LAUSR

The fragility of photosystem I (PSI) was observed in transgenic rice plants overproducing Rubisco activase (RCA) (RCA-ox). This study examined the effects of RCA overproduction on PSI photoinhibition sensitivity in three lines of RCA-ox plants. The quantum yield of PSI [Y(I)] decreased, and the quantum yield of acceptor-side limitation of PSI [Y(NA)] conversely increased in all lines of RCA-ox plants, especially under low light conditions. In RCA-ox plants with the highest RCA content (RCA-ox 1), the quantum yield of PSII [Y(II)] and CO2 assimilation also decreased under low light. When leaves were exposed to high light (2,000 μmol photon m -2 s -1) for 60 min, the maximal activity of PSI (Pm) drastically decreased in RCA-ox 1. These results suggested that RCA overproduction disturbs PSI electron transport control, increasing PSI photoinhibition susceptibility. When flavodiiron protein (FLV), which functions as a large electron sink downstream of PSI, was introduced into RCA-ox 1 (RCA-FLV), PSI, PSII parameters, and CO2 assimilation in RCA-FLV plants were recovered to wild-type plant levels. Thus, FLV introduction restored PSI robustness in RCA-ox plants.