Mechanism And Functional Characterization Of The Mitogen-Activated Protein Kinase Genes In Rice Disease Resistance

Rice is the most important economic crops all over the world. Enhancing the disease resistance of rice is important to our research Strategy. Mitogen-activated protein kinase (MAPK) cascades play important roles in diverse developmental and physiological processes of plants, including pathogen-induced defense responses.OsMPK6 is an important component in rice MAPK cascade. Yuan et al (2007) found that OsMPK6 negative the disease resistance of rice to Xoo strain PXO61, and suppression/knockout of OsMPK6 would develop lesion mimics on the leaves. The present results suggest that OsMPK6 is also an activator of rice resistance to Xoo. OsMPK6 kinase activity and its expression were rapidly induced in(R) gene-mediated disease resistance. Activation of OsMPK6 resulted in formation of lesion mimics and local resistance to Xoo, accompanied by accumulation of salicylic acid (SA) and jasmonic acid (JA) and induced expression of SA-and JA-signaling genes. We found that OsMPK6 protein is located in nucleus, and nuclear localization of OsMPK6 was essential for local resistance, positive regulation mediated by OsMPK6 is related the shifting of OsMPK6 from cytoplasm to nucleus. All these data suggesting that modulating the expression of defense-responsive genes through transcription regulators may be the primary mechanism of OsMPK6-mediated local resistance. Knockout of OsMPK6 resulted in enhanced Xoo resistance, accumulation of SA, and induced expression of PR1a, the marker gene of systemic acquired resistance (SAR) in systemic health tissues, suggesting that OsMPK6 negatively regulates SAR. OsMPK6-regulated local resistance signaling and SAR signaling partially overlapped, but SAR signaling did not accompany activation of JA signaling. Overexpression of OsMPK6 can enhance the disease resistance of rice to rice blast disease caused by Magnaporthe grisea. These results suggest that OsMPK6 is a two-faced player in the rice-Xoo interaction. It may first activate local resistance initiated by the R gene and then suppress SAR through different signaling pathways. In order to find out the interaction protein of OsMPK6, we screen the rice cDNA library through yeast-two-hybrid. We found 17 putative OsMPK6-interaction candidate genes. We analysis some of them and supply information for further analysis.The enhanced disease resistance 1 (EDR1) encodes a Ctr1-like kinase and was previously reported to function as a negative regulator of defense responses and ethylene-related senescence in Arabidopsis. Here, we identified that OsEDR1 is an important component in rice disease resistance and ethylene biosynthesis. Suppressing OsEDR1 and OsEDR1 mutant, mutated by T-DNA insertion, displays enhanced resistance to rice bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) with the development of lesion mimics on the leaves. Overexpression of OsEDR1 results in decreasing the resistance to Xoo and early-death of the transgenic rice. The resistance plants showed different expression of a subset of defense-responsive genes functioning in salicylic acid defense-pathway and jasmonic acid defense-pathway. We also measured the SA&JA concentration in OsEDR1 mutant and wild type after inoculated with PXO61. It shows that OsEDR1 may mediate defense response via SA-JA antagonism. OsEDR1 mutant plants enhance disease resistance to Xoc comparing to the wild type zhonghua 11. These data indicat that disease resistance mediated by OsEDR1 is a broad-spectrum resistance. Suppression of OsEDR1 blocks ethylene biosynthesis through the modulation of 1-Aminocyclopropane-1-carboxylic acid synthase (ACS) family, the key component of ethylene biosynthesis. The formation of lesion mimic would be delayed after spraying 250 um ACC onto leaves and it also would be intensified after spraying 200um AVG. We also found that OsEDR1 mutant plants enhance sensitivity to drought stress. These results suggest that OsEDR1 can negative regulates defense responses as well as regulates the ethylene biosynthesis as a positive component. OsEDR1 also positive regulates drought stress in rice.