Abstract
Plants possess exuberant plasticity that facilitates its ability to adapt and survive under challenging environmental conditions. The developmental plasticity largely depends upon cellular elongation which is governed by a complex network of environmental and phytohormonal signals. Here, we report role of glucose (Glc) and Glc-regulated factors in controlling elongation growth and shade response in Arabidopsis. Glc controls shade induced hypocotyl elongation in a dose dependent manner. We have identified a Glc repressed factor REGULATED BY SUGAR AND SHADE1 (RSS1) encoding for an atypical basic helix-loop-helix (bHLH) protein of unknown biological function that is required for normal Glc actions. Phenotype analysis of mutant and overexpression lines suggested RSS1 to be a negative regulator of elongation growth. RSS1 affects overall auxin homeostasis. RSS1 interacts with the elongation growth-promoting proteins HOMOLOG OF BEE2 INTERACTING WITH IBH 1 (HBI1) and BR ENHANCED EXPRESSION2 (BEE2) and negatively affects the transcription of their downstream targets such as YUCs, INDOLE-3-ACETIC ACID INDUCIBLE (IAAs), LONG HYPOCOTYL IN FAR-RED1 (HFR1), HOMEOBOX PROTEIN 2 (ATHB2), XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASES (XTHs) and EXPANSINS. We propose, Glc signals might maintain optimal hypocotyl elongation under multiple signals such as light, shade and phytohormones through the central growth regulatory bHLH/HLH module.
Highlights
Shade has been shown to increase HY5 protein stability and decrease LONG HYPOCOTYL IN FAR-RED1 (HFR1) protein stability suggesting that enhanced nuclear localization of COP1 under shade differentially regulates its targets[8]
The hypocotyl cell elongation under shade avoidance response is tightly regulated via cooperative interactions among the tripartite HLH/basic helix-loop-helix (bHLH) module which is formed through antagonistic interactions among DNA-binding bHLH factors, such as PREs with non-DNA-binding HLH factors such as INCLINATION1 binding bHLH 1 (IBH1), HFR1, PHYTOCHROME RAPIDLY REGULATED1 (PAR1) and PAR213
We have previously shown that Glc signaling could regulate hypocotyl elongation growth in both light grown as well as etiolated seedlings of Arabidopsis by affecting a phytohormonal signaling cascade[43]
Summary
Shade has been shown to increase HY5 protein stability and decrease LONG HYPOCOTYL IN FAR-RED1 (HFR1) protein stability suggesting that enhanced nuclear localization of COP1 under shade differentially regulates its targets[8]. In Arabidopsis, perception of low R/FR leads to expression of several non-DNA binding atypical bHLH factors, which act as negative regulators of shade induced plant growth. Plants perceive more levels of FR light (ca.[735] nm) and due to a subsequent reduction in R:FR ratio, a signal is triggered through phytochromes (mainly phyB) Generated sugars, such as sucrose (Suc) and glucose (Glc) serve as basic elements regulating cellular metabolism but can act as signal molecules by a coordinated modulation of gene expression and enzyme activities in both source and sink tissues[19,20]. Sugars/Glc can act like hormones in translating nutrient status to regulate growth and floral transition[19,28,31,33,34] Multiple signals, such as light, shade, nutrients and phytohormones integrate and regulate common transcriptional signatures to orchestrate growth and development in plants. RSS1 encodes for atypical bHLH protein that acts as a negative regulator of cell elongation in response to multiple signals such as light, shade, phytohormones and temperature
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