A MAPKKK gene from rice, RBG1res, confers resistance to Burkholderia glumae through negative regulation of ABA

Burkholderia glumae causes bacterial seedling rot (BSR) of rice and is a threat to a consistent food supply. When previously screening for resistance against B. glumae in the resistant cultivar Nona Bokra (NB) versus the susceptible cultivar Koshihikari (KO), we detected a gene, Resistance to Burkholderia glumae 1 (RBG1), at a quantitative trait locus (QTL). Here, we found that RBG1 encodes a MAPKKK gene whose product phosphorylates OsMKK3. We also found that the kinase encoded by the RBG1 resistant (RBG1res) allele in NB presented higher activity than did that encoded by the RBG1 susceptible (RBG1sus) allele in KO. RBG1res and RBG1sus differ by three single-nucleotide polymorphisms (SNPs), and the G390T substitution is essential for kinase activity. Abscisic acid (ABA) treatment of inoculated seedlings of RBG1res-NIL (a near-isogenic line (NIL) expressing RBG1res in the KO genetic background) decreased BSR resistance, indicating that RBG1res conferred resistance to B. glumae through negative regulation of ABA. The results of further inoculation assays showed that RBG1res-NIL was also resistant to Burkholderia plantarii. Our findings suggest that RBG1res contributes to resistance to these bacterial pathogens at the seed germination stage via a unique mechanism.