STOP1 Regulates Transcription and Coordinates K/NH Balance in Response to Low-K Stress.

Potassium and nitrogen are essential mineral elements for plant growth and development. The protein kinase LKS1/CIPK23 is involved in both K and NH uptake in root. The transcripts of can be induced by low K (0.1 mM) and high NH (30 mM); however, the molecular mechanism is still unknown. In this study, we isolated the transcription factor STOP1 that positively regulates transcription in responses to both low-K and high-NH stresses. STOP1 proteins can directly bind to the promoter, promoting its transcription. The mutants displayed a leaf chlorosis phenotype similar to mutant when grown on low-K and high-NH medium. On the other hand, overexpressing plants exhibited a similar tolerant phenotype to overexpressing plants. The transcript level of was only upregulated by low K rather than high NH; however, the accumulation of STOP1 protein in the nucleus was required for the upregulation of transcripts in both low-K and high-NH responses. Our data demonstrate that STOP1 positively regulates transcription under low-K and high-NH conditions; therefore, LKS1 promotes K uptake and inhibits NH uptake. The STOP1/LKS1 pathway plays crucial roles in K and NH homeostasis, which coordinates potassium and nitrogen balance in plants in response to external fluctuating nutrient levels.