WRKY2 and WRKY10 regulate the circadian expression of PIF4 during the day through interactions with CCA1/LHY and phyB

Abstract WRKY transcription factors are mostly known for their function in plant defense, abiotic stress responses, senescence, pollen, embryo and seed development and seed germination. Here, we report regulatory function of two WRKY proteins on photomorphogenesis and PIF4 expression. PIF4 is a critical signaling hub in light, temperature and hormonal signaling pathways. Either its expression or accumulation peak in the morning and afternoon. WRKY2 and WRKY10 form heterodimers and recognize their target site in the PIF4 promoter near the MYB element bound by CCA1 and LHY under red and blue light. WRKY2 and WRKY10 interact directly with CCA1/LHY to enhance their targeting but indirectly with SHB1. The two WRKY proteins also interact with phyB and their interaction enhances the targeting of CCA1 and LHY to the PIF4 promoter. Moreover, SHB1 associates with the WRKY2 and WRKY10 loci and enhances their expression parallel to the PIF4 expression peaks. This forward regulatory loop further sustains the accumulation of the two WRKY proteins and targeting of CCA1/LHY to the PIF4 locus. In summary, interactions of the two WRKY proteins with CCA1/LHY and phyB maintain an optimal expression level of PIF4 toward noon and afternoon, essential to sketch the circadian pattern of PIF4 expression.