Evolutionarily conserved but not indispensable for survival: plastic UV RESISTANCE LOCUS8 functionality in Marchantia polymorpha under laboratory and field conditions.

The functionality of the UV-B photoreceptor UV RESISTANCE LOCUS8 (UVR8) has been widely demonstrated in angiosperms under controlled conditions, but studies in nature or using bryophytes are scarce. To fill this gap, we compared the UV responses of the model liverwort Marchantia polymorpha and a mutant lacking a functional UVR8, to provide for the first time an evolutionary perspective of UVR8 performance in both controlled and field conditions. Under exposure to UV in the laboratory, UVR8 in wild type plants was essential to induce the expression of genes (CHS1, FNSI, HY5, SIG5, and ELIP2) and the accumulation of metabolites (apigenin and luteolin derivatives, particularly apigenin 7-O-glucuronide) that contributed to conserving photosynthetic performance. In the mutant, gene and metabolite responses were much weaker, occurred only under high UV-B in a UVR8-independent response, and did not confer photoprotection to the photosynthetic apparatus. In a field seasonal cycle, differences between UV treatments and genotypes were attenuated and the UVR8 functionality was less clear, probably due to the low UV-B levels required to trigger plant responses and the interaction of other abiotic factors. Given the similarity of UVR8 functionality in M. polymorpha and Arabidopsis under both controlled and field conditions, we conclude that UVR8 has been conserved across plant evolution.