| Plants and algae employ a number of defense mechanisms to protect themselves from photo-oxidative damage. The goal of my research was to learn more about how these defenses are activated in response to oxidative stress. To this end, I took three complementary approaches, each using the model alga Chlamydomonas reinhardtii. The first was to evaluate responses to oxidative stress in a high-light-sensitive, antioxidant-deficient mutant (Chapter 2). Studies of this mutant, npq1 lor1, pointed to the possible importance of singlet oxygen during high-light stress, as evidenced by enhanced sensitivity of npq1 lor1 to singlet oxygen and increased expression of a singlet-oxygen-induced gene in both the mutant and the wild type following a shift from low light to high light.;To evaluate responses to specific reactive oxygen species, I assayed C. reinhardtii for the ability to acclimate to several different forms of oxidative stress (Chapter 3). I found a previously undescribed acclimation response to singlet oxygen that is not present in the non-photosynthetic organisms Saccharomyces cerevisiae and Escherichia coli. This response can be activated both by exogenous application of singlet-oxygen-generating compounds and by high-light exposure, again suggesting an overlap between photo-oxidative stress and singlet oxygen. To investigate this link further, I initiated two genetic screens to isolate mutants that exhibit altered responses to singlet oxygen---the first a screen for resistance to singlet oxygen produced by the photosensitizing dye rose bengal (Chapter 4), and the second a screen for singlet-oxygen resistance in strains that accumulate protochlorophyllide, an endogenous photosensitizer (Chapter 5). With the tools established here, future work can delineate the process by which photosynthetic organisms sense and respond to singlet-oxygen stress. |
