Photosynthetic consequences of impaired Photosystem II : antisense reductions in the PsbO proteins in Arabidopsis thaliana

Abstract

Photosystem (PS) II is the multi-sub unit complex which uses light energy to split water, providing the reducing equivalents needed for photosynthesis. The complex is susceptible to damage from environmental stresses such as excess excitation energy and high temperature. This thesis investigates the in vivo photosynthetic consequences of impairments to PSII in Arabidopsis thaliana expressing an antisense construct to the PsbO proteins of PSII. Transgenic lines were obtained with between 25 and 100% of wild type (WT) PsbO protein content per leaf area, and this decrease coincided with a proportional decrease in functional PSII content. Low PsbO (less than 50% WT) plants had reduced quantum yields, grew more slowly and had lower chlorophyll content per leaf area. On a PSII centre basis however, the chlorophyll and light harvesting content was increased. Fast fluorescence measurements of DCMU-treated leaves indicated that the majority of PSII centres in the low-PsbO transgenics exhibited slow phase kinetics. It is suggested that the low-PsbO plants had a smaller pool of functional PSII centres, which mostly consists of centres inefficient in excitation energy transfer from the antenna to the core. The antisense construct reduced RNA expression for both PsbO isoforms expressed in arabidopsis, but expression of other photosynthetic genes was largely unaffected. Likewise, there was little change detected in leaf proteins representative of the major photosynthetic complexes (cytochrome b{u2086}f, ATP synthase, Rubsico) on an area basis, with the exception of a moderate decrease in PSI content. Oxygen evolution measurements showed that, while the reduced quantum yield limits photosynthetic rate at low light, the low-PsbO plants matched the photosynthetic oxygen evolution rates of WT plants at saturating irradiance. At high irradiance the oxygen evolution rate was sustained by maintaining a greater proportion of 'open' PSII centres, and as such the estimated catalytic rates of oxygen production per PSII centre were not different between WT and low-PsbO plants. PSII per se appears not to be a significant rate-limiting step to maximum light-saturated photosynthetic rates in arabidopsis, however in nature light induced inactivation of PSII may reduce maximum photosynthetic rates by impeding the diffusion of the lipid-soluble electron carrier plastoquinone through the thylakoid membrane. Show more