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Dear Editor,Guard cell ABA signaling is central for the regulation of plant stomatal conductance (gs) and responses to environmental factors (Merilo et al.,2013);however,the source of ABA is still debatable.Generally,ABA levels are regulated by (1) biosynthesis:de novo and recycling from the inactive conjugates by β-glucosidases BG1 and BG2;(2) catabolism:irreversible hydroxylation and reversible conjugation to ABA glucosyl ester (ABA-GE);and (3) transport from elsewhere in the plant.Here,we report that the stomatal responses to elevated CO2 concentration,reduced air humidity,and exogenous ABA in different mutants of ABA transport and recycling were generally intact except in abcg22 mutant.ABCG22 was required for initiating reduced air humidityinduced stomatal closure.Thus,either ABA transport into guard cells is not critical for the studied responses or there are several proteins with redundant functions that are responsible for ABA transport.