Photoinhibition and loss of photosystem II reaction centre proteins during senescence of soybean leaves. Enhancement of photoinhibition by the 'stay-green' mutation cytG.

The 'stay-green' mutation cytG in soybean (Glycine max) partially inhibits the degradation of the light-harvesting complex II (LHCII) and the associated chlorophyll during monocarpic senescence. cytG did not alter the breakdown of the cytochrome b6/f complex, thylakoid ATP synthase or components of Photosystem I. In contrast, cytG accelerated the loss of oxygen evolution activity and PSII reaction-centre proteins. These data suggest that LHCII and other thylakoid components are degraded by separate pathways. In leaves induced to senesce by darkness, cytG inhibited the breakdown of LHCII and chlorophyll, but it did not enhance the loss of PSII-core components, indicating that the accelerated degradation of PSII reaction centre proteins in cytG was light dependent. Illumination of mature and senescent leaves of wild-type soybean in the presence of an inhibitor (lincomycin) of chloroplast protein synthesis revealed that senescence per se did not affect the rate of photoinhibition in leaves. Likewise, mature leaves of the cytG mutant did not show more photoinhibition than wild-type leaves. However, in senescent cytG leaves, photoinhibition proceeded more rapidly than in the wild-type. We conclude that the cytG mutation enhances photoinhibition in senescing leaves, and photoinhibition causes the rapid loss of PSII reaction-centre proteins during senescence in cytG.