A Sucrose‐Utilisation Gene Cluster Contributes to Colonisation of Horse Chestnut by <i>Pseudomonas syringae</i> pv. <i>aesculi</i>

Pseudomonas syringae pathovar aesculi (E-Pae) causes bleeding canker disease in the woody tissue of European horse chestnut (HC). Comparative genomic analysis of E-Pae with a related leaf-infecting strain (I-Pae) and other P. syringae strains identified candidate virulence genes for colonisation of woody tissue, including a sucrose uptake and utilisation system (scrYABCDBR cluster) found in 162 of 206 P. syringae strains spanning the pangenome. Growth analysis using sucrose as sole carbon source showed that I-Pae (lacking the gene cluster) was unable to grow whereas E-Pae could grow. P. savastanoi pv. phaseolicola 1448A and P. syringae pv. morsprunorum R15244 were compromised in growth despite the presence of the gene cluster. Sucrose utilisation assays using scrB and scrY mutants and complemented strains confirmed the importance of the cluster for sucrose metabolism in vitro. Pathogenicity assays in HC revealed the sucrose gene cluster is important for symptom development in the woody tissue. While the scr genes contribute to disease causation, they were not essential for pathogen fitness when compared to hrpL and hopAB1 mutants. E-Pae caused disease symptoms in HC leaves, suggesting the strain has the potential to infect leaves as well. However, it was notable that the scrB mutant of E-Pae caused increased disease symptoms, possibly highlighting a niche adaptation strategy for I-Pae to cause leaf spots in HC as well as constraining E-Pae to predominantly infect the woody tissue.