Vacuolar Transport of Abscisic Acid Glucosyl Ester Is Mediated by ATP-Binding Cassette and Proton-Antiport Mechanisms in Arabidopsis

Abstract

Abscisic acid (ABA) is a key plant hormone involved in diverse physiological and developmental processes, including abiotic stress responses and the regulation of stomatal aperture and seed germination. Abscisic acid glucosyl ester (ABA-GE) is a hydrolyzable ABA conjugate that accumulates in the vacuole and presumably also in the endoplasmic reticulum. Deconjugation of ABA-GE by the endoplasmic reticulum and vacuolar β-glucosidases allows the rapid formation of free ABA in response to abiotic stress conditions such as dehydration and salt stress. ABA-GE further contributes to the maintenance of ABA homeostasis, as it is the major ABA catabolite exported from the cytosol. In this work, we identified that the import of ABA-GE into vacuoles isolated from Arabidopsis (Arabidopsis thaliana) mesophyll cells is mediated by two distinct membrane transport mechanisms: proton gradient-driven and ATP-binding cassette (ABC) transporters. Both systems have similar Km values of approximately 1 mm. According to our estimations, this low affinity appears nevertheless to be sufficient for the continuous vacuolar sequestration of ABA-GE produced in the cytosol. We further demonstrate that two tested multispecific vacuolar ABCC-type ABC transporters from Arabidopsis exhibit ABA-GE transport activity when expressed in yeast (Saccharomyces cerevisiae), which also supports the involvement of ABC transporters in ABA-GE uptake. Our findings suggest that the vacuolar ABA-GE uptake is not mediated by specific, but rather by several, possibly multispecific, transporters that are involved in the general vacuolar sequestration of conjugated metabolites.