Phosphorylation of ARF2 Relieves Its Repression of Transcription of the K+ Transporter Gene HAK5 in Response to Low Potassium Stress.

Potassium (K+) plays crucial roles in plant growth and development. In natural environments, K+ availability in soils is relatively low and fluctuating. Transcriptional regulation of K+ transporter genes is one of the most important mechanisms in the plant's response to K+ deficiency. In this study, we demonstrated that the transcription factor ARF2 (Auxin Response Factor 2) modulates the expression of the K+ transporter gene HAK5 (High Affinity K+ transporter 5) in Arabidopsis thaliana The arf2 mutant plants showed a tolerant phenotype similar to the HAK5-overexpressing lines on low-K+ medium, whose primary root lengths were longer than those of wild-type plants. High-affinity K+ uptake was significantly increased in these plants. ARF2-overexpressing lines and the hak5 mutant were both sensitive to low-K+ stress. Disruption of HAK5 in the arf2 mutant abolished the low-K+-tolerant phenotype of arf2 As a transcriptional repressor, ARF2 directly bound to the HAK5 promoter and repressed HAK5 expression under K+ sufficient conditions. ARF2 can be phosphorylated after low-K+ treatment, which abolished its DNA binding activity to the HAK5 promoter and relieved the inhibition on HAK5 transcription. Therefore, HAK5 transcript could be induced, and HAK5-mediated high-affinity K+ uptake was enhanced under K+ deficient conditions. The presented results demonstrate that ARF2 plays important roles in the response to external K+ supply in Arabidopsis and regulates HAK5 transcription accordingly.