Plant matrix metalloproteinases (MMPs) have some function in development, but their function in plant immunity remained little understood. The aim of my study in this part is to analyze the role of Arabidopsis thaliana At-MMPs in plant immunity. The studies showed that triple mutant at2, 3, 5-mmp exhibited lower level of ROS production and callose deposition than wild type plants. Furthermore, the double mutant at2, 3-mmp and triple mutant at2, 3, 5-mmp exhibited more susceptibility to necrotrophic and biotrophic fungal pathogens than single mutants at2-mmp and at3-mmp, respectively. At2-MMP overexpression could restore the resistance to necrotrophic fungal pathogen and exhibited higher levels of ROS production and callose deposition than WT plants. These results indicated that At2-MMP plays an important role in plant disease resistance and might have an additive positive effect with At3-MMP and At5-MMP on the plant immunity, suggesting that At2-MMP, At3-MMP and At5-MMP have at least partially overlapping function in plant immunity, which might be due to the overlapping of enzyme activity.MORC1 showed some function in plant immunity and gene silencing. However, whether the other homologs of Arabidopsis MORC have similar functions in resistance and gene silencing remained unclear. In this part, atmorc1 and atmorc6 mutants exhibited more susceptibility to Pst AvrRpt2 than WT plants. In contrast, atmorc2, atmorc5 and atmorc7 mutants showed a similar phenotype to Pst AvrRpt2 as WT. Similar to that displayed in DNA glycosylase REPRESSOR OF SILENCING 1 (ROS1) mutant, the expression of R gene RESISTANCE METHYLATED GENE 1 (RMG1), which is negatively regulated by RdDM, was down-regulated in atmorc mutants. It is speculated that AtMORC activate demethylation resulting in the induction of R genes. Meanwhile, the expression of a transposon gene (ATCOPIA28) and silencing-related gene SDC (suppressor of drm2 cmt3) were significantly up-regulated in atmorc1 and atmorc6 mutants in comparison to WT. However, atmorc2, atmorc5 and atmorc7 showed little or no induction of transposon gene ATCOPIA28 and silencing-related gene SDC, which was similar to that displayed in WT. Taken together, AtMORC1 and AtMORC6 might repress transcriptional gene silencing through affecting RdDM and further induce specific pathogenesis-related genes, which could affect plant immune responses to microbial infection.
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