Rice MPK17 Plays a Negative Role in the Xa21 -Mediated Resistance Against Xanthomonas oryzae pv. oryzae

Rice bacterial blight, caused by Xanthomonas oryzae pv . oryzae ( Xoo ), is one of the most serious diseases affecting rice production worldwide. Xa21 was the first disease resistance gene cloned in rice, which encodes a receptor kinase and confers broad resistance against Xoo stains. Dozens of components in the Xa21 -mediated pathway have been identified in the past decades, however, the involvement of mitogen-activated protein kinase (MAPK) genes in the pathway has not been well described. To identify MAPK involved in Xa21 -mediated resistance, the level of MAPK proteins was profiled using Western blot analysis. The abundance of OsMPK17 (MPK17) was found decreased during the rice- Xoo interaction in the background of Xa21 . To investigate the function of MPK17, MPK17-RNAi and over-expression (OX) transgenic lines were generated. The RNAi lines showed an enhanced resistance, while OX lines had impaired resistance against Xoo , indicating that MPK17 plays negative role in Xa21 -mediated resistance. Furthermore, the abundance of transcription factor WRKY62 and pathogenesis-related proteins PR1A were changed in the MPK17 transgenic lines when inoculated with Xoo . We also observed that the MPK17-RNAi and -OX rice plants showed altered agronomic traits, indicating that MPK17 also plays roles in the growth and development. On the basis of the current study and published results, we propose a “ Xa21 -MPK17-WRKY62-PR1A” signaling that functions in the Xa21- mediated disease resistance pathway. The identification of MPK17 advances our understanding of the mechanism underlying Xa21 -mediated immunity, specifically in the mid- and late-stages.