Abstract:The phytohormone abscisic acid(ABA) has a wide range of important roles in plant growth and development as well as abiotic stress response. Previous studies identified the enormous components involved in ABA responses in plants. We identified a chaperon AtJ3(Arabidopsis thaliana DnaJ homolog 3; heat shock protein 40-like) using yeast two-hybrid assays in which PKS5 was used as bait. The AtJ3 T-DNA mutants atj3-1 and atj3-2 displayed ABA phenotypes. Seed germinations of atj3-1 and atj3-2 decreased, and the seedlings of them showed stunted growth and leaf chlorotic symptoms under ABA. Double mutants of atj3-1pks5-1, atj3-1pks5-3 and atj3-1pks5-4 had similar ABA phenotypes as mutants of AtJ3 or PKS5. Moreover, the assays of subcellular location and transgenic plants showed that AtJ3 has overlapping expression pattern with PKS5. By co-immunoprecipitation and in vitro phosphorylation, AtJ3 physically interacts with PKS5 and represses the PKS5 kinase activity. Therefore, AtJ3 interacts with PKS5 in response to ABA by repressing the PKS5 kinase activity.