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.
Highlights
The basic helix–loop–helix family constitutes one of the largest groups of transcription factors (TFs) in plants with around 160 members in Arabidopsis thaliana (Arabidopsis)
Instead of directly influencing gene expression, they act as second tier repressors, inhibiting the activity of the ATBS1INTERACTING FACTOR (AIF), HYPOCOTYL IN FAR-RED 1 (HFR1) and the PHYTOCHROME RAPIDLY REGULATED (PAR), thereby releasing PHYTOCHROME INTERACTING FACTOR (PIF), BES1-INTERACTING MYC-LIKE (BIM) and BR ENHANCED EXPRESSION (BEE) to bind DNA (Hyun and Lee 2006, Mara et al 2010, Hao et al 2012, Hong et al 2013, Ikeda et al 2013)
It has been argued that degradation of the inactive pool of BRASSINAZOLE RESISTANT 1 (BZR1) enhances the relative concentration of the active form, and thereby enhances BZR1 function (Kim et al 2014)
Summary
The basic helix–loop–helix (bHLH) family constitutes one of the largest groups of transcription factors (TFs) in plants with around 160 members in Arabidopsis thaliana (Arabidopsis). They are involved in virtually all aspects of plant development from germination onwards, and in stress tolerance, pathogen defence, nutrient uptake and the transition to reproductive growth. We wish to highlight several of the bHLH interaction networks that have been identified in recent years We emphasise how these networks shape plant architecture in response to the threat of neighbour shade, in the context of hypocotyl elongation. We decided to use the bHLH subgroups described in Carretero-Paulet et al (2010) as these groupings generally coincide with each family’s physiological role
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