Molecular Mechanism Of Arabidopsis Transcription Factor MYB40 In Plant Responses To Arsenate And Low-Phosphate Stresses

Arsenic is a ubiquitous non-metallic element in the environment.Arsenate is the most prevalent chemical form of arsenic,and its chemical similarity to phosphate(Pi)renders it easily incorporated into plant cells through Pi transporters.Arsenate is reduced to arsenite in plant cells,and then complexed with phytochelatins(PCs)and sequestered into vacuoles,which protecting cells from arsenic toxicity.During arsenic stress,transcript levels of genes encoding Pi transporter,arsenate reductase and phytochelatin synthase are obviously induced.Until recently,the transcript regulation mechanisms of these genes are still unknown.The MYB family is the largest transcription factor family in Arabidopsis,and some members of this family have been reported to be involved in plant responses to biotic and abiotic stresses.In this study,MYB40 is demonstrated to be a key factor in Arabidopsis responses to arsenate and low-Pi stresses.MYB40 protein is localized in the nucleus and has transcription activity.MYB40 is mainly expressed in roots and induced during arsenate stress.Compared with wild-type plants,the MYB40-overexpressing lines show arsenate-tolerant phenotypes,with longer primary roots and heavier biomass,and the myb40 mutants are sensitive to arsenate stress.And the arsenate and phosphate uptake rates of MYB40-overexpressing lines are lower than that of wild-type plants.Further qRT-PCR results show that the expression of PHT1;1 is repressed in the MYB40-overexpressing lines,and ChIP and EMSA results display that MYB40 can bind to the P2 site within the PHT1;1 promoter.Moreover,the overexpression of PHTl;1 in MYB40-overexpressing line abolishes the arsenate-tolerant phenotypes of MYB40-overexpressing line,indicating that PHT1;1 is epistatic to MYB40.The qRT-PCR results show that the transcript level of phytochelatin synthase gene(PCS1)is elevated in the MYB40-overexpressing lines.And ChIP and EMSA results show that MYB40 protein can also bind to PCS1 promoter,indicating that MYB40 directly up-regulates PCS1 expression.MYB40 protein also plays important roles in plants responses to low-Pi stress.When grown under Pi-deficient condition,the MYB40-overexpressing lines show higher Pi uptake rate and Pi content compared with wild-type plants.qRT-PCR results display that the transcript level of PHT1;1 in MYB40-overexpressing lines is higher than that in wild-type seedlings when grown on Pi-deficient condition.And ChIP assay results show that MYB40 can bind to P3 site within the PHT1;1 promoter during Pi starvation.These data suggest that MYB40 can precisely modulate phosphate uptake by directly regulating PHT1;1 expression to adapt to environmental changes in Pi availability.In summary,these data demonstrate that MYB40 enhances Arabidopsis arsenate resistance by directly regulating the expression of PHT1;1 and PCS1.And MYB40 can precisely modulate PHT1;1 expression relevant to Pi availability.