Beck Sebastian, Marlow Victoria L, Woodall Katy, Doerrler William T, James Euan K, Ferguson Gail P
Institute of Cell Biology and Centre for Science at Extreme Conditions, School of Biological Sciences, King's Buildings, University of Edinburgh, Edinburgh EH9 3JR, UK.
School of Medicine, Department of Medicine and Therapeutics, Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, UK.
Microbiology (Reading). 2008 Apr;154(Pt 4):1258-1270. doi: 10.1099/mic.0.2007/014894-0.
Sinorhizobium meliloti is a beneficial legume symbiont, closely related to Brucella species, which are chronic mammalian pathogens. We discovered that the S. meliloti MsbA2 protein is essential to ensure the symbiotic interaction with the host plant, alfalfa. S. meliloti invades plant cells via plant-derived structures known as infection threads. However, in the absence of MsbA2, S. meliloti remains trapped within abnormally thickened infection threads and induces a heightened plant defence response, characterized by a substantial thickening of the nodule endodermis layer and the accumulation of polyphenolic compounds. The S. meliloti MsbA2 protein is homologous to the Escherichia coli lipopolysaccharide/phospholipid trafficking protein MsbA. However, MsbA2 was not essential for the membrane transport of either lipopolysaccharide or phospholipids in S. meliloti. We determined that the msbA2 gene is transcribed in free-living S. meliloti and that in the absence of MsbA2 the polysaccharide content of S. meliloti is altered. Consequently, we propose a model whereby the altered polysaccharide content of the S. meliloti msbA2 mutant could be responsible for its symbiotic defect by inducing an inappropriate host response.
苜蓿中华根瘤菌是一种有益的豆科共生菌，与作为慢性哺乳动物病原体的布鲁氏菌属密切相关。我们发现苜蓿中华根瘤菌的MsbA2蛋白对于确保与宿主植物苜蓿的共生相互作用至关重要。苜蓿中华根瘤菌通过植物来源的称为感染丝的结构侵入植物细胞。然而，在缺乏MsbA2的情况下，苜蓿中华根瘤菌仍被困在异常增厚的感染丝内，并引发增强的植物防御反应，其特征是根瘤内皮层显著增厚和多酚类化合物的积累。苜蓿中华根瘤菌的MsbA2蛋白与大肠杆菌脂多糖/磷脂转运蛋白MsbA同源。然而，MsbA2对于苜蓿中华根瘤菌中脂多糖或磷脂的膜转运并非必不可少。我们确定msbA2基因在自由生活的苜蓿中华根瘤菌中被转录，并且在缺乏MsbA2的情况下苜蓿中华根瘤菌的多糖含量会发生改变。因此，我们提出了一个模型，即苜蓿中华根瘤菌msbA2突变体改变的多糖含量可能通过诱导不适当的宿主反应而导致其共生缺陷。
