Degradation of the transcription factors PIF4 and PIF5 under UV‐B promotes UVR8‐mediated inhibition of hypocotyl growth in Arabidopsis

Abstract

Summary Inhibition of hypocotyl growth is a well‐established UV‐B‐induced photomorphogenic response that is mediated by the UV‐B photoreceptor UV RESISTANCE LOCUS 8 (UVR8). However, the molecular mechanism by which UVR8 signaling triggers inhibition of hypocotyl growth is poorly understood. The bZIP protein ELONGATED HYPOCOTYL 5 (HY5) functions as the main positive regulatory transcription factor in the UVR8 signaling pathway, with HY5‐HOMOLOG (HYH) playing a minor role. However, here we demonstrate that hy5 hyh double mutants maintain significant UVR8‐dependent hypocotyl growth inhibition. We identify UVR8‐dependent inhibition of the activities of bHLH transcription factors PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5 as part of the UVR8 signaling pathway, which results in inhibition of hypocotyl growth. The UVR8‐mediated repression of several hypocotyl elongation‐related genes is independent of HY5 and HYH but largely associated with UVR8‐dependent degradation of PIF4 and PIF5, a process that consequently diminishes PIF4/5 target promoter occupancy. Taken together, our data indicate that UVR8‐mediated inhibition of hypocotyl growth involves degradation of PIF4 and PIF5. These findings contribute to our mechanistic understanding of UVR8‐induced photomorphogenesis and further support the function of PIFs as integrators of different photoreceptor signaling pathways.

Light signaling and UV‐B‐mediated plant growth regulation

“…The shorter hypocotyls developed during UV-B photomorphogenesis is partly due to cell cycle arrest caused by UV-B-mediated DNA photo dimer accumulation (Biever et al 2014). A recent report also reveals the role of PIF4/PIF5 in controlling the hypocotyl length under UV-B (Tavridou et al 2020). However, uvr8 while being insensitive to UV-B growth signal, exhibits hypersensitivity to high doses of UV-B, a stress signal (Favory et al 2009).…”

CONSTITUTIVELY PHOTOMORPHOGENIC1 promotes ABA‐mediated inhibition of post‐germination seedling establishment

“…Another recent study revealed that COP1 physically interacts with and stabilizes PHYOCHROME INTERACTING FACTOR5 (PIF5), which promotes hypocotyl elongation under shade. This stabilization is disrupted in UVB by the sequestration of COP1 by UVR8 (Sharma et al, 2019;Tavridou et al, 2019). In addition, COP1/SPA complex stabilizes PIF3, a repressor of photomorphogenesis, by blocking its interaction with BRASSINOSTEROID INSENSITIVE 2 (BIN2) that phosphorylates and mediates the degradation of PIF3 (Bauer et al, 2004;Ling et al, 2017).…”

Degrading to inhibit: a role for Phytochrome Interacting Factors in determining hypocotyl growth under UV‐B

“…However, the mechanisms by which UVR8 triggers hypocotyl growth inhibition have only just begun to be clarified. In their study in this issue of The Plant Journal, (Tavridou et al, 2019) show that UVR8-mediated inhibition of PIF4 and PIF5 is an important and intrinsic part of UVR8 photomorphogenic signalling.The laboratory of Roman Ulm, Professor at the Department of Botany and Plant Biology at the University of Geneva (Geneva, Switzerland), has been carrying out seminal work on the plant responses to UV-B radiation for several years. Roman joined the University of Geneva in 2010 and has been the Director of the department since 2017, previously having been an independent group leader at the University of Freiburg (Freiburg, Germany).…”

“…However, the mechanisms by which UVR8 triggers hypocotyl growth inhibition have only just begun to be clarified. In their study in this issue of The Plant Journal, (Tavridou et al, 2019) show that UVR8-mediated inhibition of PIF4 and PIF5 is an important and intrinsic part of UVR8 photomorphogenic signalling.…”

“…Tavridou and colleagues achieved this by discovering of a role for UVR8 photoreceptor-induced destabilization of PIF4 and PIF5 in UV-B-induced gene repression and hypocotyl growth inhibition (Tavridou et al, 2019). PIF4 and PIF5 are bHLH transcription factors that play crucial roles in the regulation of genes associated with hypocotyl elongation.…”