Myrosinases, TGG1 and TGG2, redundantly function in reactive carbonyl species signaling in Arabidopsis guard cells.

Myrosinase (thioglucoside glucohydrolase, EC 3.2.1.147, TGG) is a highly abundant protein in Arabidopsis guard cells and TGG1 and TGG2 redundantly function in abscisic acid (ABA)- and methyl jasmonate (MeJA)-induced stomatal closure. Reactive carbonyl species (RCS) are α,β-unsaturated aldehydes and ketones, which function downstream of reactive oxygen species (ROS) production in ABA signal pathway in guard cells. Among the RCS, acrolein is the most highly reactive that is significantly produced in the ABA-treated guard cells. In order to clarify the ABA signal pathway downstream of ROS production, we investigated the responses of tgg mutants (tgg1-3, tgg2-1, and tgg1-3 tgg2-1) to acrolein. Acrolein induced stomatal closure and triggered cytosolic alkalization in the wild type (WT), the tgg1-3 single mutant, and the tgg2-1 single mutant, but not in the tgg1-3 tgg2-1 double mutant. Exogenous Ca2+ induced stomatal closure and cytosolic alkalization not only in WT but also in all the mutants. Acrolein- and Ca2+-induced stomatal closure were inhibited by an intracellular acidifying agent, butyrate, a Ca2+ chelator, EGTA, and a Ca2+ channel blocker, LaCl3. Acrolein induced cytosolic free calcium concentration ([Ca2+]cyt) elevation in guard cells of the WT plant but not in the tgg1-3 tgg2-1 double mutant. Exogenous Ca2+ elicited [Ca2+]cyt elevation in guard cells of the WT and the tgg1-3 tgg2-1. Our results suggest that TGG1 and TGG2 redundantly function not between ROS production and RCS production but downstream of RCS production in ABA signal pathway in Arabidopsis guard cells.