Improved survival of very high light and oxidative stress is conferred by spontaneous gain-of-function mutations in Chlamydomonas |
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Authors: | Britta Förster |
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Institution: | ARC Centre of Excellence in Plant Energy Biology, School of Biochemistry and Molecular Biology, Bldg. 41, The Australian National University, Canberra, ACT 0200, Australia |
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Abstract: | Investigations into high light and oxidative stress in photosynthetic organisms have focussed primarily on genetic impairment of different photoprotective functions. There are few reports of “gain-of-function” mutations that provide enhanced resistance to high light and/or oxidative stress without reduced productivity. We have isolated at least four such very high light resistant (VHLR) mutations in the green alga, Chlamydomonas reinhardtii, that permit near maximal growth rates at light intensities lethal to wild type. This resistance is not due to an alteration in electron transport rate or quantity and functionality of the two photosystems that could have enhanced photochemical quenching. Nor is it due to reduced excitation pressure by downregulation of the light harvesting antennae or increased nonphotochemical quenching. In fact, photosynthetic activity is unaffected in more than 30 VHLR isolates. Instead, the basis of the VHLR phenotype is a combination of traits, which appears to be dominated by enhanced capacity to tolerate reactive oxygen species generated by excess light, methylviologen, rose bengal or hydrogen peroxide. This is further evidenced in lower levels of ROS after exposure to very high light in the VHLR-S9 mutant. Additionally, the VHLR phenotype is associated with increased zeaxanthin accumulation, maintenance of fast synthesis and degradation rates of the D1 protein, and sustained balanced electron flow into and out of PSI under very high light. We conclude that the VHLR mutations arose from a selection pressure that favors changes to the regulatory system(s) that coordinates several photoprotective processes amongst which repair of PSII and enhanced detoxification of reactive oxygen species play seminal roles. |
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Keywords: | Chl chlorophyll D1 D1 reaction center protein DPS de-epoxidation state of xanthophyll cycle pigments DAB 3 3&prime diaminobenzidine Fo and Fm minimum and maximum fluorescence Fv/Fm maximum dark-adapted efficiency of photosystem II Lhcp light harvesting complex protein MV methylviologen NBT nitroblue tetrazolium NPQ nonphotochemical quenching of chlorophyll fluorescence PSI photosystem I PSII photosystems II ROS reactive oxygen species VHLR very high light resistant mutations |
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