OsRAR1 and OsSGT1 physically interact and function in rice basal disease resistance

The RAR1 and SGT1 proteins function synergistically or antagonistically in plant innate immune responses. Here, we show that the rice orthologs OsRAR1 and OsSGT1 physically interact in vivo and in yeast. They displayed conserved roles in Arabidopsis disease resistance through ectopic expression in the Arabidopsis rar1 and sgt1 mutants. Overexpression of OsRar1 and OsSGT1 in rice significantly increased basal resistance to a virulent bacterial blight Xanthomonas oryzae pv. oryzae PXO99 but not to another virulent strain DY89031, suggesting race-specific-like basal resistance conferred by OsRar1 and OsSGT1. OsRar1-OE and OsSGT1-OE plants also enhanced resistance to all four virulent blast fungal Magnaporthe oryzae races. Overexpression of the OsSGT1-green fluorescent protein (GFP) fusion most likely caused a dominant negative phenotype which led to race-specific-like basal resistance. Transgenic plants overexpressing OsSGT1-GFP show enhanced resistance to DY89031 but decreased resistance to PXO99, implying that OsSGT1 might be the target of a component required for DY89031 virulence or OsSGT1-GFP might stabilize weak resistance proteins against DY89031. Consistent with the hypothesis of the dominant negative regulation, we observed the reduced sensitivity to auxin of OsSGT1-GFP plants compared with the wild-type ones, and the curling-root phenotype in OsSGT1-OE plants. These results collectively suggest that OsRar1 and OsSGT1 might be differentially required for rice basal disease resistance. Our current study also provides new insight into the roles of OsSGT1 in basal disease resistance.