Membrane lipids in plant-associated bacteria: their biosyntheses and possible functions

Membrane lipids in most bacteria generally consist of the glycerophospholipids phosphatidylglycerol, cardiolipin, and phosphatidylethanolamine (PE). A subset of bacteria also possesses the methylated derivatives of PE, monomethylphosphatidylethanolamine, dimethylphosphatidylethanolamine, and phosphatidylcholine (PC). In Sinorhizobium meliloti, which can form a nitrogen-fixing root nodule symbiosis with Medicago spp., PC can be formed by two entirely different biosynthetic pathways, either the PE methylation pathway or the recently discovered PC synthase pathway. In the latter pathway, one of the building blocks for PC formation, choline, is obtained from the eukaryotic host. Under phosphorus-limiting conditions of growth, S. meliloti replaces its membrane phospholipids by membrane-forming lipids that do not contain phosphorus; namely, the sulfolipid sulfoquinovosyl diacylglycerol, ornithine-derived lipids, and diacylglyceryl-N,N,N-trimethylhomoserine. Although none of these phosphorus-free lipids is essential for growth in culture media rich in phosphorus or for the symbiotic interaction with the legume host, they are expected to have major roles under free-living conditions in environments poor in accessible phosphorus. In contrast, sinorhizobial mutants deficient in PC show severe growth defects and are completely unable to form nodules on their host plants. Even bradyrhizobial mutants with reduced PC biosynthesis can form only root nodules displaying reduced rates of nitrogen fixation. Therefore, in the cases of these microsymbionts, the ability to form sufficient bacterial PC is crucial for a successful interplay with their host plants.