Plant UV-B photoreceptor signalling

Lead Research Organisation: University of Glasgow
Department Name: College of Medical, Veterinary &Life Sci

Abstract

Ultraviolet-B (UV-B) radiation is a minor but very energetic component of sunlight. Exposure to UV-B wavelengths (280-315 nm) has numerous effects on plants, including changes in metabolism and development. Importantly, UV-B stimulates responses in plants that protect them against the potentially damaging consequences of UV-B exposure. The effects of UV-B are due to its ability to regulate the expression of numerous plant genes, including those involved in UV-protection, biosynthesis and chloroplast function. However, little is known about how UV-B is perceived by plants and how it initiates responses. The first component involved in these processes has been identified, a protein called UVR8. UVR8 acts as a photoreceptor to detect UV-B light. UVR8 interacts with another protein, COP1, to initiate responses to UV-B in plants. The aim of this project is to enhance understanding of the processes through which UVR8 and COP1 function to regulate plant responses. Establishing the molecular mechanisms of plant UV-B perception will help us to understand how UV-B regulates aspects of plant growth and development and how plants survive in sunlight.

Technical Summary

UV-B wavelengths (280-315 nm) induce a range of physiological responses in plants, but the mechanisms of UV-B perception and signal transduction are poorly understood. Our research has shown that the Arabidopsis protein UV RESISTANCE LOCUS8 (UVR8) acts in a UV-B-specific manner to regulate the expression of a range of genes involved in processes that protect the plant against damage by UV-B, including secondary metabolism, anti-oxidant defence and DNA repair. We found that UVR8 associates with chromatin to regulate the expression of target genes. UVR8 interacts with the COP1 protein in a UV-B dependent manner in plants. Recently we discovered that UVR8 acts as a UV-B photoreceptor. UV-B absorption converts UVR8 from a dimer to a monomer. The aim of this project is to enhance understanding of how UVR8 functions with COP1 in signal transduction.

Planned Impact

Beneficiaries The beneficiaries and users of the research will include: organisations in the commercial sector interested in novel strategies to improve crop productivity; agencies and policy makers interested in the effects of UV-B on organisms and ecosystems in relation to depletion of the ozone layer and effects of increasing UV-B radiation (including the relevant United Nations Environmental Programme - UNEP - panel); individuals (including text book authors) and organisations (e.g. Glasgow Science Centre and the University's Technology and Science Alliance) involved in science communication to schools and the wider public. The general public, in so far as they are interested in the effects of UV-B on organisms and ecosystems.
Benefits The impact of the research to the beneficiaries derives both from (i) its potential relevance to crop plant improvement and agricultural practice and (ii) the relevance to understanding the impact of UV-B on the biosphere. (i) UV-B impacts on agricultural as well as natural ecosystems and therefore has direct relevance to crop plants. UV-B signalling pathways regulate biosynthetic activities (and hence plant biochemical composition), prime defence responses (e.g. UV-B exposure reduces damage by herbivorous insects in a range of species) and regulate aspects of morphogenesis and development of relevance to crops (e.g. leaf expansion, extension growth and branching). Furthermore there is evidence that UV-B interacts with a number of signalling pathways to modify responses to a variety of abiotic factors (e.g. drought, low temperature and various mineral nutrients). Research to understand UV-B perception, signalling and response therefore has the potential to generate novel strategies for crop plant improvement that could benefit farmers, consumers and the environment and contribute to the economic competitiveness of the UK. In addition, there are examples where manipulation of the UV-B environment is being used in agriculture to help control pests. Thus it is important to understand how altering the UV-B environment may affect plant processes and investigation of the mechanisms of UV-B perception and signalling is key to this. (ii) Plants are key components of natural ecosystems and UV-B has broad impacts on ecosystem function. Concern over depletion of the stratospheric ozone layer by human activities has promoted an interest in understanding how plants perceive UV-B. This information is being used to inform policy makers who are concerned with maintaining human health and the quality of life. The general public will benefit in a cultural sense from the increase in knowledge and understanding of the effects of UV-B on plants. The public can relate to effects of UV-B such as sunburn and skin cancer and so the idea that plants manage to avoid damage by UV-B through perceiving and responding to UV-B is accessible. The above impacts of the research will be realised over the short to medium term. Staff working on the project will obtain knowledge and expertise that can be applied in related research or more widely in the commercial or public sectors.

Publications

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Hayes S (2014) UV-B detected by the UVR8 photoreceptor antagonizes auxin signaling and plant shade avoidance. in Proceedings of the National Academy of Sciences of the United States of America

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Heilmann M (2016) Dimer/monomer status and in vivo function of salt-bridge mutants of the plant UV-B photoreceptor UVR8. in The Plant journal : for cell and molecular biology

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Jenkins GI (2014) Structure and function of the UV-B photoreceptor UVR8. in Current opinion in structural biology

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Liao X (2019) A FRET method for investigating dimer/monomer status and conformation of the UVR8 photoreceptor. in Photochemical & photobiological sciences : Official journal of the European Photochemistry Association and the European Society for Photobiology

 
Description Relationship between UVR8 monomerization and nuclear accumulation: Both the UVR8 dimer and monomer can accumulate in the nucleus. We produced the GFP-UVR8W285F transgenic line. GFP-UVR8W285F, which is constitutively dimeric, is present in both the cytosol and nucleus. The constitutively monomeric GFPUVR8R286A and GFP-UVR8W285A lines also show localisation in both compartments.
Interaction of UVR8 with COP1: Co-immunoprecipitation experiments with GFP-UVR8W285A (and other mutants) show that the protein constitutively interacts with COP1 but it is present in both the cytosol and nucleus. Therefore COP1 does not interact with UVR8 only in the nucleus. ?C27UVR8, which lacks 27 amino acids towards the C-terminus and only weakly interacts with COP1, still accumulates in the nucleus. GFP-UVR8R286A and GFP-UVR8W285A are constitutively monomeric in plants. GFP-UVR8W285A constitutively interacts with COP1 in transgenic plants but GFPUVR8R286A does not interact. Neither mutant shows UV-B induction of HY5 and CHS expression. Therefore monomerisation and COP1 interaction are not sufficient for function.
Regeneration of the active photoreceptor: We succeeded in demonstrating that reversion of the monomer to the dimer, rather than de novo synthesis following rapid degradation of the monomer, is the mechanism of dimer regeneration. Furthermore we showed that rapid reversion requires intact cells, protein synthesis, the presence of the C27 region of UVR8 (which interacts with COP1 and RUP proteins) and COP1. We published a paper describing this work (Heilmann & Jenkins, 2013).
Interaction of UVR8 and COP1 with chromatin: Through a combination of ChIP experiments with wild-type and uvr8 mutant plants combined with sequencing of ChIP products, we obtained evidence that UVR8 regulates the accumulation of a specific histone modification (H3 K9,K14 diacetylation) at UVR8-regulated loci (Velanis et al. 2016)
How is UVR8 signalling modulated by other light signalling pathways?: We addressed this question by initiating collaboration with Dr Kerry Franklin (Bristol). We found that UV-B exposure inhibits the phytochrome and blue light regulated shade avoidance response and that the effect of UV-B is mediated by UVR8. We further showed that the effect of UVR8 is mediated through the control of auxin and gibberellic acid signalling and that UV-B acts through regulation of the abundance of PIF and DELLA proteins. These findings were published in PNAS (Hayes et al. 2014).
Exploitation Route The findings provide a basis for further studies of the mechanisms through which UVR8 regulates plant processes. We are pursuing several aspects of this research. Since UV-B light has a wide ranging impact on plant development, metabolism, defence and responses to abiotic stimuli, knowledge of how UVR8 functions has potential applications in the agricultural sector. In addition, fundamental knowledge of how UVR8 protein functions has application in providing ontogenetic tools for biotechnological applications.
Sectors Agriculture, Food and Drink