Quorum Sensing Regulation in Phytopathogenic Bacteria

Abstract

Quorum sensing is a type of chemical communication by which bacterial populations control expression of their genes in a coordinated manner. This regulatory mechanism is commonly used by pathogens to control the expression of genes encoding virulence factors and that of genes involved in the bacterial adaptation to variations in environmental conditions. In phytopathogenic bacteria, several mechanisms of quorum sensing have been characterized. In this review, we describe the different quorum sensing systems present in phytopathogenic bacteria, such as those using the signal molecules named N-acyl-homoserine lactone (AHL), diffusible signal factor (DSF), and the unknown signal molecule of the virulence factor modulating (VFM) system. We focus on studies performed on phytopathogenic bacteria of major importance, including Pseudomonas, Ralstonia, Agrobacterium, Xanthomonas, Erwinia, Xylella,Dickeya, and Pectobacterium spp. For each system, we present the mechanism of regulation, the functions targeted by the quorum sensing system, and the mechanisms by which quorum sensing is regulated.

Keywords: Agrobacterium tumefaciens; Dickeya spp.; Erwinia amylovora; N-acyl-homoserine lactone; Pectobacterium spp.; Pseudomonas syringae; Ralstonia solanacearum; Xanthomonas spp.; Xylella spp.; diffusible signal factor.

Figure 1

General structure of the N-acyl-homoserine lactone (AHL). A fatty acyl chain is linked to an homoserine lactone ring through an amide bond at the α position. The acyl chain ranges from 4 to 18 carbons in length and varies in its degree of saturation, oxidation, or substitution at the third carbon of the chain. Turnover of the QS mechanisms involves the degradation of AHL using lactonases in Agrobacterium or AHL acylase in P. syringae and R. solanacearum.

Figure 2

The N-acyl-homoserine lactone-mediated quorum sensing system. A LuxI-like synthetase forms an amide bond between a fatty acyl chain and an S-adenosylmethionine (SAM) to produce AHLs. Acyl–Acyl carrier protein (ACP). At a low cell density, AHLs are diluted in growth medium, whereas at a high cell density, AHLs accumulate and reach a threshold. Signal molecules diffuse across the cell envelope and bind to the LuxR-like regulator. Then, the LuxR–AHL complex regulates the expression of target genes such as luxI. Genes encoding the LuxI and LuxR protein families have different names depending on the strain and the species, with some variations in their functions.

Figure 3

General structure of the Diffusible Signal Factor (DSF). Signal molecules are cis-2-unsaturated fatty acids. Fatty acid carbon chains vary in their lengths, double-bond configurations, and side-chain modifications, particularly methylation. Fatty acid carbon chains range from 8 to 14 carbons. A given species is able to produce different molecules. Methylation occurs at the first carbon for R. solanacearum signal molecules 3-OH-PAME or 3-OH-MAME.

Figure 4

The Diffusible Signal Factor-mediated quorum-sensing (DSF-QS) system. In phytopathogenic bacteria, the DSF system is encoded by the rpf gene cluster. RpfF is a bifunctional enzyme involved in the production of DSF molecules. RpfB is proposed to be involved in DSF turnover. RpfC–RpfG is a two-component regulatory system that is involved in signal perception and transduction. RpfC is a DSF sensor that uses a phospho-relay mechanism to transfer the signal to the response regulator, RpfG. The N-terminal RR response domain of RpfG interacts directly with RpfC. Its HD-GYP domain then degrades cyclic di-GMP. RpfC can also bind to RpfF using its C-terminal REC domain and negatively regulates DSF biosynthesis. At a low cell density, (i) RpfC forms a complex with RpfF, blocking its enzymatic activity and inhibiting DSF signal biosynthesis, and (ii) cyclic di-GMP binds to the global transcription factor Clp, which represses rpfB expression. At a high cell density, RpfF is released and produces DSF signals, which allow the induction of QS regulation. Cyclic di-GMP is degraded by the HD-GYP domain of RpfG, and rpfB is expressed, like several genes encoding virulence factors activated by Clp.

Figure 5

The VFM quorum-sensing (QS) system. (A) The vfm locus is composed of 26 genes. Genes vfmY-vfmK-W and vfmAZBCD encode proteins involved in the biosynthesis of the signal molecule, while vfmFG encodes an ABC transporter involved in the signal molecule export. VfmIH is a two-component system, where the sensor VfmI perceives the signal molecule (shown in red), and the regulator VfmH induces the expression of vfmE, which encodes a transcriptional activator of virulence factors and the vfm locus. In D. zeae, vfmE was shown to be regulated by Fis; (B) Genetic organization of vfmE and vfmAZBCD. In strains from D. chrysanthemi, D. aquatica, and D. paradisiaca, the coding sequences of vfmD and vfmE overlap. Genetic organization is different in D. poaceiphila.