PIF4 Integrates Multiple Environmental and Hormonal Signals for Plant Growth Regulation in Arabidopsis

Abstract

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.

Keywords: PIF4; brassinosteroid; high temperature; light; signaling integration.

Fig. 1.

PIF4-mediated light signaling pathways. (A) PIF4 positively regulates skotomorphogenesis and shade response. HBL, high blue light; LBL, low blue light; LRF, low red:far-red ratio; HRF, high red:far-red ratio; PBE, PIF-binding E-box. “P” in a circle indicates phosphorylated status. A group of “U” in a circle indicates polyubiquitination. (B) PIF4 is involved in the blue light-dependent phototropism by regulating auxin signaling. (C) High light promotes stomatal development, which is mediated by phyB and PIF4. Solid lines indicate post-translational modifications or regulation at protein levels. Dashed lines show transcriptional regulation. Question marks indicate unknown or unconfirmed mechanism.

Fig. 2.

PIF4-mediated hormone signaling pathways. (A) Integration of auxin, gibber-ellic acid (GA), and brassinosteroid (BR) signaling pathways in the regulation of hypocotyl elongation. (B) PIF4 transcriptionally regulates auxin and ethylene signaling and biosynthetic genes, which mediates phototropism, cell elongation, and leaf senescence.

Fig. 3.

PIF4-mediated high temperature signaling pathways EC indicates the Evening Complex consisting of ELF3, ELF4 and LUX. Solid lines indicate post-translational modification or regulation at protein levels. Dashed lines show transcriptional regulation. A question mark indicates unconfirmed mechanism.