UVR8 is an UV-B and UV-A photoreceptor

Our two recent papers:

Neha Rai, Susanne Neugart, Yan Yan, Fang Wang, Sari M Siipola, Anders V Lindfors, Jana Barbro Winkler, Andreas Albert, Mikael Brosché, Tarja Lehto, Luis O Morales, Pedro J Aphalo (2019) How do cryptochromes and UVR8 interact in natural and simulated sunlight? Journal of Experimental Botany, 70, 4975–4990. https://doi.org/10.1093/jxb/erz236

Neha Rai, Andrew O’Hara, Daniel Farkas, Omid Safronov, Khuanpiroon Ratanasopa, Fang Wang, Anders V. Lindfors, Gareth I. Jenkins, Tarja Lehto, Jarkko Salojärvi, Mikael Brosché, Åke Strid, Pedro J. Aphalo, Luis O. Morales (2020) The photoreceptor UVR8 mediates the perception of both UV‐B and UV‐A wavelengths up to 350 nm of sunlight with responsivity moderated by cryptochromes. Plant, Cell & Environment, https://doi.org/10.1111/pce.13752

In these two recent publications we have shown that UVR8, previously described as an ultravioltet-B (UV-B, 280-315 nm) photoreceptor, in sunlight functions both as an ultraviolet-A (UV-A, 315-400 nm) and UV-B photoreceptor. Although UVR8 presents maximal absorption at the boundary between ultraviolet-C (UV-C, <280 nm) and UV-B, the shape of the solar spectrum in the ultraviolet region, characterized by a very steep slope, allows the UVR8 protein to absorb nearly as many UV-A photons as UV-B photons, and obviously no photons in the UV-C as they are not present in sunlight at ground level.

Normalized spectral absoorbance of UVR8 protein in vitro (From Rai et al.. 2020).

The longer the wavelength the less energy photons carry, and this may limit their ability to drive photochemical reactions, such as the activation of a photoreceptor. We show that somewhere near 340 to 350 nm there is a transition, with photons at longer wavelengths, even if absorbed not leading to monomerization and activation of the UVR8 photoreceptor.

Plants have other photoreceptors capable of absorbing ultraviolet-A radiation: cryptochromes (cry1, cry2), phototropins (phot1, phot2) and proteins in the Zeitloup family (zl). In sunlight, cry1 and cry2 , are mainly activated by blue light (BL) and they seem to play a smaller direct role in the UV-A region of sunlight. However, the action of cry1 and/or cry2 very strongly down-regulates responses to UV-A and UV-B mediated by UVR8.

Finally we also showed that both the cry1 cry2 and uvr8-2 mutants survive and grow almost normally in full sunlight, while full sunlight  kills within a few days of germination most individuals of the cry1 cry uvr8-2 triple mutant.

Clockwise from top left: uvr8-2 cry1 cry2 mutant; uvr8-2 mutant; cry1 cry2 mutant; Ler wild type.

Take home message: UVR8 should be in the future described as a UV-B/UV-A photoreceptor. When studying plants, for measurements and treatments to be informative need to divide the UV-A range into two regions UV-Asw and UV-Alw with a split at 350 nm as we have used, or following CIEs definitions of UV-A1 and UV-A2 with a split at 340 nm. Fully understanding the mechanisms of perception of ultraviolet radiation by plants will require additional studies aiming at disentangling the many signalling interactions downstream of these and other photoreceptors.

How we did it: Pedro J. Aphalo and Luis O. Morales lead this research and generated the main hypotheses, but the success of these studies was made possible by intellectual and practical contributions from several other research groups from Finland, Sweden, Germany, Singapore and Great Britain. It took nearly six years since we started suspecting  that UVR8 played a role in UV-A perception in sunlight and the publication of these articles were we demonstrate why and how UVR8 functions as UV-B/UV-A photoreceptor in sunlight. Both the process and the achievement were highly rewarding intellectually in spite of the lack of enthusiasm shown by several grant-application reviewers along the way and the slow-down this caused.

Perspectives on plant UV-research and its applications

An article, titled “A perspective on ecologically relevant plant-UV research and its practical application”, to be included in the PPS special issue, has been published on-line. It originated on discussions at the second UV4Plants Network meeting held in Bled last year, but writing and editing continued for several months. The article has been published under open access and is available through PPS’ web site. Several members of our research group and some of our collaborators are co-authors.

The graphical and text abstracts are reproduced below.

Graphical abstract from the article. Copyrighted (c) 2019.

Abstract

Plants perceive ultraviolet-B (UV-B) radiation through the UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8), and initiate regulatory responses via associated signalling networks, gene expression and metabolic pathways. Various regulatory adaptations to UV-B radiation enable plants to harvest information about fluctuations in UV-B irradiance and spectral composition in natural environments, and to defend themselves against UV-B exposure. Given that UVR8 is present across plant organs and tissues, knowledge of the systemic signalling involved in its activation and function throughout the plant is important for understanding the context of specific responses. Fine-scale understanding of both UV-B irradiance and perception within tissues and cells requires improved application of knowledge about UV-attenuation in leaves and canopies, warranting greater consideration when designing experiments. In this context, reciprocal crosstalk among photoreceptor-induced pathways also needs to be considered, as this appears to produce particularly complex patterns of physiological and morphological response. Through crosstalk, plant responses to UV-B radiation go beyond simply UV-protection or amelioration of damage, but may give cross-protection over a suite of environmental stressors. Overall, there is emerging knowledge showing how information captured by UVR8 is used to regulate molecular and physiological processes, although understanding of upscaling to higher levels of organisation, i.e. organisms, canopies and communities remains poor. Achieving this will require further studies using model plant species beyond Arabidopsis, and that represent a broad range of functional types. More attention should also be given to plants in natural environments in all their complexity, as such studies are needed to acquire an improved understanding of the impact of climate change in the context of plant-UV responses. Furthermore, broadening the scope of experiments into the regulation of plant-UV responses will facilitate the application of UV radiation in commercial plant production. By considering the progress made in plant-UV research, this perspective highlights prescient topics in plant-UV photobiology where future research efforts can profitably be focussed. This perspective also emphasises burgeoning interdisciplinary links that will assist in understanding of UV-B effects across organisational scales and gaps in knowledge that need to be filled so as to achieve an integrated vision of plant responses to UV-radiation.

 

Titta’s latest paper is on-line

The reprint of Titta’s latest paper “Seasonal fluctuations in leaf phenolic composition under UV manipulations reflect contrasting strategies of alder and birch trees” by “Titta Kotilainen, Riitta Tegelberg, Riitta Julkunen-Tiitto, Anders Lindfors, Robert B. O’Hara, Pedro J. Aphalo” is available at http://onlinelibrary.wiley.com/doi/10.1111/j.1399-3054.2010.01398.x/pdf, as it has been published on-line in Physiologia Plantarum.