Redox regulation of variegation in the immutans mutant of Arabidopsis thaliana

IMMUTANS protein has been suggested to act as an oxidase of the intersystem electron transport carrier plastoquinone. IMMUTANS has been proposed to prevent over-reduction of the electron transport chain. This hypothesis was examined through the use of wild-type Arabidopsis plants compared to plants in which the IMMUTANS gene is over-expressed and immutans mutant. The abundance of IMMUTANS did not alter the flux of electrons between photosystem II and photosystem I and did not confer resistance to photoinhibition.;The validity of this model was examined by investigating chloroplast biogenesis under high and low excitation pressure. I hypothesized that immutans exposed to high excitation pressure would be delayed in chloroplast biogenesis and IMMUTANS expression in wild-type would be highest. Etiolated immutans seedlings exposed to high excitation pressure exhibited a 38% decrease in photosystem II function, whereas immutans grown under low excitation pressure exhibited a 13% decrease when compared to wild-type. However, IMMUTANS expression decreased in wild-type grown under high compared to low excitation pressure. These results do not support the threshold model of photo-oxidation. I propose the threshold model of excitation pressure, where if more than 15% of photosystem II are closed than variegation will result.;Keywords: Chloroplast biogenesis, Chlororespiration, Excitation pressure, IMMUTANS, Photoprotection, Redox regulation, Variegation.;Lack of IMMUTANS results in a variegated phenotype consisting of green and white sectored leaves. Variegation has been hypothesized to occur by the threshold model of photo-oxidation, where the lack of immutans results in an over-reduced plastoquinone pool, which disrupts carotenoid biosynthesis resulting in the formation of reactive oxygen species and photo-bleaching of chlorophyll. Excitation pressure was used to estimate the redox state of the plastoquinone pool and has been shown to be modulated by irradiance and temperature. I hypothesized that the redox state of the plastoquinone pool regulated variegation. Development of an imaging technique to quantify variegation determined that immutans grown under 50, 150 and 450 μmol photons m-2s-1 at 25°C exhibited 0, 30 and 60% increased variegation and which was correlated with an increase in excitation pressure. However, by maintaining irradiances and decreasing the growth temperature to 12°C, variegation increased 20 to 40% further compared to growth at 25°C. Therefore, excitation pressure modulates variegation.