| Summary: | In <i>Arabidopsis thaliana</i>, cysteine-rich receptor-like kinases (CRKs) constitute a large group of membrane-localized proteins which perceive external stimuli and transduce the signal into the cell. Previous reports based on their loss-of-function phenotypes and expression profile support their role in many developmental and stress-responsive pathways. Our study revealed that one member of this family, CRK5, acts as a negative regulator of leaf aging. Enrichment of the <i>CRK5</i> promoter region in W-box <i>cis</i>-elements demonstrated that WRKY transcription factors control it. We observed significantly enhanced <i>WRKY53</i> expression in <i>crk5</i> and reversion of its early-senescence phenotype in the <i>crk5 wrky53</i> line, suggesting a negative feedback loop between these proteins antagonistically regulating chlorophyll <i>a</i> and <i>b</i> contents. Yeast-two hybrid assay showed further that CRK5 interacts with several proteins involved in response to water deprivation or calcium signaling, while gas exchange analysis revealed a positive effect of CRK5 on water use efficiency. Consistent with that, the <i>crk5</i> plants showed disturbed foliar temperature, stomatal conductance, transpiration, and increased susceptibility to osmotic stress. These traits were fully or partially reverted to wild-type phenotype in <i>crk5 wrky53</i> double mutant. Obtained results suggest that WRKY53 and CRK5 are antagonistic regulators of chlorophyll synthesis/degradation, senescence, and stomatal conductance.
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