Photoprotection and acclimation of mutants with defects in nonphotochemical quenching and antioxidant synthesis

Abstract

Algae and plants have evolved numerous mechanisms to cope with the potential for photo-oxidative damage that accompanies oxygenic photosynthesis. Photosynthetic light harvesting is regulated by processes such as nonphotochemical quenching (NPQ) that help to minimize production of reactive oxygen species, and various antioxidant molecules and enzymes are involved in scavenging or quenching of reactive molecules that are inevitably generated. NPQ and antioxidants are being investigated using mutants of Arabidopsis thaliana and Chlamydomonas reinhardtii. Characterization of an Arabidopsis mutant revealed an essential role for the photosystem II protein PsbS in NPQ. Based on the results of site-directed mutagenesis experiments, we hypothesize that PsbS is involved in sensing light intensity through changes in thylakoid lumen pH. NPQ has an important photoprotective role during short-term fluctuations in light intensity, because NPQ-deficient mutants were more photoinhibited than the wild type during a short treatment with high light, but in the long term the mutants were able to acclimate to high light conditions. In contrast to its Arabidopsis counterpart, the npq1 lor1 double mutant of Chlamydomonas, which is deficient in zeaxanthin, lutein, and NPQ, was unable to acclimate to high light and exhibited photo-oxidative bleaching. Extragenic suppressors of the light sensitivity of npq1 lor1 identified new mutations that affect antioxidant synthesis, providing insights into the overlapping functions of chloroplast antioxidants. By studying acclimation of npq and antioxidant mutants to high light, we also hope to uncover other processes involved in photoprotection