UV-B Signaling Pathways with Different Fluence-Rate Response Profiles Are Distinguished in Mature Arabidopsis Leaf Tissue by Requirement for UVR8, HY5, and HYH

Abstract

UV-B signaling is an important but poorly understood aspect of light responsiveness in plants. Arabidopsis (Arabidopsis thaliana) UV RESISTANCE LOCUS8 (UVR8) is a recently identified UV-B-specific signaling component that regulates UV-protective responses. Using the uvr8 mutant, we defined genetically distinct UVR8-dependent and UVR8-independent pathways that stimulate different sets of genes in mature Arabidopsis leaf tissue. Both pathways operate at 1 mmol m 22 s 21 UV-B and above, but the UVR8-dependent pathway is able to stimulate UV-protective genes even in response to 0.1 mmol m 22 s 21 UV-B. Both pathways function in mutants lacking phytochromes, cryptochromes, or phototropins. Genes encoding the ELONGATED HYPOCOTYL5 (HY5) and HY5 HOMOLOG (HYH) transcription factors are induced at low UV-B fluence rates (0.1 mmol m 22 s 21). Experiments with hy5 and hyh mutants reveal that both these factors mediate responses of the UVR8-dependent pathway, acting with partial or complete redundancy to stimulate expression of particular genes. Furthermore, evidence is presented that all UVR8 pathway genes are likely to be regulated by HY5/HYH and that these transcription factors do not mediate UV-B responses independent of UVR8. Finally, we highlight the functions of HY5 and HYH in UV protection and show that HY5 plays the more critical role. This research provides evidence that, in UV-B signaling, UVR8, HY5, and HYH act together in a photoregulatory pathway and demonstrates a new role for HYH in UV-B responses.

UV-B-Responsive Association of the Arabidopsis bZIP Transcription Factor ELONGATED HYPOCOTYL5 with Target Genes, Including Its Own Promoter

“…Sequence conservation, the ability to form heterodimers, and partial functional redundancy between HY5 and HYH predicted a possible involvement of HYH in regulating the activity of the HY5 promoter (Holm et al, 2002;Brown and Jenkins, 2008;Stracke et al, 2010). Accordingly, ChIP data clearly demonstrated a specific association of endogenous HYH with chromatin in the HY5 promoter region in wild-type seedlings ( Figure 4C).…”

“…However, the molecular events and the identity of the components mediating the transcriptional regulation of target genes by UVR8 remained elusive. It is known that the bZIP transcription factor ELONGATED HYPOCOTYL5 (HY5) mediates UV-B-induced gene expression changes downstream of UVR8, in partial redundancy with its homolog HYH (Ulm et al, 2004;Brown et al, 2005;Oravecz et al, 2006;Brown and Jenkins, 2008;Stracke et al, 2010;Fehér et al, 2011;Huang et al, 2012). Indeed, HY5 and HYH are thought to govern the majority of the UVR8-mediated UV-B transcriptional responses (Tilbrook et al, 2013;Jenkins, 2014).…”

A novel pathway of direct methane production and emission by eukaryotes including plants, animals and fungi: An overview

“…Pectin may interact with UV photosensitisers instead of UV radiation directly. It is noteworthy that UV-B radiation can cause biochemical and physiological changes in plants, some of which are linked to ROS generation (Brown and Jenkins, 2008;Caldwell et al, 1998;Jenkins, 2009). ROS can react with various biomolecules, causing irreversible damage to plants.…”

UV-B Perception and Acclimation in Chlamydomonas reinhardtii

“…Two separate domains of UVR8 participate in the interaction with COP1: The b-propeller core of UVR8 mediates UV-Bdependent interaction with COP1 and the UVR8 C-terminal C27 domain further stabilizes the interaction and regulates COP1 activity (Cloix et al, 2012;Yin et al, 2015). UVR8-COP1 interaction induces UV-B signaling governed by the bZIP transcription factor ELONGATED HYPOCOTYL5 (HY5) and its homolog HYH (Ulm et al, 2004;Brown and Jenkins, 2008;Stracke et al, 2010;Binkert et al, 2014), which leads to UV-B acclimation (Brown et al, 2005;Oravecz et al, 2006;Favory et al, 2009). The UVR8 UV-B photocycle is completed via the action of negative feedback regulators REPRESSOR OF UV-B PHOTOMORPHOGENESIS1 (RUP1) and RUP2 , which directly interact with UVR8 and facilitate redimerization to downregulate UV-B signaling Yin et al, 2015).…”