Evidence for a disease-resistance pathway in rice similar to the NPR1-mediated signaling pathway in Arabidopsis

The Arabidopsis NPR1/NIM1 gene is a key regulator of systemic acquired resi stance (SAR). Overexpression of NPR1 leads to enhanced resistance in Arabid opsis. To investigate the role of NPR1 in monocots, we over-expressed the A rabidopsis NPR1 in rice and challenged the transgenic plants with Xanthomon as oryzae pv. oryzae (Xoo), the rice bacteria] blight pathogen. The transge nic plants displayed enhanced resistance to Xoo. RNA blot hybridization ind icates that enhanced resistance requires expression of NPR1 mRNA above a th reshold level in rice. To identify components mediating the resistance cont rolled by NPR1, we used NPR1 as bait in a yeast two-hybrid screen. We isola ted four cDNA clones encoding rice NPR1 interactors (named rTGA2.1, rTGA2.2 , rTGA2.3 and rLG2) belonging to the bZIP family. rTGA2.1, rTGA2.2 and rTGA 2.3 share 75, 76 and 78% identity with Arabidopsis TGA2, respectively. In c ontrast, rLG2 shares highest identity (81%) to the maize liguleless (LG2) g ene product, which is involved in establishing the leaf blade-sheath bounda ry. The interaction of NPR1 with the rice bZIP proteins in yeast was impair ed by the npr1-1 and npr1-2 mutations, but not by the nim1-4 mutation. The NPR1-rTGA2.1 interaction was confirmed by an in vitro pull-down experiment. In gel mobility shift assays, rTGA2.1 binds to the rice RCH10 promoter and to a cis-element required sequence-specifically for salicylic acid respons iveness. This is the first demonstration that the Arabidopsis NPR1 gene can enhance disease resistance in a monocot plant. These results also suggest that monocot and dicot plants share a conserved signal transduction pathway controlling NPR1-mediated resistance.