UV-B signaling pathways with different fluence-rate response profiles are distinguished in mature Arabidopsis leaf tissue by requirement for UVR8, HY5, and HYH.

Bobby A Brown,Gareth I Jenkins

doi:10.1104/pp.107.108456

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 micromol m(-2) s(-1) UV-B and above, but the UVR8-dependent pathway is able to stimulate UV-protective genes even in response to 0.1 micromol m(-2) s(-1) 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 micromol m(-2) s(-1)). 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.

Highlights

UV-B signaling is an important but poorly understood aspect of light responsiveness in plants
reverse transcription (RT)-PCR experiments were undertaken with a selection of genes chosen because they were likely to be UV RESISTANCE LOCUS8 (UVR8) dependent or UVR8 independent, based on their rank products (RP) score and FDR values (Breitling et al, 2004) in the microarray analysis (Table I)
It is important to define the nature of the UVR8 pathway and to understand how it relates to other UV-B signaling pathways

Summary

Introduction

UV-B signaling is an important but poorly understood aspect of light responsiveness in plants. High fluence rates of UV-B produce reactive oxygen species and may cause damage to DNA, proteins, membranes, and lipids (A-H-Mackerness et al, 2001; Broscheand Strid, 2003; Frohnmeyer and Staiger, 2003; Ulm and Nagy, 2005; Jenkins and Brown, 2007). These higher fluence rates can cause leaf curling and growth inhibition and initiate the expression of genes characteristic of stress responses via signaling pathways that are not specific to UV-B. Ulm et al (2004) found that different UV-B wavebands induce distinct gene expression responses in Arabidopsis (Arabidopsis thaliana) and that shorter wavelength UV-B antagonizes the response to longer wavelengths

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