Mechanism Underlying The Turnip Yellow Mosaic Virus P69-induced Symptom And Mechanism Underlying NRPC7-promoted MIR Gene Transcription

Turnip yellow mosaic virus(TYMV)is a positive-strand RNA virus that infects Arabidopsis and other plants in the Brassica genus.TYMV infection or the expression of its suppressor of RNA silencing(VSR),P69,in Arabidopsis induces a pale-green symptom.However,the molecular mechanism underlying this P69-elicited phenotype is unknown.Here,by yeast two-hybrid assays,we identified that P69 interacts with GLK transcription factors,which play an important role in regulating photo synthetic gene expression and chloroplast development.The P69-overexpressing plants and the glklglk2 mutant plants showed similar pale-green phenotype and similar patterns of photosynthetic gene downregulation,suggesting that P69 may inhibit GLK proteins'function.By ChIP-PCR analysis,we found that P69 suppresses transcriptional activation of photosynthetic genes by antagonizing GLK proteins' binding to their target promoters.Our study thus reveals a novel mechanism by which a virus VSR causes disease symptoms.In Arabidopsis,microRNAs(miRNAs)are mainly associated with AGO1 to post-transcriptionally regulate gene expression.EMA1,an importin ? family member,negatively regulates miRNA loading into AGO1 and the emma1-1 mutant exhibits a highly clustered trichome phenotype due to enhanced activity of an artificial miRNA.Through a forward genetic screen for suppressors of ema1-1,we identified the mutant soe7(suppressor of ema1),which displays strongly reduced trichome clustering and multiple developmental defects.The soe7 mutation was mapped to the NRPC7 gene,which encodes one subunit of RNA Polymerase ?(Pol ?).Further analysis revealed that the levels of a panel of mature miRNAs are greatly reduced and the expression levels of their target genes are up-regulated in soe7.Since the levels of corresponding pri-miRNAs are decreased,we concluded that NRPC7 regulates MIR gene expression at the transcription level.However,our ChIP-qPCR results showed that NRPC7 does not directly associate with the MIR gene regions,excluding the possibility that Pol ?transcribes these MIR genes.Here we propose that Pol ? may positively regulate MIR gene transcription through a Pol ?-dependent mechanism.