Possible roles for phytohormones in controlling the stomatal behavior of Mesembryanthemum crystallinum during the salt-induced transition from C to crassulacean acid metabolism.

The halophyte ice plant (Mesembryanthemum crystallinum) converts its mode of photosynthesis from C to crassulacean acid metabolism (CAM) during severe water stress. During the transition to CAM, the plant induces CAM-related genes and changes its diurnal stomatal behavior to take up CO efficiently at night. However, limited information concerning this signaling exists. Here, we investigated the changes in the diurnal stomatal behavior of M. crystallinum during its shift in photosynthesis using a detached epidermis. M. crystallinum plants grown under C conditions opened their stomata during the day and closed them at night. However, CAM-induced plants closed their stomata during the day and opened them at night. Quantitative analysis of endogenous phytohormones revealed that trans-zeatin levels were high in CAM-induced plants. In contrast, the levels of jasmonic acid (JA) and JA-isoleucine were severely reduced in CAM-induced plants, specifically at night. CAM induction did not alter the levels of abscisic acid; however, inhibitors of abscisic acid synthesis suppressed CAM-induced stomatal closure. These results indicate that M. crystallinum regulates the diurnal balance of cytokinin and JA during CAM transition to alter stomatal behavior.