Regulation of UVR8 photoreceptor dimer/monomer photo‐equilibrium in Arabidopsis plants grown under photoperiodic conditions

The UV RESISTANCE LOCUS 8 (UVR8) photoreceptor specifically mediates photomorphogenic responses to UV‐B. Photoreception induces dissociation of dimeric UVR8 into monomers to initiate responses. However, the regulation of dimer/monomer status in plants growing under photoperiodic conditions has not been examined. Here we show that UVR8 establishes a dimer/monomer photo‐equilibrium in plants growing in diurnal photoperiods in both controlled environments and natural daylight. The photo‐equilibrium is determined by the relative rates of photoreception and dark‐reversion to the dimer. Experiments with mutants in REPRESSOR OF UV‐B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2 show that these proteins are crucial in regulating the photo‐equilibrium because they promote reversion to the dimer. In plants growing in daylight, the UVR8 photo‐equilibrium is most strongly correlated with low ambient fluence rates of UV‐B (up to 1.5 μmol m(−2) s(−1)), rather than higher fluence rates or the amount of photosynthetically active radiation. In addition, the rate of reversion of monomer to dimer is reduced at lower temperatures, promoting an increase in the relative level of monomer at approximately 8–10 °C. Thus, UVR8 does not behave like a simple UV‐B switch under photoperiodic growth conditions but establishes a dimer/monomer photo‐equilibrium that is regulated by UV‐B and also influenced by temperature.