, in silico analysis revealed 16 candidate genes (putative orthologous genes) that carried sequences homologous to the resistance genes RPM1, FLS2, RPG1/RPG1-B, and Pto-all of which confer resistance to P. syringae in different species. The results confirm that, apart from the major genes implicated in resistance to Psp, specific bean genotypes exhibit a quantitative mode of inheritance of resistance to Psp.