Abstract
Sun-loving plants perceive the proximity of potential light-competing neighboring plants as a reduction in the red:far-red ratio (R:FR), which elicits a suite of responses called the "shade avoidance syndrome" (SAS). Changes in R:FR are primarily perceived by phytochrome B (phyB), whereas UV-B perceived by UV RESISTANCE LOCUS 8 (UVR8) elicits opposing responses to provide a counterbalance to SAS, including reduced shade-induced hypocotyl and petiole elongation. Here we show at the genome-wide level that UVR8 broadly suppresses shade-induced gene expression. A subset of this gene regulation is dependent on the UVR8-stabilized atypical bHLH transcription regulator LONG HYPOCOTYL IN FAR-RED 1 (HFR1), which functions in part redundantly with PHYTOCHROME INTERACTING FACTOR 3-LIKE 1 (PIL1). In parallel, UVR8 signaling decreases protein levels of the key positive regulators of SAS, namely the bHLH transcription factors PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5, in a COP1-dependent but HFR1-independent manner. We propose that UV-B antagonizes SAS via two mechanisms: degradation of PIF4 and PIF5, and HFR1- and PIL1-mediated inhibition of PIF4 and PIF5 function. This work highlights the importance of typical and atypical bHLH transcription regulators for the integration of light signals from different photoreceptors and provides further mechanistic insight into the crosstalk of UVR8 signaling and SAS.
Highlights
Plants have evolved sophisticated sensory systems to respond and adapt appropriately to the environment
We have discovered an important role for an atypical transcriptional regulator HYPOCOTYL IN FARRED 1 (HFR1) that is stabilized under UV-B, strongly contributing to the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor-mediated repression of shade responses by counteracting PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5 activities
As HFR1 is a CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) substrate and activated UVR8 inactivates COP1 [17,18,42,43,44,45], we hypothesized that HFR1 stability is regulated by UVR8 in response to UV-B, which may contribute to the antagonistic effect of UV-B on shade avoidance syndrome (SAS)
Summary
Plants have evolved sophisticated sensory systems to respond and adapt appropriately to the environment. In addition to being an energy source, is an important source of information about the environment for plants. Plants growing in close proximity to their neighbors are able to sense that light resources may become limiting and preemptively elicit a suite of responses to avoid future shading, collectively known as shade avoidance syndrome (SAS) [3,4]. SAS promotes petiole and stem elongation so that a plant can outcompete its neighbors for light. Photosynthetic pigments strongly absorb red (R) and blue (B) light and preferentially reflect or transmit far-red (FR) light. Plants perceive a reduction in R:FR due to the scattering of FR from non-shading neighbors and initiate SAS [3,5]
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