Histone variant H2A.Z substitution mediated by the SWR1-like complex is a novel transcriptional regulatory mechanism controlling defense genes and immunity in plants
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Plants have evolved a complex immune system as a result of an evolutionary arms race between the host and various pathogens. One of the most vigorous immune responses in plants involves resistance (R) proteins, which detect the presence of effector molecules from pathogens and trigger a large-scale reprogramming in RNA transcription of defense genes. MORC1 is one of few proteins that have been shown to interact with several of these R-proteins and is required for optimum immune responses in Arabidopsis against a wide range of pathogens. MORC1 has been speculated to be involved in remodeling chromatin in response to pathogen infection as it has been shown to exhibit ATPase and endonuclease activity and its nuclear population increases after pathogen infection. Through a co-immunoprecipitation, we found that MORC1 physically interacts with several chromatin-remodeling factors including ACT1, ARP4, SWC2, SWC6, SUF3, PIE1, RVB1, and YAF9. These factors belong to the SWR1-like complex in Arabidopsis whose yeast homolog functions to exchange the histone H2A for its variant, H2A.Z. This replacement has been speculated to be involved in transcription regulation as it occurs in the promoter and/or genic region of actively transcribed genes. Interestingly, mutations in a few SWR1-like components led to altered resistance to the bacterial pathogen P. syringae, suggesting that the SWR1-like complex functions in plant immunity. In fact, a previous study revealed that RVB1, also known as Resistance to Pseudomonas syringae pv maculicola Interactor 1 (RIN1), interacts with the R-proteins, RPM1 and RPP5, and has been shown to be a negative regulator of plant defense responses. Interestingly, RVB1 and MORC1 interacted in both the nucleus and microsomes. To further gain insight into the molecular mechanism of this H2A.Z replacement in defense signaling, I performed chromatin immunoprecipitation with H2A.Z and found that infection with P. syringae leads to an elevated level of chromatin associated with H2A.Z including defense genes such as PR-5. Furthermore, an Arabidopsis mutant line lacking three genes encoding H2A.Z showed altered transcriptional induction of defense genes in response to pathogen infection. Together, these results establish that the histone replacement with H2A.Z by the SWR1-like complex modulates the transcription of defense genes and thereby likely affects immunity in plants.