R-R-Type MYB-Like Transcription Factor, AtMYBL, is Involved in Promoting Leaf Senescence and Modulates an Abiotic Stress Response in Arabidopsis

Functional analysis of a putative novel transcription factor Arabidopsis MYB-like protein designated AtMYBL, which contains two predicted DNA-binding domains, was performed. The physiological role of the R-R-type MYB-like transcription factor has not been reported in any plant. Analyses of an AtMYBL promoter-β-glucuronidase (GUS) construct revealed substantial gene expression in old leaves and induction of GUS activity by ABA and salt stress. AtMYBL-overexpressing plants displayed a markedly enhanced leaf senescence phenotype. Moreover, the ectopic expression of the AtMYBL gene was very significantly influential in senescence parameters including Chl content, membrane ion leakage and the expression of senescence-related genes. Although the seed germination rate was improved under ABA and saline stress conditions in the AtMYBL-overexpressing plants, decreased salt tolerance was evident compared with the wild type and atmybl RNA interference lines during later seedling growth when exposed to long-term salt stress, indicating that AtMYBL protein is able to developmentally regulate stress sensitivity. Furthermore, AtMYBL protein activated the transcription of a reporter gene in yeast. Green fluorescent protein-tagged AtMYBL was localized in the nuclei of transgenic Arabidopsis cells. Taken together, these results suggest that AtMYBL functions in the leaf senescence process, with the abiotic stress response implicated as a putative potential transcription factor.