Phosphorus-free membrane lipids of Sinorhizobium meliloti are not required for the symbiosis with alfalfa but contribute to increased cell yields under phosphorus-limiting conditions of growth.
The microsymbiont of alfalfa, Sinorhizobium meliloti, possesses phosphatidylglycerol, cardiolipin, phosphatidylethanolamine, and phosphatidylcholine as major membrane phospholipids, when grown in the presence of sufficient accessible phosphorus sources. Under phosphate-limiting conditions of growth, S. meliloti replaces its phospholipids by membrane lipids that do not contain any phosphorus in their molecular structure and, in S. meliloti, these phosphorus-free membrane lipids are sulphoquinovosyl diacylglycerols (SL), ornithine-containing lipids (OL), and diacylglyceryl-N,N,N-trimethylhomoserines (DGTS). In earlier work, we demonstrated that neither SL nor OL are required for establishing a nitrogen-fixing root nodule symbiosis with alfalfa. We now report the identification of the two structural genes btaA and btaB from S. meliloti required for DGTS biosynthesis. When the sinorhizobial btaA and btaB genes are expressed in Escherichia coli, they cause the formation of DGTS in this latter organism. A btaA-deficient mutant of S. meliloti is unable to form DGTS but can form nitrogen-fixing root nodules on alfalfa, demonstrating that sinorhizobial DGTS is not required for establishing a successful symbiosis with the host plant. Even a triple mutant of S. meliloti, unable to form any of the phosphorus-free membrane lipids SL, OL, or DGTS is equally competitive for nodule occupancy as the wild type. Only under growth-limiting concentrations of phosphate in culture media did mutants that could form neither OL nor DGTS grow to lesser cell densities.
López-Lara
Isabel M
IM
Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Apdo. Postal 565-A, Cuernavaca, Morelos, CP62210, Mexico.
Gao
Jun-Lian
JL
Soto
María José
MJ
Solares-Pérez
Alhondra
A
Weissenmayer
Barbara
B
Sohlenkamp
Christian
C
Verroios
George P
GP
Thomas-Oates
Jane
J
Geiger
Otto
O
D001483:Base Sequence; D003470:Culture Media; D004269:DNA, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D005798:Genes, Bacterial; D000455:Medicago sativa / Q000378:metabolism / Q000382:microbiology*; D008563:Membrane Lipids / Q000737:chemistry / Q000378:metabolism*; D009586:Nitrogen Fixation; D010758:Phosphorus / Q000378:metabolism*; D018517:Plant Roots / Q000378:metabolism / Q000382:microbiology; D016962:Sinorhizobium meliloti / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism*; D013559:Symbiosis / Q000502:physiology
D003470:Culture Media; D004269:DNA, Bacterial; D008563:Membrane Lipids; D010758:Phosphorus