Difference in the action spectra for UVR8 monomerisation and HY5 transcript accumulation in Arabidopsis

Abstract

The photoreceptor UV RESISTANCE LOCUS 8 (UVR8) activates photomorphogenic responses when plants are exposed to ultraviolet-B (UV-B) light. However, whereas the absorption spectrum of UVR8 peaks at 280 nm, action spectra for several photomorphogenic UV-B responses show maximal photon effectiveness at 290-300 nm. To investigate this apparent discrepancy we measured the effectiveness of UV wavelengths in initiating two responses in Arabidopsis: photoconversion of homodimeric UVR8 into the monomeric form, which is active in signaling, and accumulation of transcripts of the ELONGATED HYPOCOTYL 5 (HY5) transcription factor, which has a key role in UVR8-mediated responses. When purified UVR8 or Arabidopsis leaf extracts were exposed to UV light monomerisation was maximal at approximately 280 nm, which correlates with the UVR8 absorption spectrum. When intact plants were exposed to UV, monomerisation was most strongly initiated at approximately 290 nm, and this shift in maximal effectiveness could be explained by strong absorption or reflectance at 280 nm by leaf tissue. Notably, the action spectrum for accumulation of HY5 transcripts in the same leaf tissue samples used to assay UVR8 dimer/monomer status peaked at approximately 300 nm. Possible reasons for the difference in maximal photon effectiveness of UVR8 monomerisation and HY5 transcript accumulation in leaf tissue are discussed.

A dynamic model of UVR8 photoreceptor signalling in UV‐B‐acclimated Arabidopsis

“…One possibility is that the rate of dimer dissociation, and hence monomer formation (number of monomers formed per unit time), determines the level of response. The rate of monomerisation will increase in proportion to the increase in UV-B fluence rate, as shown in previous dose-response analyses (D ıaz-Ramos et al, 2018). However, as the steady-state monomer/total UVR8 fraction remains the same, there must be a corresponding increase in the rate of New Phytologist (2020) 227: 857-866 Ó 2020 The Authors New Phytologist Ó 2020 New Phytologist Trust www.newphytologist.com re-dimerisation, mediated by RUP proteins.…”

A perspective on ecologically relevant plant-UV research and its practical application

“…Arabidopsis uvr8 null mutants retain a residual response to UV-B, even at low fluence rates, so the involvement of another photoreceptor is conceivable. It is interesting that the peak of the UVR8 action spectrum in vivo differs from that of UVR8 dimer-to-monomer conversion, 94 and while there are several reasonable explanations for this discrepancy (see section h), the possibility that plants possess an additional UV-B photoreceptor, perhaps dependent on the presence of UVR8 or co-acting with UVR8, needs to be considered. Other known photoreceptors, which are able to absorb UV-B wavelengths, or flavin-binding proteins that absorb short wavelength UV-A, could also potentially interact with UVR8 to mediate responses, and such photoreceptor interactions are discussed further below (section f ).…”

Regulation of Arabidopsis gene expression by low fluence rate UV-B independently of UVR8 and stress signaling

“…† Assays of expression at different UV wavelengths were undertaken using a pulsed Opolette 355 + UV II tuneable laser (Opotek Inc., USA) with a half bandwidth of 0.4 nm as described in ref. 35. Following UV exposure with the tuneable laser, plant material was left in darkness for 1 hour to allow transcripts to accumulate and then harvested and snap frozen in liquid nitrogen.…”