MODIFICATION OF THE WATER OXIDIZING COMPLEX IN LEAVES OF THE DGD1 MUTANT OF ARABIDOPSIS-THALIANA DEFICIENT IN THE GALACTOLIPID DIGALACTOSYLDIACYLGLYCEROL

The primary biochemical defect in the genetically well characterized d gdl mutant of Arabidopsis thaliana causes a 90% reduction in the relat ive amount of the galactolipid digalactosyldiacylglycerol (DGDG). To s tudy the effect of this DGDG deficiency on photosystem II (PS II), tim e-resolved transients of laser-flash-induced changes of the relative f luorescence quantum yield F-var,F-rel(t) were measured in whole leaves from wild-type and the dgdl mutant. The results obtained reveal (i) i n untreated leaves the decay kinetics of F-var,F-rel(t) reflecting Q(A )(.-) reoxidation by endogenous plastoquinone are very similar in wild -type and the dgdl mutant at room temperature, (ii) the Arrhenius plot of the temperature dependence of electron transfer from Q(A)(.-) to Q (B) exhibits a break point at about 19 degrees C in wild-type and abou t 12 degrees C in the dgdl mutant, (iii) in leaves treated with DCMU t he slow reoxidation of Q(A)(.-) by the PS II donor side is blocked to a much higher extent in the dgdl mutant (about 50%) compared to wild-t ype (about 10%), and iv) the normalized amplitude of F-var,F-rel(t = 1 mu s) reflecting the percentage of fast P680(.+) reduction by Y-Z exh ibits a characteristic period four oscillation in wild-type while this feature is strongly damped in the dgdl mutant, Presumably, the severe DGDG deficiency is causing the thermal down shift of a lipid phase tr ansition that affects the Q(A)(.-) reoxidation by Q(B). Most strikingl y, the properties of the WOC are modified as a result of reduced DGDG content. Thus, the lipid DGDG appears to be of structural relevance fo r the WOC.