QTL mapping for nodule number and common bacterial blight in Phaseolus vulgaris L.

A recently developed bean RFLP linkage map was used to identify genetic ele ments affecting quantitative trait loci (QTLs) in two contrasting common be an genotypes, BAT-93 and Jalo EEP558, under two levels of mineral nitrogen: low - 0.25 mM NH4NO3 and a high - 6 mM NH4NO3. QTLs affecting nodule numbe r (NN) and response to Xanthomonas campestris by. phaseoli, which causes co mmon bacterial blight (CBB) were identified and mapped. Analyses of 70 F-2- derived F-3 families, using the F-1, the two parents, and a nodulation-defe ctive mutant (Nod(-)) inoculated with R. tropici UM1899 under both levers o f N showed significant differences (P < 0.0001) among the F-3 families for NN. Under low N, three genomic regions influenced both traits, with seven linke d markers. In three of the six regions influencing NN, higher NN was associ ated with the Jalo EEP-558 allele, whereas in only two regions was the BAT- 93 allele associated with higher NN. One-way analysis of variance, with eac h marker as the independent variable and NN as the dependent variable, and interval mapping analysis identified four QTLs, which accounted for 45% of the total variation, and two additional QTLs near to yet unassigned loci. I n linkage group D7, one QTL mapped to the same region as a QTL for CBB. Under high N, three additional regions were linked to NN, one where the BAT -93 allele was closely associated with CH18 (chitinase), and the others whe re the Jalo EEP-558 allele was associated with CHS (chalcone synthetase) an d PAL-1 (phenylalanine ammonia lyase). Four regions for CBB were mapped adj acent to or in the same region as a QTL for NN. Thus, N showed dual and opp osite effects on the expression of NN and CBB. Analysis of these RFLP marke rs revealed these 'hidden' favorable alleles and can serve as an indirect s election tool to increase NN and resistance to CBB.