Cytokinin receptor-dependent and receptor-independent pathways in the dehydration response of Arabidopsis thaliana

Cytokinin signaling in Arabidopsis thaliana utilizes a multi-step two-component signaling (TCS) system comprised of sensor histidine kinases (AHKs), histidine phosphotransfer proteins (AHPs), and response regulators (ARRs). Recent studies have suggested that the cytokinin TCS system is involved in a variety of other signaling and metabolic pathways. To further explore a potential function of the cytokinin TCS in the Arabidopsis dehydration stress response, we investigated the expression of all type-A ARR genes and a type-C ARR, ARR22, in both wild type and ahk single, double, and triple mutants in response to dehydration compared to cytokinin as well as dehydration tolerance of ahk mutants. We found that drought significantly induced the expression of a subset of ARR genes, ARR5, ARR7, ARR15, and ARR22. The results of expression analyses in ahk single, double, and triple mutants demonstrated that the cytokinin receptors AHK2 and AHK3 are redundantly involved in dehydration-inducible expression of ARR7, but not that of ARR5, ARR15, or ARR22. Dehydration tolerance assays showed that ahk2 and ahk3 single mutants exhibited enhanced dehydration tolerance compared with that of wild-type plants and ahk4 mutants, and that ahk2 ahk3 double mutants exhibited stronger drought tolerance than that of ahk3 ahk4, which exhibited more enhanced drought tolerance than that of wild-type plants and ahk single mutants. Taken together, these results demonstrate that while the cytokinin receptors AHK2 and AHK3 are critically involved in the dehydration tolerance response, both cytokinin receptor-dependent pathway and receptor-independent pathway occur in the dehydration response regulating ARR gene expression. In addition, preincubating ahk2, ahk3, ahk4, and the wild-type plants with cytokinin induced enhanced dehydration stress tolerance in these plants, demonstrating that cytokinins are involved in regulating plant response to dehydration stress.