A Gas-and-Brake Mechanism of bHLH Proteins Modulates Shade Avoidance.

Sara Buti,Emilie Reinen,Ronald Pierik,Valérie Hoogers,Kasper Van Gelderen,Chrysoula K Pantazopoulou

doi:10.1104/pp.20.00677

Abstract

Plants detect proximity of competitors through reduction in the ratio between red and far-red light that triggers the shade avoidance syndrome, inducing responses such as accelerated shoot elongation and early flowering. Shade avoidance is regulated by PHYTOCHROME INTERACTING FACTORs, a group of basic helix-loop-helix (bHLH) transcription factors. Another (b)HLH protein, KIDARI (KDR), which is non-DNA-binding, was identified in de-etiolation studies and proposed to interact with LONG HYPOCOTYL IN FAR-RED1 (HFR1), a (b)HLH protein that inhibits shade avoidance. Here, we established roles of KDR in regulating shade avoidance in Arabidopsis (Arabidopsis thaliana) and investigated how KDR regulates the shade avoidance network. We showed that KDR is a positive regulator of shade avoidance and interacts with several negative growth regulators. We identified KDR interactors using a combination of yeast two-hybrid screening and dedicated confirmations with bimolecular fluorescence complementation. We demonstrated that KDR is translocated primarily to the nucleus when coexpressed with these interactors. A genetic approach confirmed that several of these interactions play a functional role in shade avoidance; however, we propose that KDR does not interact with HFR1 to regulate shade avoidance. Based on these observations, we propose that shade avoidance is regulated by a three-layered gas-and-brake mechanism of bHLH protein interactions, adding a layer of complexity to what was previously known.

Highlights

Plants harvest light energy during photosynthesis, especially, blue (B) (~400-500 nm waveband) and red (R) (~600-700 nm waveband) light, whilst mostly reflecting far[45] red light (FR) (~700-800 nm waveband)
We measured the elongation of hypocotyls in seedlings 120 and of petioles in rosette plants upon exposure to low R:far45 red light (FR)
Since AIF2, AIF4, ILI1 BINDING BHLH 1 (IBH1) and IBH1-LIKE 1 (IBL1) were never associated with shade responses, we verified if their expression level was differentially regulated upon exposure to low R:FR and we found that this was the case (Figure 8A)

Summary

Introduction

Plants harvest light energy during photosynthesis, especially, blue (B) (~400-500 nm waveband) and red (R) (~600-700 nm waveband) light, whilst mostly reflecting far[45] red light (FR) (~700-800 nm waveband). The ratio of R to FR is reduced by light reflected or transmitted through plant leaves and neighbors use this to detect the presence of nearby plants. Shade intolerant plants, such as Arabidopsis thaliana, respond to this lowered R:FR ratio with the shade avoidance syndrome (SAS). The main shade avoidance characteristics in A. thaliana are: hypocotyl, internode and petiole elongation, early flowering, and upward leaf movement called hyponasty (de Wit et al, 2016; Ballaré et al, 1991; de Wit et al, 2015; Galvāo et al, 2019; Pantazopoulou et al, 2017). SAS is typical of most plants, including crops, and it improves individual plant fitness, it may compromise total crop yield (Robson et al, 1996; Boccalandro et al, 2003). Shade tolerant species, such as those from forest understories, have developed alternative strategies to cope with shade conditions without investing in shade avoidance growth (Gommers et al, 2013, 2017; Molina-Contreras et al, 2019)

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Conclusion