CHANGES IN THE COMPOSITION OF THE PHOTOSYNTHETIC APPARATUS IN THE GALACTOLIPID-DEFICIENT DGD1 MUTANT OF ARABIDOPSIS-THALIANA

The glycerolipid digalactosyl diacylglycerol (DGDG) is exclusively ass ociated with photosynthetic membranes and thus may play a role in the proper assembly and maintenance of the photosynthetic apparatus. Here we employ a genetic approach based on the dgd1 mutant of Arabidopsis t haliana to investigate the function of DGDC in thylakoid membranes. Th e primary defect in the genetically well-characterized dgd1 mutant res ulted in a 90% reduction of the DGDC content. The mutant showed a decr eased photosystem II (PSII) to photosystem I ratio. In vivo room-and l ow-temperature (77 K) chlorophyll fluorescence measurements with thyla koid preparations are in agreement with a drastically altered excitati on energy allocation to the reaction centers. Quantification of pigmen t-binding apoproteins and pigments supports an altered stoichiometry o f individual pigment-protein complexes in the mutant. Most strikingly, an increase in the amount of peripheral light-harvesting complexes of PSII relative to the inner antenna complexes and the PSII reaction ce nter/core complexes was observed. Regardless of the severe alterations in thylakoid organization, photosynthetic oxygen evolution was virtua lly not compromised in dgd1 mutant leaves.