Phosphatidylcholine levels in Bradyrhizobium japonicum membranes are critical for an efficient symbiosis with the soybean host plant

Phosphatidycholine (PC), the major membrane phospholipid in eukaryotes, is found in only some bacteria including members of the family Rhizobiaceae. F or this reason, it has long been speculated that rhizobial PC might be requ ired for a successful interaction of rhizobia with their legume host plants in order to allow the formation of nitrogen-fixing root nodules. A major p athway for PC formation in prokaryotes involves a threefold methylation of the precursor phosphatidylethanolamine (PE). Here, we report on the isolati on of a Bradyrhizobium japonicum gene (pmtA) encoding the phospholipid N-me thyltransferase PmtA. Upon expression of the bradyrhizobial pmtA gene in Es cherichia coli, predominantly monomethylphosphatidylethanolamine was formed from PE. PmtA-deficient B. japonicum mutants still produced low levels of PC by a second methylation pathway. The amount of PC formed in such mutants (6% of total phospholipids) was greatly decreased compared with the wild t ype (52% of total phospholipids). Root nodules of soybean plants infected w ith B. japonicum pmtA mutants showed a nitrogen fixation activity of only 1 8% of the wild-type level. The interior colour of the nodules was beige ins tead of red, suggesting decreased amounts of leghaemoglobin. Moreover, ultr astructure analysis of these nodules demonstrated a greatly reduced number of bacteroids within infected plant cells. These data suggest that the bios ynthesis of wild-type amounts of PC are required to allow for an efficient symbiotic interaction of B. japonicum with its soybean host plant.