Genetic analysis and molecular mapping of QTLs associated with resistance to bacterial blight in a rice mutant, SA0423

Bacterial blight disease, caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most serious diseases in rice producing areas. SA0423 is a broad-range resistance mutant selected from a popular japonica-type variety, TNG67, using sodium azide mutagenesis. Genetic analysis and QTL mapping of SA0423 were performed using the descendants obtained from crossing with Taichung Native 1, a susceptible and well-known indica variety, by challenging with a Taiwanese Xoo isolate, XF89b. Genetic analysis displayed that the resistance of SA0423 is regulated by quantitative trait loci (QTLs) with incomplete dominance. A linkage map covering 12 chromosomes and consisting of 148 SSR as well as 3 InDel markers was constructed. Three QTLs are identified on chromosomes 11, 8 and 6 and account for 21.1, 11 and 9.6 % of the observed phenotypic variance, respectively. Three QTLs are localized to 6, 7 and 14 confidence intervals, respectively. These QTLs contribute to approximately 47 % of the total phenotypic variation of the F-2 population. No epistatic effect could be detected among the three QTLs. Our results provide a suitable source of potential disease resistance genes and establish a system for improving rice bacterial blight resistance through marker-assisted selection

PHO2-Dependent Degradation of PHO1 Modulates Phosphate Homeostasis in Arabidopsis

The Arabidopsis thaliana pho2 mutant, which is defective in a ubiquitin-conjugating E2 enzyme, displays inorganic phosphate (Pi) toxicity as a result of enhanced uptake and root-to-shoot translocation of Pi. To elucidate downstream components of the PHO2-dependent regulatory pathway, we identified two pho2 suppressors as carrying missense mutations in PHO1, which has been implicated in Pi loading to the xylem. In support of the genetic interaction between PHO1 and PHO2, we found that the protein level of PHO1 is increased in pho2, whereas such accumulation is ameliorated in both pho2 suppressors. Results from cycloheximide and endosomal Cys protease inhibitor E-64d treatments further suggest that PHO1 degradation is PHO2 dependent and involves multivesicular body–mediated vacuolar proteolysis. Using the transient expression system of tobacco (Nicotiana tabacum) leaves, we demonstrated that PHO1 and PHO2 are partially colocalized and physically interact in the endomembranes, where the ubiquitin conjugase activity of PHO2 is required for PHO1 degradation. In addition, reduced PHO1 expression caused by PHO1 mutations impede Pi uptake, indicating a functional association between xylem loading and acquisition of Pi. Together, our findings uncover a pivotal molecular mechanism by which PHO2 modulates the degradation of PHO1 in the endomembranes to maintain Pi homeostasis in plants