| Plants and algae have evolved a set of defense mechanisms to protect themselves from photooxidative stress and its consequences. Among these protective mechanisms, the harmless dissipation of excitation energy as heat (qE), carotenoids, and vitamin E (tocopherols) are of particular interest. To understand the function of carotenoids and tocopherols and the regulation of their content in high light (HL) stress, a combination of genetics, molecular biology, biophysics, and metabolic engineering approaches were taken with two model organisms: Arabidopsis thaliana and Chlamydomonas reinhardtii.;To investigate the xanthophyll dependence of qE, I did a suppressor screen of the Arabidopsis npq1 mutant which lacks zeaxanthin (Chapter 2). A suppressor named szl1 npq1 that lacks zeaxanthin but accumulates more lutein (also a xanthophyll) was isolated. A correlation between lutein cation formation and qE was demonstrated in the suppressor, suggesting that, like zeaxanthin, lutein can act as a quencher of excited chlorophyll. The results also support the requirement of xanthophylls for qE in general.;To test the functional overlap between carotenoids (especially xanthophylls) and tocopherols, I engineered a Chlamydomonas strain that accumulates high levels of tocopherols in a background lacking both zeaxanthin and lutein (Chapter 3). The new strain was able to grow in HL for up to 48 hours, had higher photosytem II efficiency, and was resistant to various oxidative stresses, demonstrating that tocopherols have a role in photoprotection and have functional overlap with carotenoids as antioxidants.;HL induces an increase in the xanthophyll cycle pool size and the tocopherol content in both Arabidopsis and Chlamydomonas. To get some insight into the regulatory mechanisms involved in this acclimation process, I tested the involvement of candidate factors by monitoring the change of the xanthophyll cycle pool size in Arabidopsis mutants. A thylakoid protein kinase that depends on plastoquinone redox state did not seem to be involved. A mutant deficient in two carotenoid beta-ring hydroxylase genes (CHYB1 and CHYB2) showed a loss of HL-induced xanthophyll cycle pool size increase, indicating that these two enzymes are involved in the regulation of carotenoid content. |
