Very high light resistant mutants of Chlamydomonas reinhardtii: Responses of Photosystem II, nonphotochemical quenching and xanthophyll pigments to light and CO2

We have isolated very high light resistant nuclear mutants (VHLR) in Chlamy domonas reinhardtii, that grow in 1500-2000 mu mol photons m(-2) s(-1) (VHL ) lethal to wildtype. Four nonallellc mutants have been characterized in te rms of Photosystem II (PS II) function, nonphotochemical quenching (NPQ) an d xanthophyll pigments in relation to acclimation and survival under light stress. In one class of VHLR mutants isolated from wild type (S4 and S9), V HL resistance was accompanied by slower PS II electron transfer, reduced co nnectivity between PS II centers and decreased PS II efficiency. These lesi ons in PS II function were already present in the herbicide resistant D1 mu tant A251L (L*) from which another class of VHLR mutants (L4 and L30) were isolated, confirming that optimal PS II function was not critical for survi val in very high light. Survival of all four VHLR mutants was independent o f CO2 availability, whereas photoprotective processes were not. The de-epox idation state (DPS) of the xanthophyll cycle pigments in high light (HL, 60 0 mu mol photons m(-2) s(-1)) was strongly depressed when all genotypes wer e grown in 5% CO2. In S4 and S9 grown in air under HL and VHL, high DPS was well correlated with high NPQ. However when the same genotypes were grown in 5% CO2, high DPS did not result in high NPQ, probably because high photo synthetic rates decreased thylakoid Delta pH. Although high NPQ lowered the reduction state of PS II in air compared to 5% CO2 at HL in wildtype, S4 a nd S9, this did not occur during growth of S4 and S9 in VHL. L* and VHLR mu tants L4 and L30, also showed high DPS with low NPQ when grown air or 5% CO 2, possibly because they were unable to maintain sufficiently high Delta pH due to constitutively impaired PS II electron transport. Although dissipat ion of excess photon energy through NPQ may contribute to VHL resistance, t here is little evidence that the different genes conferring the VHLR phenot ype affect this form of photoprotection. Rather, the decline of chlorophyll per biomass in all VHLR mutants grown under VHL suggests these genes may b e involved in regulating antenna components and photosystem stoichiometries .