The Use of Lipid Headgroup Mutants to Explore the Function of Thylakoid Lipids in Photosynthesis

Abstract

A large number of different mutants of the small cruciferous plant Arabidopsis thaliana with an altered fatty acid composition of polar membrane lipids has become available during the course of the past 10 years [1]. These have proven to be extremely useful for testing different hypotheses on the role of individual fatty acid constituents of thylakoid lipids with regard to the assembly and function of the photosynthetic apparatus [2]. Genetic procedures such as repeated back-crossing or genetic complementation of biochemical mutants using the respective gene have been well established and ensure that the observed physiological effects on the photosynthetic apparatus or photosynthetic performance are a direct or indirect consequence of the biochemical defect in lipid metabolism. More recently, lipid head group mutants of A. thaliana with alterations in the composition of complex membrane lipids have been isolated or identified in our laboratory. These include the dgdl mutant lacking 90% of the galactolipid digalactosyl diacylglycerol (DGDG) [3], the pho 1 mutant with a decrease in phospholipids and an increase in the amounts of the sulfolipid sulfoquinovosyl diacylglycerol (SQDG) and DGDG [4], and the phg 1 mutant with a specific decrease in the phospholipid phosphatidylglycerol (PG; Hartel and Benning, unpublished). The lipid profiles of these mutants are compared in Figure 1. The changes in lipid composition of the pho 1 mutant are consistent with the proposal of a substitution of plastidic PG with SQDG and to some extent with DGDG in the thylakoid membrane under phosphate starvation [5]. On the contrary, the phg 1 mutant is not phosphate depleted and PG is decreased specifically, suggesting a defect in PG biosynthesis. Thus far, the best studied of the three is the dgd 1 mutant. It has been genetically characterized down to the level of the gene (Dormann and Benning, unpublished) leading to the construction of nearly isogenic lines. Therefore, the observed structural changes in the photosynthetic apparatus [6], functional alterations in the water-oxidizing complex [7], and altered light utilization [8] can be traced back directly to the DGDG deficiency in the mutant, permitting important conclusions with regard to the function of DGDG in the photosynthetic membrane. Here we provide, as an example for the general approach, data derived from the analysis of the dgd 1 mutant, which illustrate the use of lipid head group mutants for the analysis of thylakoid lipid function.