The phytopathogen Xanthomonas campestris scavenges hydroxycinnamic acids in planta via the hca cluster to increase virulence on its host plant

Xanthomonas campestris pv. campestris ( Xcc ) is the causal agent of black rot of cruciferous plants, which harbor high levels of hydroxycinnamic acids (HCAs) in their above-ground parts. Thus, upon infection of the host plant, the pathogen experiences a complex cocktail of HCAs. The present study shows that Xcc can efficiently degrade the HCAs, 4-hydroxycinnamic acid (4-HCA), ferulic acid (FA) and sinapic acid (SiA), via an hca cluster which encodes putative genes for a 4-hydroxycinnamoyl-CoA synthetase/4-HCA ligase HcaL, a benzaldehyde dehydrogenase HcaD, a 4-hydroxycinnamoyl-CoA hydratase/lyase HcaH and a member of the MarR-family of transcriptional factors, HcaR. Xcc also degrades the HCA caffeic acid, but with an alternative mechanism. RT-PCR and subsequent GUS assays show that the hca cluster is transcribed within a single operon, and its transcription is specifically induced by 4-HCA, FA and SiA. Furthermore, we show that HcaR negatively regulates hca transcription when its ligand, the proposed degradation pathway intermediate HCA-CoA, is not present. HcaR specifically binds to a 25-bp site, which encompasses the -10 elements of the hca promoter. Finally, GUS histochemical staining and subsequent quantitative analysis shows that the hca cluster is transcribed in planta during pathogenesis of Chinese radish, and hca deletion mutant strains exhibit compromised virulence in cabbage. Together, these results suggest that the ability to degrade HCAs contributes to Xcc virulence by facilitating its growth and spread, and by protecting the pathogen from HCA toxicity. A working model to explain Xcc HCA sensing and subsequent induction of the HCA degradation process is proposed.