Max-Planck-Institute of Molecular Plant Physiology, Potsdam-Golm 14476, Germany.
Glycobiology. 2013 Jan;23(1):69-80. doi: 10.1093/glycob/cws124. Epub 2012 Aug 24.
Phosphate deficiency is characteristic for many natural habitats, resulting in different physiological responses in plants and bacteria including the replacement of phospholipids by glycolipids and other phosphorous-free lipids. The plant pathogenic bacterium Agrobacterium tumefaciens, which is free of glycolipids under full nutrition, harbors an open reading frame (ORF) coding for a processive glycosyltransferase (named as Pgt). This glycosyltransferase was previously shown to synthesize glucosylgalactosyldiacylglycerol (GGD) and digalactosyldiacylglycerol (DGD) after heterologous expression. The native function of this enzyme and the conditions for its activation remained unknown. We show here that Pgt is active under phosphate deprivation synthesizing GGD and DGD in Agrobacterium. A corresponding deletion mutant (Δpgt) is free of these two glycolipids. Glycolipid accumulation is mainly regulated by substrate (diacylglycerol) availability. Diacylglycerol and the total fatty acid pool are characterized by an altered acyl composition in dependence of the phosphate status with a strong decrease of 18:1 and concomitant increase of 19:0 cyclo during phosphate deprivation. Furthermore, Agrobacterium accumulates two additional unknown glycolipids and diacylglycerol trimethylhomoserine (DGTS) during phosphate deprivation. Accumulation of all these lipids is accompanied by a reduction in phospholipids from 75 to 45% in the wild type. A further non-phosphorous lipid, ornithine lipid, was not increased but its degree of hydroxylation was elevated under phosphate deprivation. The lack of GGD and DGD in the Δpgt mutant has no effect on growth and virulence of Agrobacterium, suggesting that these two lipids are functionally replaced by DGTS and the two unknown glycolipids under phosphate deprivation.
磷酸盐缺乏是许多自然栖息地的特征，导致植物和细菌产生不同的生理反应，包括用糖脂和其他无磷脂质替代磷脂。植物病原细菌根癌农杆菌在充分营养条件下不含糖脂，它含有一个开放阅读框（ORF），编码一个连续糖基转移酶（命名为 Pgt）。该糖基转移酶先前在异源表达后被证明能合成葡萄糖基半乳糖基二酰基甘油（GGD）和双半乳糖基二酰基甘油（DGD）。该酶的天然功能及其激活条件尚不清楚。我们在这里表明，Pgt 在磷酸盐缺乏时是活跃的，能在根癌农杆菌中合成 GGD 和 DGD。相应的缺失突变体（Δpgt）不含这两种糖脂。糖脂的积累主要受底物（二酰基甘油）可用性的调节。二酰基甘油和总脂肪酸库的酰基组成在磷酸盐状态的依赖性方面发生改变，在磷酸盐缺乏时 18:1 的含量强烈减少，同时 19:0 环增加。此外，根癌农杆菌在磷酸盐缺乏时还积累两种未知的糖脂和二酰基甘油三甲羟肟酸（DGTS）。所有这些脂质的积累伴随着磷脂从野生型的 75%减少到 45%。在磷酸盐缺乏时，还有另外一种非磷脂质鸟氨酸脂质没有增加，但它的羟化程度升高。Δpgt 突变体中缺乏 GGD 和 DGD 对根癌农杆菌的生长和毒力没有影响，这表明在磷酸盐缺乏时，这两种脂质被 DGTS 和两种未知的糖脂替代。
