In this study, we examined the involvement of endogenous abscisic acid (ABA) in methyl jasmonate (MeJA)-induced stomatal closure using an inhibitor of ABA biosynthesis, fluridon (FLU), and an ABA-deficient Arabidopsis (Arabidopsis thaliana) mutant, aba2-2. We found that pretreatment with FLU inhibited MeJA-induced stomatal closure but not ABA-induced stomatal closure in wild-type plants. The aba2-2 mutation impaired MeJA-induced stomatal closure but not ABA-induced stomatal closure. We also investigated the effects of FLU and the aba2-2 mutation on cytosolic free calcium concentration ([Ca2+]cyt) in guard cells using a Ca2+-reporter fluorescent protein, Yellow Cameleon 3.6. In wild-type guard cells, FLU inhibited MeJA-induced [Ca2+]cyt elevation but not ABA-induced [Ca2+]cyt elevation. The aba2-2 mutation did not affect ABA-elicited [Ca2+]cyt elevation but suppressed MeJA-induced [Ca2+]cyt elevation. We also tested the effects of the aba2-2 mutation and FLU on the expression of MeJA-inducible VEGETATIVE STORAGE PROTEIN1 (VSP1). In the aba2-2 mutant, MeJA did not induce VSP1 expression. In wild-type leaves, FLU inhibited MeJA-induced VSP1 expression. Pretreatment with ABA at .1 µm, which is not enough concentration to evoke ABA responses in the wild type, rescued the observed phenotypes of the aba2-2 mutant. Finally, we found that in wild-type leaves, MeJA stimulates the expression of 9-CIS-EPOXYCAROTENOID DIOXYGENASE3, which encodes a crucial enzyme in ABA biosynthesis. These results suggest that endogenous ABA could be involved in MeJA signal transduction and lead to stomatal closure in Arabidopsis guard cells.
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