Recent Breakthroughs in the Characterization of Abscisic Acid Efflux Transporters: Shedding New Light on Abscisic Acid Dynamics and Regulation

The 15-carbon terpenoid abscisic acid (ABA) acts in vascular plants as a versatile hormone playing essential roles in reproductive development, vegetative development and growth, stress-development interactions, and physiological responses to abiotic and biotic stresses. Over the past 60 years, ABA dynamics, regulation, and responses have been progressively characterized: synthesis, transport and translocation, conjugation and deconjugation, metabolism, sensing, signal transduction, and downstream responses. In this context, the discovery of ABA exporters and importers has added novel dimensions to the understanding of ABA regulation. Moreover, since the initial discovery of the adenosine triphosphate-binding cassette (ABC) AtABCG25 exporter and AtABCG40 importer, several transporters of ABA have been characterized. As ABA is synthesized within cells, efflux transport of ABA to extracellular spaces is an important controlling step before ABA movements from cell to cell and from source to target. Until now, structural and functional mechanistic interactions between ABA and its transporters have remained poorly understood. Recent structure-function studies have unveiled key features of the mechanisms and functional dynamics of ABA-transporter interactions in the case of the ABCG25 exporter. ABCG25 has been established as a bona fide ABA efflux transporter involved in signal transmission leading to stomatal closure. It has also been shown to transport ABA-glucosyl ester (ABA-GE) conjugated form, thus implying involvement in conjugation and deconjugation dynamics. All of these mechanisms are essential for distribution throughout source tissues, source-to-target transmission, distribution throughout target tissues, and signal shape kinetics in target cells. This opinion piece emphasizes the pivotal role of transport discoveries in the history of ABA research, and how recent studies on ABA and ABA-GE transporters yield a more integrative vision of ABA regulation and adaptive responses, open new avenues of research on signaling crosstalks, and constitute opportunities as well as challenges for crop improvement strategies.