MPK4 mediated phosphorylation of PIF4 controls thermosensing by regulation of H2A.Z deposition in Arabidopsis

Plants have the ability to perceive a slight upsurge in ambient temperature and respond by undergoing morphological changes, such as elongated hypocotyls and early flowering. The dynamic functioning of PHYTOCHROME INTERACTING FACTOR4 (PIF4) in thermomorphogenesis has been well established, although the regulatory pathway involved in thermosensing is not deciphered completely. In our study, we demonstrate that an increase in temperature from 22 to 28 induces the phosphorylation of PIF4 by MITOGEN-ACTIVATED PROTEIN KINASE 4 (MPK4) which shows high expression and activation at 28. Apparently, phosphorylated PIF4 represses the expression of ACTIN-RELATED PROTEIN 6 (ARP6) that is required for mediating histone variant H2A.Z deposition at its target gene loci. We demonstrate that variation of ARP6 expression in PIF4 phosphor -null and phosphor-mimetic seedlings affects hypocotyl growth and flowering at 22 and 28. Further, we show that change in ARP6 expression affects H2A.Z deposition at the loci of genes involved in hypocotyl elongation using PIF4 phosphor -null and phosphor-mimetic seedlings. Interestingly, the expression of MPK4 is also controlled by H2A.Z deposition in temperature dependent manner. Taken together, our findings highlight the cumulative molecular interplay between MPK4, PIF4, and chromatin modification by ARP6-mediated H2A.Z deposition as a regulatory mechanism of thermosensing.