Zeaxanthin has enhanced antioxidant capacity with respect to all other xanthophylls in arabidopsis leaves and functions independent of binding to PSII antennae

The ch1 mutant of Arabidopsis thaliana lacks chlorophyll b. Leaves of this mutant are devoid of PSII chlorophyll-protein antenna complexes (LHCII) and have a very low capacity of nonphotochemical energy quenching (NPQ). Lhcb5 was the only PSII antenna protein that accumulated to significant level in ch1 mutant leaves, but the apoprotein did not assemble in vivo with chlorophylls to form a functional antenna. The abundance of Lhca proteins was also reduced, to ~20% of wild-type level. Ch1 was crossed with various xanthophyll mutants to analyze the antioxidant activity of carotenoids unbound to PSII antenna. Suppression of zeaxanthin by crossing ch1 with npq1 resulted in oxidative stress in high light, while removing other xanthophylls or the PSII protein PsbS had no such effect. The tocopheroldeficient ch1 vte1 double mutant was as sensitive to high hight as ch1 npq1, and the triple mutant ch1 npq1 vte1 exhibited an extreme sensitivity to photooxidative stress, indicating that zeaxanthin and tocopherols have cumulative effects. Conversely, constitutive accumulation of zeaxanthin in the ch1 npq2 double mutant led to an increased phototolerance relative to ch1. Comparison of ch1 npq2 with another zeaxanthin-accumulating mutant (ch1 lut2) that lacks lutein suggests that protection of polyunsaturated lipids by zeaxanthin is enhanced when lutein is also present. During photooxidative stress, α-tocopherol noticeably decreased in ch1 npq1 and increased in ch1 npq2 relative to ch1, suggesting protection of vitamin E by high zeaxanthin levels. Our results indicate that the antioxidant activity of zeaxanthin, distinct from NPQ, can occur in the absence of LHCII. The capacity of zeaxanthin to protect thylakoid membrane lipids is comparable to that of vitamin E, but noticeably higher than that of all other xanthophylls of Arabidopsis leaves.