Expression of the Arabidopsis transcription factor AtMYB30 is post-transcriptionally regulated

The Arabidopsis transcription factor AtMYB30 was previously identified as a positive regulator of plant hypersensitive cell death and defence responses to inoculation with bacterial pathogens. In this study, we attempted to generate Arabidopsis transgenic lines that overexpress AtMYB30 under the control of the constitutive 35S promoter. However, no transgenic lines overexpressing AtMYB30 could be obtained, suggesting the existence of a molecular mechanism that negatively regulates AtMYB30 expression in planta. Our results suggest that RNA silencing directly mediates downregulation of AtMYB30 expression, both in young seedlings and in adult plants. In contrast, an indirect RNA silencing mechanism is responsible for the induction of AtMYB30 expression after bacterial inoculation, possibly via the degradation of a yet unknown negative regulator of its expression. These results underline the importance of RNA silencing in the regulation of the activity of transcription factors both during plant development and in response to microbes.