Composition of photosystem II antenna in light-harvesting complex II antisense tobacco plants at varying irradiances.

Plants with genes coding for chlorophyll a/b-binding proteins of light-harvesting complex II (LHCII) in antisense orientation (Lhcb) that are characterized by severely reduced Lhcb transcript levels (below 10% of wild type) do not show a bleached phenotype due to a specific loss of the polypeptide. To produce such a phenotype, a conceptually different antisense approach was tested with a dual-functional transcript encoding the gene for hygromycin phosphotransferase and the transit sequence of Lhcb1-2 in the antisense orientation. Using increasing concentrations of hygromycin, transformants with Lhcb steady-state levels as low as 9% of wild type were regenerated and grown in a growth chamber. Together with Lhcb antisense plants obtained in an earlier study, these antisense plants were analyzed biochemically for their photosystem II (PSII) antenna composition under varying light conditions. All antisense plants showed a characteristic low-irradiance-induced increase of their PSII antenna size as determined by higher chlorophyll concentrations, an increased content of LHCII, and a constant chlorophyll b-to-lutein ratio in comparison with control plants. One to 5% of the total Lhcb transcript amount was sufficient to allow unrestricted formation of the PSII antenna at low irradiance, suggesting that LHCH biogenesis is not controlled primarily by transcription.