PIFs: pivotal components in a cellular signaling hub

Abstract

A small subset of basic helix-loop-helix transcription factors called PIFs [phytochrome (phy)-interacting factors] act to repress seed germination, promote seedling skotomorphogenesis and promote shade-avoidance through regulated expression of over a thousand genes. Light-activated phy molecules directly reverse these activities by inducing rapid degradation of the PIF proteins. Here, we review recent advances in dissecting this signaling pathway and examine emerging evidence that indicates that other pathways also converge to regulate PIF activity, including the gibberellin pathway, the circadian clock and high temperature. The PIFs thus have broader roles than previously appreciated, functioning as a cellular signaling hub that integrates multiple signals to orchestrate regulation of the transcriptional network that drives multiple facets of downstream morphogenesis. The relative contributions of the individual PIFs to this spectrum of regulatory functions ranges from quantitatively redundant to qualitatively distinct. Phytochrome signal perception and transductionThe perception of light signals by the phytochrome (phy) family of sensory photoreceptors [phyA through phyE in Arabidopsis (Arabidopsis thaliana)] initiates an intracellular transduction process that culminates in the altered expression of nuclear genes that direct growth and developmental responses, termed photomorphogenesis, appropriate to the prevailing environment, throughout the plant life cycle [1,2]. Current data indicate that the transduction process involves rapid translocation of the light-activated photoreceptor molecule (the Pfr conformer) from the cytoplasm into the nucleus, where it induces transcriptional responses in target genes [3]. The pathway by which the signaling information is propagated to the transcriptional network involves direct, physical interaction of the translocated Pfr conformer with a small subset of constitutively nuclear, basic helixloop-helix (bHLH) transcription factors, designated Phytochrome-Interacting Factors (PIFs) [4,5]. Several comprehensive articles have examined various aspects of this overall process in recent years [6][7][8][9][10][11]. This review focuses predominantly on recent advances in defining the phy-PIF signaling mechanism and the function of the PIFs in regulating the primary,

A Triple Helix-Loop-Helix/Basic Helix-Loop-Helix Cascade Controls Cell Elongation Downstream of Multiple Hormonal and Environmental Signaling Pathways inArabidopsis

“…Light signaling mediated by photoreceptors affects the accumulation and activity of several transcription factors that directly regulate primary light-responsive genes, leading to developmental responses. Among the lightsignaling transcription factors, phytochrome-interacting factors (PIFs), a small family of basic helix-loop-helix (bHLH) transcription factors, accumulate in the dark or shade to promote cell elongation but are degraded upon light activation of phytochromes (Leivar and Quail, 2011). Additional environmental and endogenous signals, including temperature, the circadian clock, and ethylene, control cell elongation at least partly by regulating the expression levels of PIF family members (Leivar and Quail, 2011;Zhong et al, 2012), whereas GA signaling activates PIF proteins by inducing ubiquitination-mediated degradation of the DELLA proteins, which directly interact with PIFs to inhibit their DNA binding (de Lucas et al, 2008;Feng et al, 2008).…”

The Energy-Signaling Hub SnRK1 Is Important for Sucrose-Induced Hypocotyl Elongation

“…The PIFs are signaling hubs that control plant development through genomewide transcriptional alterations. One outcome of these PIF-mediated transcriptional changes is the alteration in phytohormone signaling that regulates hypocotyl elongation Leivar and Quail, 2011).…”

CUL4 forms an E3 ligase with COP1 and SPA to promote light-induced degradation of PIF1

“…In addition, phytochromes promote nuclear exclusion of COP1 under light and thereby stabilizing the positive regulators of photomorphogenesis 12,13 . By contrast, the light-activated forms of phytochromes directly interact with PIFs and induce rapid phosphorylation, polyubiquitylation and UPS-mediated degradation of PIFs to promote photomorphogenesis 14,15 .…”