<i>Arabidopsis</i> ABCG21 is an ABA efflux transporter involved in seed germination under salt stress

Abscisic acid (ABA) is a key phytohormone that regulates stomatal movement, seed germination, root development, and stress responses. Here, we report that Arabidopsis (Arabidopsis thaliana) ATP-binding cassette (ABC) transporter G family member 21 (AtABCG21) functions as an ABA transporter that plays a role in seed germination under salt stress. AtABCG21 exhibits efflux transporter activity toward ABA in tobacco (Nicotiana benthamiana) and yeast (Saccharomyces cerevisiae) cells. β-glucuronidase (GUS) staining showed that AtABCG21, predominantly expressed in embryos and vascular tissues, is induced by ABA and salt stress, respectively. Subcellular localization analysis of GFP-AtABCG21 in N. benthamiana leaves indicated that AtABCG21 localizes to the plasma membrane. The inflorescence intensity of the ABSCISIC ACID RESPONSIVE ELEMENT (ABRE)-based synthetic promoter-driven GFP reporter ABRE::GFP was significantly reduced in the embryos of the atabcg21 mutant under salt stress, suggesting that disruption of AtABCG21 compromised ABA signaling in the embryo. Consistently, quantification of ABA and ABA-GE revealed that the levels of ABA, but not ABA-GE, in the seeds of the atabcg21 mutant were significantly reduced compared to those in wild-type plants under salt stress. Consequently, the seed germination rate of the atabcg21 mutants was increased compared to that of wild-type plants under salt stress. These results demonstrate that AtABCG21 acts as an ABA efflux transporter, facilitating ABA translocation and influencing seed germination under salt stress. Our findings provide new insights into the mechanisms of ABA-mediated stress adaptation in plants.