Abstract
The photoreceptor UVR8 mediates numerous photomorphogenic responses of plants to UV-B wavelengths by regulating transcription. Studies with purified UVR8 and seedlings not previously exposed to UV-B have generated a model for UVR8 action in which dimeric UVR8 rapidly monomerises in response to UV-B exposure to initiate signalling. However, the mechanism of UVR8 action in UV-B-acclimated plants growing under photoperiodic conditions, where UVR8 exists in a dimer/monomer photo-equilibrium, is poorly understood. We examined UVR8 dimer/monomer status, gene expression responses, amounts of key UVR8 signalling proteins and their interactions with UVR8 in UV-B-acclimated Arabidopsis. We show that in UV-B-acclimated plants UVR8 can mediate a response to a 15-fold increase in UV-B without any increase in abundance of UVR8 monomer. Following transfer to elevated UV-B, monomers show increased interaction with both COP1, to initiate signalling and RUP2, to maintain the photo-equilibrium when the dimer/monomer cycling rate increases. Native RUP1 is present in low abundance compared with RUP2. We present a model for UVR8 action in UV-B-acclimated plants growing in photoperiodic conditions that incorporates dimer and monomer photoreception, dimer/monomer cycling, abundance of native COP1 and RUP proteins, and interactions of the monomer population with COP1, RUP2 and potentially other proteins.
Highlights
UV-B wavelengths (280–315 nm) regulate numerous aspects of plant morphogenesis, physiology, biochemical composition and defence, principally by controlling the expression of hundreds of genes (Jenkins, 2009)
We have found that UV RESISTANCE LOCUS 8 (UVR8) functions differently in plants growing in photoperiodic conditions with supplementary, low level UV-B (Findlay & Jenkins, 2016)
All experiments were undertaken with the Arabidopsis thaliana (Ler) uvr8-1/CaMV35Spro:GFP-UVR8 transgenic line described by Cloix & Jenkins (2008)
Summary
UV-B wavelengths (280–315 nm) regulate numerous aspects of plant morphogenesis, physiology, biochemical composition and defence, principally by controlling the expression of hundreds of genes (Jenkins, 2009). Many responses to UV-B are mediated by the photoreceptor UV RESISTANCE LOCUS 8 (UVR8) (Jenkins, 2017; Yin & Ulm, 2017). Research in Arabidopsis has revealed that UVR8 regulates a wide range of processes, including metabolite biosynthesis, stem extension, leaf expansion, phototropism, photosynthetic competence, stomatal density, stomatal closure, circadian rhythmicity, flowering, and resistance to pathogens (Jenkins, 2017; Yin & Ulm, 2017). UVR8 is involved in responses to other stimuli, including osmotic stress (Fasano et al, 2014) and UV-A light (Morales et al, 2013), and inhibits thermomorphogenesis (Hayes et al, 2017), and shade-avoidance responses (Hayes et al, 2014). UVR8 is highly conserved in diverse plant taxa and is likely to be pivotal in mediating responses to UV-B in numerous species. It is important to understand how UVR8 functions in plants growing in natural growth environments
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