Regulation of miR399f Transcription by AtMYB2 Affects Phosphate Starvation Responses in Arabidopsis

Although a role for microRNA399 (miR399) in plant responses to phosphate (Pi) starvation has been indicated, the regulatory mechanism underlying miR399 gene expression is not clear. Here we report that AtMYB2 functions as a direct transcriptional activator for miR399 in Arabidopsis Pi starvation signaling. Compared to untransformed control plants, transgenic plants constitutively overexpressing AtMYB2 showed increased miR399f expression and tissue Pi contents under high Pi growth, and exhibited elevated expression of a subset of Pi starvation-induced (PSI) genes. Pi-starvation-induced root architectural changes were more exaggerated in AtMYB2 overexpressing transgenic plants compared to wild type. AtMYB2 directly binds to a MYB-binding site in the miR399f promoter in vitro, as well as in vivo, and stimulates miR399f promoter activity in Arabidopsis protoplasts. Transcription of AtMYB2 itself is induced in response to Pi deficiency, and the tissue expression pattern of miR399f and AtMYB2 are similar. Both genes are expressed mainly in vascular tissues of cotyledons and in roots. Our results suggest that AtMYB2 regulates plant responses to Pi-starvation by regulating the expression of the miR399 gene.