Pseudomonas syringae pv.
tomato DC3000 (
Pst DC3000), which causes bacterial speck disease of tomato, has been used as a model pathogen to investigate the molecular basis of plant–pathogen interactions. The function of many potential virulence factors encoded in the
Pst DC3000 genome and their modes of action are not fully understood.
P. syringae is known to produce the exopolysaccharide alginate. Although AlgU, a sigma factor, is known to regulate the expression of genes such as
algD related to alginate biosynthesis, the molecular mechanisms of AlgU in the virulence of
Pst DC3000 is still unclear. To investigate the function of AlgU and alginate in plant–bacterial pathogen interactions, we generated Δ
algU and Δ
algD mutants. After inoculation with Δ
algU but not Δ
algD, host plants of
Pst DC3000 including tomato and
Arabidopsis had milder disease symptoms and reduced bacterial populations. Expression profiles of
Pst DC3000 genes revealed that AlgU can regulate not only the expression of genes encoding alginate biosynthesis, but also the expression of genes related to type III effectors and the phytotoxin coronatine (COR). We also demonstrated that the Δ
algU mutant showed full virulence in the
Arabidopsis fls2 efr1 double mutant, which is compromised in the recognition of PAMPs. Further, the application of COR was able to restore the phenotype of the Δ
algU mutant in the stomatal response. These results suggest that AlgU has an important role in the virulence of
Pst DC3000 by regulating COR production.
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