A Stress-Responsive CaM-Binding Transcription Factor, bZIP4, Confers Abiotic Stress Resistance in Arabidopsis

Abstract

The basic leucine zipper (bZIP) transcription factors (TFs) function as regulators of many key developmental and physiological processes in all eukaryotes. In this study, we characterized the function of Arabidopsis bZIP4, a group S bZIP, whose function was not known. We confirmed that bZIP4 localizes to the nucleus and has DNA-binding affinity. By qRT-PCR and GUS histochemical analysis, we showed that bZIP4 is specifically expressed in root and that its expression is induced by abiotic stress and ABA. By phenotypic analysis, we demonstrated that the root length and the germination rate of bZIP4 overexpression (bZIP4-Ox) were significantly longer and higher than those of the WT and bZIP4-SRDX under higher salt and glucose concentrations, indicating that bZIP4-Ox is insensitive and tolerant to abiotic stress. Despite that, we found that bZIP4-Ox had enhanced expression of genes encoding protein phosphatases suppressing ABA responsiveness. We also confirmed that bZIP4 interacts with CaM1 and showed that its DNA-binding affinity is inhibited by interaction with CaM1. We propose a model in which the increased cytosolic calcium concentration under stress conditions activates CaM1 to bind bZIP4 to remove it from promoters of genes encoding ABA negative regulators, allowing the plants to operate on a typical ABA signaling pathway.