Regulation of stomatal opening and histone modification by photoperiod in Arabidopsis thaliana

Stomatal movements are regulated by many environmental signals, such as light, CO2, temperature, humidity, and drought. Recently, we showed that photoperiodic flowering components have positive effects on light-induced stomatal opening in Arabidopsis thaliana. In this study, we determined that light-induced stomatal opening and increased stomatal conductance were larger in plants grown under long-day (LD) conditions than in those grown under short-day (SD) conditions. Gene expression analyses using purified guard cell protoplasts revealed that FT and SOC1 expression levels were significantly increased under LD conditions. Interestingly, the enhancement of light-induced stomatal opening and increased SOC1 expression in guard cells due to LD conditions persisted for at least 1 week after plants were transferred to SD conditions. We then investigated histone modification using chromatin immunoprecipitation-PCR, and observed increased trimethylation of lysine 4 on histone 3 (H3K4) around SOC1. We also found that LD-dependent enhancement of light-induced stomatal opening and H3K4 trimethylation in SOC1 were suppressed in theft-2 mutant. These results indicate that photoperiod is an important environmental cue regulating stomatal opening, and that LD conditions enhance light-induced stomatal opening and epigenetic modification (H3K4 trimethylation) around SOC1, a positive regulator of stomatal opening, in an FT-dependent manner. Thus, this study provides novel insights into stomatal responses to photoperiod.