How Plants Take the Bad with the Good: Conserved UV-B Perception and Signaling in Chlamydomonas.

The sun gives light, but it’s a package deal: Sunlight contains “good” wavelengths that plants can use for photosynthesis, but it also contains “bad” wavelengths that can damage cells. For example, the ozone layer absorbs most UV-B (wavelengths 280 to 315 nm) from sunlight; the remaining UV-B makes up less than 0.5% of the total light energy but can damage DNA and the photosynthetic apparatus. Not surprisingly, in addition to havingmechanisms to sense longer, photosynthetically active wavelengths (phytochromes, cryptochromes, etc.), plants have also evolved mechanisms to sense shorter, potentially damaging wavelengths (reviewed in Heijde and Ulm, 2012). UV RESISTANCE LOCUS8 (UVR8) absorbs UV-B light via conserved tryptophan residues (rather than an exogenous chromophore as in other photoreceptors). Absorbing UV-B causes the UVR8 apparent homodimer to dissociate into monomers, which interact with CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1), an E3 ubiquitin ligase (Rizzini et al., 2011). This interaction initiates a transcriptional response that induces genes encoding protective factors such as enzymes in the phenylpropanoid biosynthesis pathway and damage-repair factors such as photolyases. The COP1-UVR8 interaction also induces the expression of negative regulators that interact directly with UVR8 to help it transition back into a homodimer, thus turning off the response and completing the cycle (see figure). To examine the evolution of UV-B acclimation responses, Tilbrook et al. (2016) lookedatwhether theUVR8-COP1pathway exists in Chlamydomonas reinhardtii. By querying the Chlamydomonas proteome with Arabidopsis thaliana UVR8, they identified a likely ortholog with 45% amino acid sequence identity, including the conserved tryptophan residues, and good predictedstructural conservation.Moreover, Chlamydomonas UVR8 undergoes a similar UV-B-dependentmonomerizationthateffects interactionwithChlamydomonasCOP1and even can complement an Arabidopsis uvr8 null mutant. A conserved UVR8-COP1 UV-B pathway has functional consequences for Chlamydomonas cells. Chlamydomonas cultures grown under low-level UV-B show acclimation: They can survive UV-B stress better than nonacclimated cultures. Acclimated cells alsoshowed lessphotoinhibition,measured by examining the chlorophyll fluorescence spectrum and the levels of the photosystem II core proteins D1 and D2. This acclimation response does not occur in Chlamydomonas cells of the high light tolerant1 (hit1) mutant line, which carry a mutation in Cr-COP1. The authors also used RNA-sequencing to show that acclimation to UV-B involves large changes to the Chlamydomonas transcriptome, which includes homologs of Arabidopsis genes involved in UV-B signaling and genes encoding stress-related light-harvesting complex proteins. These observations show that plants have adapted to take the bad with the good, and that photosynthetic cells evolved mechanisms to acclimate to the UV-B radiation present in natural sunlight early in the green lineage. A further intriguing aspect of this work is the conserved role of COP1 in responding to light andpossibly in integrating responses to different wavelengths. In addition to interacting with UVR8, in Arabidopsis COP1 also interacts with photoreceptors that respond to longer wavelengths. The evolution and function of the mechanisms producing specific outputs remains an intriguing topic for future research.