As sessile organisms, plants must be able to adapt to the environment. Plants respond to the environment by adjusting their growth and development, which is mediated by sophisticated signaling networks that integrate multiple environmental and endogenous signals. Recently, increasing evidence has shown that a bHLH transcription factor PIF4 plays a major role in the multiple signal integration for plant growth regulation. PIF4 is a positive regulator in cell elongation and its activity is regulated by various environmental signals, including light and temperature, and hormonal signals, including auxin, gibberellic acid and brassinosteroid, both transcriptionally and post-translationally. Moreover, recent studies have shown that the circadian clock and metabolic status regulate endogenous PIF4 level. The PIF4 transcription factor cooperatively regulates the target genes involved in cell elongation with hormone-regulated transcription factors. Therefore, PIF4 is a key integrator of multiple signaling pathways, which optimizes growth in the environment. This review will discuss our current understanding of the PIF4-mediated signaling networks that control plant growth.
FIGURE 4. The expression of YUC8 and IAA29 is hyposensitive to high temperature in lngq seedlings. (A) Diagram showing plant growth conditions used for the qRT-PCR analyses in (B–D) . Seedlings were maintained under 12 h light/12 h dark at 20°C for 4 days, and then transferred to 24 h light conditions. The seedlings were then exposed for 4 h to high temperature (28°C) at ZT20 and harvested for RNA extraction. (B–D) The qRT-PCR analysis of the expression level of YUC8 and IAA29 . Gene expression levels were normalized to PP2A and presented as values relative to those of the WT seedlings at 20°C. Error bars indicate standard deviation (SD, n = 3). Numbers indicate ratios of the expression levels (28°C/20°C). Different letters above each bar indicate statistically significant differences (ANOVA and Tukey’s HSD; P < ). (E) Western blotting with anti-PIF4 antibody showed that PIF4 protein levels were increased by high temperatures in both wild type and lngq seedlings. Total protein was extracted from the seedlings grown in the conditions described in (A) . Equal loading of samples is shown by Ponceau S staining. (F) A hypothetical model depicting PIF4-LNGs-mediated thermomorphogenic growth. At elevated temperatures, PIF4 transcription factor binds to the promoters of LNG1 and LNG2 as well as auxin-related genes ( YUC8 and IAA29 ), and activate their expression. The increased LNG proteins further activate the expression of YUC8 and IAA29 through unknown mechanisms. The resulting increased auxin biosynthesis and signaling induce thermomorphogenic growth.