Water shortage reduces <i>PHYTOCHROME INTERACTING FACTOR 4</i> , <i>5</i> and <i>3</i> expression and shade avoidance in Arabidopsis

In agricultural crops, forests and grasslands, water deficit often occurs in the presence of cues from neighbouring vegetation. However, most studies have addressed separately the mechanisms of plant growth responses to these two aspects of the environment. Here we show that transferring Arabidopsis thaliana seedlings to agar containing polyethylene glycol (PEG) to restrict water availability reduces hypocotyl growth responses to shade without simultaneously affecting cotyledon expansion or its response to shade. Hypocotyl growth showed significant triple interaction among water availability, shade and the presence of PHYTOCHROME INTERACTING FACTOR 4 (PIF4), PIF5 and PIF3. Water restriction diminished auxin signalling and the activity of the PIF4, PIF5, PIF3 gene promoters and their transcript levels. The responses of PIF4 expression and hypocotyl growth to PEG were reduced in mutants of its positive morning regulators CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY). The CCA1 and LHY gene promoters also reduced their activity in response to PEG. In addition to the changes in PIF4 levels, post-transcriptional processes also contributed to the PIF4 protein response to PEG. Collectively, these results unveil PIFs as a hub that interlinks shade and drought information to control growth.