The bHLH network underlying plant shade-avoidance

Abstract

Shade is a potential threat to many plant species. When shade-intolerant plants detect neighbours, they elongate their stems and leaves in an effort to maximise their light capture. This developmental programme, known as 'shade-avoidance' is tightly controlled by specialised photoreceptors and a suite of transcriptional regulators. The basic helix-loop-helix (bHLH) family of transcription factors are particularly important for shade-induced elongation. In recent years, it has become apparent that many members of this family heterodimerise and that together they form a complex regulatory network. This review summarises recent work into the structure of the bHLH network and how it regulates elongation growth. In addition to this, we highlight how photoreceptors modulate the function of the network via direct interaction with transcription factors. It is hoped that the information integrated in this review will provide a useful theoretical framework for future studies on the molecular basis of shade-avoidance in plants.

Figure 1

Physical interactions between bHLH sub‐families in the control of hypocotyl elongation under shade. Factors that promote elongation are coloured blue, and those that inhibit elongation are coloured orange. Physical interactions that result in an enhancement of elongation are shown in blue, and those that inhibit elongation are shown in red. The PIF, BIM and BEE sub‐families (excluding HFR1 and PIL1) represent typical bHLH transcription factors that bind to DNA and directly modulate transcription. BIM and PIF activity is enhanced through heterodimerization with BES1/BZR1 family transcription factors. PIFs additionally heterodimerise with ARFs, as do members of the BEE sub‐family. Upon activation, these transcription factors promote the expression of the AIFs, PARs and HFR1/PIL1. The protein products of these genes heterodimerise with the transcription factors and inhibit their transcriptional activity. In addition to this primary negative feedback loop, the transcription factors also promote the expression of PREs. PREs heterodimerise with the AIFs, PARs and HFR1 and through this they release the transcription factors from repression.

Figure 2

Photoreceptor‐mediated inhibition of the bHLH network. Phytochromes negatively regulate the activity of PIFs and their co‐activators, BES1/BZR1 and the ARFs. BES1/BZR1 are additionally inactivated by UVR8, which also binds to and inhibits the action of BIMs. Cryptochromes antagonise several members of the PIF, BES1/BZR1, ARF, BIM and BEE sub‐families. In conditions where photoreceptor function is reduced, COP1 targets PARs, HFR1 and PIL1 for degradation alleviating their repression of elongation.