| Plants require light for life but they can also be damaged by excess light due to increased production of reactive oxygen species. As sessile organisms, plants had to evolve mechanisms, other than "walking away", to cope with excess light. To investigate the importance of various components of photoprotection, specifically the harmless dissipation of excitation energy as heat (qE), xanthophylls, and ascorbate, the sensitivity of various Arabidopsis thaliana mutants to short-term and long-term high light (HL) stress was analyzed. The following mutants were used as single mutants or in combinations: npq4, which lacks the PsbS protein and therefore all qE; npq1, which lacks zeaxanthin and qE; lut2, which lacks lutein; and vtc2, which is ascorbate-deficient.; My research showed that all components of photoprotection investigated are important for the plant, but that some are more important for certain kinds of stresses than others. qE is important during short-term HL stress, as evidenced by increased photoinhibition in npq mutants, whereas qE is less important during long-term HL stress, given the healthy growth of the qE mutants in HL. In long-term HL, increases in antioxidant levels and the capacity for utilization of absorbed light energy are more important. The ascorbate-deficient mutants were able to grow in HL, but they were more photoinhibited. They also bleached when transferred from low light to HL. This shows that ascorbate is important for acclimation, but that other components are upregulated during growth in HL. Zeaxanthin might have a possible additional photoprotective role, as vtc2npq1 mutants showed greater HL sensitivity than vtc2 alone.; The components of photoprotection are interconnected. Ascorbate is an antioxidant, but it is also required for the synthesis of zeaxanthin. I showed that ascorbate can limit the enzyme required for zeaxanthin synthesis in vivo. It is also apparent that plants have multiple levels of defense, and that different photoprotective components can compensate for each other. There also is synergism between different levels of defense, for example between zeaxanthin and ascorbate. In addition, the gene that is defective in a low qE mutant (paa1) was cloned and characterized to be a chloroplastic copper transporter. |
