Summary Xanthomonas campestris pv. campestris (Xcc), the causal agent of black rot in crucifers, produces a membrane‐bound yellow pigment called xanthomonadin to protect against photobiological and peroxidative damage, and uses a quorum‐sensing mechanism mediated by the diffusible signal factor (DSF) family signals to regulate virulence factors production. The Xcc gene XCC4003, annotated as Xcc fabG3, is located in the pig cluster, which may be responsible for xanthomonadin synthesis. We report that fabG3 expression restored the growth of the Escherichia coli fabG temperature‐sensitive mutant CL104 under non‐permissive conditions. In vitro assays demonstrated that FabG3 catalyses the reduction of 3‐oxoacyl‐acyl carrier protein (ACP) intermediates in fatty acid synthetic reactions, although FabG3 had a lower activity than FabG1. Moreover, the fabG3 deletion did not affect growth or fatty acid composition. These results indicate that Xcc fabG3 encodes a 3‐oxoacyl‐ACP reductase, but is not essential for growth or fatty acid synthesis. However, the Xcc fabG3 knock‐out mutant abolished xanthomonadin production, which could be only restored by wild‐type fabG3, but not by other 3‐oxoacyl‐ACP reductase‐encoding genes, indicating that Xcc FabG3 is specifically involved in xanthomonadin biosynthesis. Additionally, our study also shows that the Xcc fabG3‐disrupted mutant affects Xcc virulence in host plants.

Most bacteria use type II fatty acid synthesis (FAS II) systems for synthesizing fatty acids, of which the conserved FabA-FabB pathway is considered to be crucial for unsaturated fatty acid (UFA) synthesis in gram-negative bacteria. Xanthomonas campestris pv. campestris (Xcc), the phytopathogen of black-rot disease in crucifers, produces higher quantities of UFAs under low temperature conditions for increasing membrane fluidity. The fabA and fabB genes were identified in the Xcc genome by BLAST analysis; however, the growth of the Xcc fabA and fabB deletion mutants was comparable to that of the wild-type strain in nutrient and minimal media. The Xcc ΔfabA and Xcc ΔfabB strains produced large quantities of UFAs, and altogether these results indicated that the FabA-FabB pathway is not essential for growth or UFA synthesis in Xcc. We also observed that the expression of Xcc fabA and Xcc fabB restored the growth of the temperature-sensitive Escherichia coli fabA and fabB mutants, CL104 and CY242, respectively, under non-permissive conditions. The in vitro assays demonstrated that the FabA and FabB proteins of Xcc catalyzed fatty acid synthesis. Our study also demonstrated that the production of diffusible signal factor (DSF) family signals that mediate quorum sensing (QS) was higher in the Xcc ΔfabA and Xcc ΔfabB strains and greatly reduced in the complementary strains, which exhibited reduced swimming motility and attenuated host plant pathogenicity.

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