Galactolipids not associated with the photosynthetic apparatus in phosphate-deprived plants

The galactolipid digalactosyldiacylglycerol (DGGD) is one of the major constituents of thylakoids, accounting for about 25% of polar lipids found in these membranes. Although the presence of DGDG has frequently been correlated with the structural and functional integrity of the photosynthetic apparatus, it is still a matter of debate of what the in-vivo function of DGDG actually might be. To further the understanding of the role of DGDG within the photosynthetic apparatus, experiments were conducted on different Arabidopsis thaliana lines with altered DGDG content. The dgd1 mutant is characterized by a 90% reduction in the DGDG content, resulting in a severe dwarfism during growth. Complementation of the dgd1 mutant with a DGD1 cDNA completely restored the wild-type characteristics, while photosynthesis-related parameters were intermediate in transgenic plants with a partial reduction in DGD1 activity caused by post-transcription gene silencing due to over-expression of a DGD1 cDNA in wild-type plants. These data provide clear evidence for a causal relationship between the DGDG content, and the structure and function of the photosynthetic apparatus. However, a significant DGDG accumulation in the dgd1/pho1 double mutant was without any detectable effect on photosynthetic activity, indicating that the molecular DGDG species synthesized upon phosphate deprivation in leaves cannot substitute for the DGDG species present under normal nutrient supply of plants. It is suggested that depending on the environmental growth conditions different pools of DGDG species exist in plants of which one is not associated with the photosynthetic apparatus.