The interaction of the Arabidopsis response regulator ARR18 with bZIP63 mediates the regulation of PROLINE DEHYDROGENASE expression.

As the first and rate-limiting enzyme of proline degradation, PROLINE DEHYDROGENASE1 (PDH1) is tightly regulated during plant stress responses, including induction under hypoosmolarity and repression under water deficit. The plant receptor histidine kinases AHKs, elements of the two-component system (TCS) in Arabidopsis thaliana, are proposed to function in water stress responses by regulating different stress-responsive genes. However, little information is available concerning AHK phosphorelay-mediated downstream signaling. Here we show that the Arabidopsis type-B response regulator 18 (ARR18) functions as a positive osmotic stress response regulator in Arabidopsis seeds and affects the activity of the PDH1 promoter, known to be controlled by C-group bZIP transcription factors. Moreover, direct physical interaction of ARR18 with bZIP63 was identified and shown to be dependent on phosphorylation of the conserved aspartate residue in the ARR18 receiver domain. We further show that bZIP63 itself functions as a negative regulator of seed germination upon osmotic stress. Using reporter gene assays in protoplasts, we demonstrated that ARR18 interaction negatively interferes with the transcriptional activity of bZIP63 on the PDH1 promoter. Our findings provide new insight into the function of ARR18 and bZIP63 as antagonistic regulators of gene expression in Arabidopsis.