CYCLIN H;1 regulates drought stress responses and blue light-induced stomatal opening by inhibiting reactive oxygen species accumulation in arabidopsis

Arabidopsis (Arabidopsis thaliana) CYCLIN-DEPENDENT KINASE Ds (CDKDs) phosphorylate the C-terminal domain of the largest subunit of RNA polymerase II. Arabidopsis CYCLIN H; 1 (CYCH;1) interacts with and activates CDKDs; however, the physiological function of CYCH; 1 has not been determined. Here, we report that CYCH; 1, which is localized to the nucleus, positively regulates blue light-induced stomatal opening. Reduced-function cych;1 RNA interference (cych; 1 RNAi) plants exhibited a drought tolerance phenotype. CYCH; 1 is predominantly expressed in guard cells, and its expression was substantially down-regulated by dehydration. Transpiration of intact leaves was reduced in cych; 1 RNAi plants compared with the wild-type control in light but not in darkness. CYCH; 1 down-regulation impaired blue light-induced stomatal opening but did not affect guard cell development or abscisic acid-mediated stomatal closure. Microarray and real-time polymerase chain reaction analyses indicated that CYCH; 1 did not regulate the expression of abscisic acid-responsive genes or light-induced stomatal opening signaling determinants, such as MYB60, MYB61, Hypersensitive to red and blue1, and Protein phosphatase7. CYCH; 1 down-regulation induced the expression of redox homeostasis genes, such as LIPOXYGENASE3 (LOX3), LOX4, ARABIDOPSIS GLUTATHIONE PEROXIDASE 7 (ATGPX7), EARLY LIGHT-INDUCIBLE PROTEIN1 (ELIP1), and ELIP2, and increased hydrogen peroxide production in guard cells. Furthermore, loss-of-function mutations in CDKD; 2 or CDKD; 3 did not affect responsiveness to drought stress, suggesting that CYCH; 1 regulates the drought stress response in a CDKD-independent manner. We propose that CYCH; 1 regulates blue light-mediated stomatal opening by controlling reactive oxygen species homeostasis.