Loss of alpha-tocopherol in tobacco plants with decreased geranylgeranyl reductase activity does not modify photosynthesis in optimal growth conditions but increases sensitivity to high-light stress

The enzyme geranylgeranyl reductase (CHL P) catalyses the reduction of gera nylgeranyl diphosphate to phytyl diphosphate in higher-plant chloroplasts a nd provides phytol for both chlorophyll (Chl) and tocopherol synthesis. The reduction in CHL P activity in transgenic tobacco (Nicotiana tabacum L.) p lants is accompanied by the reduction in total Chl and tocopherol content a nd the accumulation of geranylgeranylated Chl (Chl(GG)). The photosynthetic performance and the susceptibility to photo-oxidative stress have been inv estigated in these transgenic plants. The reduced total Chl content in Chl P antisense plants resulted in the reduction of electron transport chains p er leaf area without a concomitant effect on the stoichiometry, composition and activity of both photosystems. However, Chl P antisense plants were mu ch more sensitive to light stress. Analyses of Chl fluorescence quenching i ndicated an increased photoinhibitory quenching at the expense of the pH-de pendent fluorescence quenching after short illumination (15 min) at moderat e light intensities. Prolonged illumination (up to I h) at saturating light intensities induced an increased photoinactivation from which the Chl P an tisense plants could not recover or could only partially recover during a s ubsequent low light phase. Our data imply that the presence of Chl(GG) has no influence on harvesting and transfer of light energy in either photosyst em. However, the reduced tocopherol content of the thylakoid membrane is a limiting factor for defensive reactions to photo-oxidative stress.