Zeaxanthin Binds to Light-Harvesting Complex Stress-Related Protein to Enhance Nonphotochemical Quenching in Physcomitrella patens.

Nonphotochemical quenching (NPQ) dissipates excess energy to protect the photosynthetic apparatus from excess light. Themoss Physcomitrella patens exhibits strong NPQ by both algal-type light-harvesting complex stress-related (LHCSR)–dependent and plant-type S subunit of Photosystem II (PSBS)-dependent mechanisms. In this work, we studied thedependence of NPQ reactions on zeaxanthin, which is synthesized under light stress by violaxanthin deepoxidase (VDE) frompreexisting violaxanthin. We produced vde knockout (KO) plants and showed they underwent a dramatic reduction in thermaldissipation ability and enhanced photoinhibition in excess light conditions. Multiple mutants (vde lhcsr KO and vde psbs KO)showed that zeaxanthin had a major influence on LHCSR-dependent NPQ, in contrast with previous reports inChlamydomonas reinhardtii. The PSBS-dependent component of quenching was less dependent on zeaxanthin, despite thenear-complete violaxanthin to zeaxanthin exchange in LHC proteins. Consistent with this, we provide biochemical evidencethat native LHCSR protein binds zeaxanthin upon excess light stress. These findings suggest that zeaxanthin played animportant role in the adaptation of modern plants to the enhanced levels of oxygen and excess light intensity of landenvironments.