The activities of natural amino acids to induce Xanthomonas campestris to degrade DSF-family quorum sensing signals

Abstract:

[Background] Xanthomonas campestris pv. campestris (Xcc) is a causal agent of black rot in cruciferous plants. The diffusible signal factor (DSF)-dependent quorum sensing (QS) system and RpfB-dependent QS exit mechanism are closely associated with Xcc pathogenicity. [Objective] We investigated the effects of 18 amino acids on the biosynthesis of DSF-family QS signals, to provide a clue for developing new strategy to control plant diseases. [Methods] The Xcc mutant strain ΔrpfC was grown in the XYS medium supplemented with 18 amino acids, respectively. The crude DSF extracts after 24 h to 36 h inoculation were then analyzed by high performance liquid chromatography (HPLC). [Results] 1) Among the 18 amino acids tested, methionine, tryptophan and cystine were found to significantly decrease DSF and BDSF levels in the ΔrpfC culture. The inhibitory effect was closely associated with amino acid concentration; 2) Additive inhibitory effects between three amino acids were observed; 3) Addition of methionine, tryptophan or cystine had no significant effect on the levels of DSF and BDSF in the ΔrpfCΔrpfB culture. [Conclusion] Methionine, tryptophan and cystine probably induce Xcc to exit from quorum sensing stage via the RpfB-dependent pathway.