Arabidopsis NPF5.1 regulates ABA homeostasis and seed germination by mediating ABA uptake into the seed coat

Abscisic acid (ABA) is a plant hormone that induces seed dormancy during seed development and inhibits seed germination after imbibition. Although ABA is synthesized in the seed coat (testa), endosperm, and embryo, the physiological roles of the hormone derived from each tissue are not fully understood. We found that the gene encoding an Arabidopsis ABA importer, NPF5.1, was expressed in the seed coat during seed development. Dry seeds of loss-of-function npf5.1 mutants contained significantly higher levels of dihydrophaseic acid (DPA), an inactive ABA metabolite, than the wild type. The npf5.1 mutant also had a slight increase in ABA content. An increase in DPA was prominent in the fraction containing the seed coat and endosperm. Seed germination of the npf5.1 mutant was similar to the wild type in the presence of ABA or the gibberellin biosynthesis inhibitor paclobutrazol. However, a mutation in NPF5.1 suppressed the paclobutrazol-resistant germination of npf4.6, a mutant impaired in an ABA importer expressed in the embryo. These results suggest that ABA uptake into the seed coat mediated by NPF5.1 is important for ABA homeostasis during seed development and for regulating seed germination.