Membrane lipids in Agrobacterium tumefaciens: biosynthetic pathways and importance for pathogenesis

Abstract

Many cellular processes critically depend on the membrane composition. In this review, we focus on the biosynthesis and physiological roles of membrane lipids in the plant pathogen Agrobacterium tumefaciens. The major components of A. tumefaciens membranes are the phospholipids (PLs), phosphatidylethanolamine (PE), phosphatidylglycerol, phosphatidylcholine (PC) and cardiolipin, and ornithine lipids (OLs). Under phosphate-limited conditions, the membrane composition shifts to phosphate-free lipids like glycolipids, OLs and a betaine lipid. Remarkably, PC and OLs have opposing effects on virulence of A. tumefaciens. OL-lacking A. tumefaciens mutants form tumors on the host plant earlier than the wild type suggesting a reduced host defense response in the absence of OLs. In contrast, A. tumefaciens is compromised in tumor formation in the absence of PC. In general, PC is a rare component of bacterial membranes but amount to ~22% of all PLs in A. tumefaciens. PC biosynthesis occurs via two pathways. The phospholipid N-methyltransferase PmtA methylates PE via the intermediates monomethyl-PE and dimethyl-PE to PC. In the second pathway, the membrane-integral enzyme PC synthase (Pcs) condenses choline with CDP-diacylglycerol to PC. Apart from the virulence defect, PC-deficient A. tumefaciens pmtA and pcs double mutants show reduced motility, enhanced biofilm formation and increased sensitivity towards detergent and thermal stress. In summary, there is cumulative evidence that the membrane lipid composition of A. tumefaciens is critical for agrobacterial physiology and tumor formation.

Keywords: Agrobacterium tumefaciens; betaine lipids; glycolipids; membrane lipids; ornithine lipids; phosphatidylcholine; phospholipid biosynthesis; phosphorus-free lipids.

FIGURE 1

General structure of phospholipids and common head groups. PLs contain two fatty acids ester-linked to glycerol at C-1 and C-2, and a polar head group attached at C-3 via a phosphodiester bond. The fatty acids in PLs can vary in carbon group length and saturation degree. The different common polar head groups and charges are indicated. PA, phosphatidic acid; PE, phosphatidylethanolamine; PC, phosphatidylcholine; PS, phosphatidylserine; PG, phosphatidylglycerol; CL, cardiolipin; PI, phosphatidylinositol.

FIGURE 2

Phospholipid pathways and enzymes in bacteria. CDP-DAG is the central precursor for synthesis of the PLs. Thick arrows and boldface letters indicate the most common pathways and enzymes in bacteria. For details see text. CMP, cytidine monophosphate; CTP, cytidine triphosphate; EA, ethanolamine; cho, choline; G3P, glycerol 3-phosphate; gly, glycerol; lys, lysine; L-ser, L-serine; myo-I-P, myo-inositol 1-phosphate; SAM, S-adenosylmethionine; SAH, S-adenosylhomocysteine.

FIGURE 3

Phospholipid synthesis in A. tumefaciens. Characterized pathways are indicated by thick arrows and enzymes in boldface letters. Dashed arrows indicate the putative pathways. For details see text.

FIGURE 4

Phosphatidylcholine biosynthesis in A. tumefaciens. In the PC synthase pathway, the integral membrane protein Pcs condenses CDP-DAG and choline to PC. Choline is taken up via the ChoXWV transporter. In the PmtA pathway, a single peripheral phospholipid N-methyltransferase (PmtA) converts PE via three successive methylations to PC. PmtA is stimulated by the anionic lipid PG and inhibited by PC and SAH. CM, cytoplasmic membrane.

FIGURE 5

Structure of the ornithine lipids OLS1/OLS2 and diacylglycerol trimethylhomoserine (DGTS) in A. tumefaciens. The OLs contain C16 3OH and C19:0 cyclo fatty acids. OLS2 is hydroxylated within the ornithine moiety. DGTS contains 18:1 and 19:0 cyclo fatty acids (Geske et al., 2013; Vences-Guzmán et al., 2013).

FIGURE 6

Phosphorus-free lipid synthesis pathways in A. tumefaciens. (A) Characterized pathways are indicated by thick arrows and enzymes in boldface letters. Putative biosynthetic pathways/enzymes are indicated with dashed arrows (Geske et al., 2013; ). For details see text. (B) Relative proportion of phosphorus-free lipids in A. tumefaciens C58C1 under phosphate-replete (+P) and phosphate depleted (-P) conditions (Geske et al., 2013). Gal, galactose; Glu, glucose; Glca, glucoronic acid.

FIGURE 7

Model of PC- or OL-dependent effects on A. tumefaciens infection efficiency. OL-lacking A. tumefaciens induces accelerated tumor formation on potato disks compared to wild type (WT) probably due to reduced plant defense (Vences-Guzmán et al., 2013). A PC-deficient mutant is unable to elicit tumors on Kalanchoë leaves since VirA/G controlled vir gene expression is impaired. As a consequence, the type4 secretion system (T4SS) is not produced which is essential for tumor formation (Wessel et al., 2006; Klüsener et al., 2010).