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
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
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DOI：
10.1094/mpmi-18-0973
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发表时间：
2005-09-01
影响因子：
3.5
通讯作者：
Geiger, O
中科院分区：
文献类型：
--
作者：
López-Lara, IM;Gao, JL;Geiger, O
紫花苜蓿微共生菌在充足的磷源存在下生长时，以磷脂酰甘油、心磷脂、磷脂酰乙醇胺和磷脂酰胆碱为主要膜磷脂。在限制磷的生长条件下，苜蓿用分子结构中不含磷的膜脂取代其磷脂，在苜蓿中，这些无磷的膜脂是磺基喹诺酮二酰甘油(SL)、含鸟氨酸的脂(OL)和二酰甘油-N，N，N-三甲基高丝氨酸(DGTS)。在早期的工作中，我们证明了SL和OL都不是与紫花苜蓿建立固氮根瘤共生所必需的。我们现在报道从苜蓿链霉菌中鉴定出DGTS生物合成所需的两个结构基因btaA和btaB。当中华根瘤菌的btaA和btaB基因在大肠杆菌中表达时，它们会导致在后者的生物体中形成DGTS。缺乏btaA的紫花苜蓿突变体不能形成DGTS，但能在紫花苜蓿上形成固氮根瘤，这表明与寄主植物建立成功的共生关系不需要中生菌DGTS。即使是不能形成任何无磷膜脂SL、OL或DGTS的紫花苜蓿的三重突变体，也与野生型一样具有竞争根瘤的能力。只有在培养基磷酸盐的生长限制浓度下，既不能形成OL也不能形成DGTS的突变株才能生长到较低的细胞密度。
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 diacyl-glycerols (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.