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AlgU contributes to the virulence of Pseudomonas syringae pv. tomato DC3000 by regulating production of the phytotoxin coronatine

  • Bacterial and Phytoplasma Diseases
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Abstract

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 ΔalgU and ΔalgD mutants. After inoculation with ΔalgU but not ΔalgD, 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 ΔalgU 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 ΔalgU 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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Acknowledgements

We thank Dr. Christina Baker for editing the manuscript. This work was supported, in part, by JST ERATO NOMURA Microbial Community Control Project, JST, Japan.

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  1. Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8572, Ibaraki, Japan

    Takako Ishiga, Yasuhiro Ishiga, Shigeyuki Betsuyaku & Nobuhiko Nomura

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  2. Yasuhiro Ishiga
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Correspondence to Yasuhiro Ishiga.

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Figure S1

. Growth curves for Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) strains including the wild-type (WT), the ∆algD mutant (∆algD), and the ∆algU (∆algU) mutant. Pst DC3000 strains were grown at 28°C for 24 h in a. King’s B (KB) or b. mannitol–glutamate (MG) broth. Strains were adjusted to OD600 of 0.1 with the respective medium, and OD600 measured after 24 h. Figure S2. Expression profiles of defense genes in 2-week-old plants of Arabidopsis after inoculation with 5 × 106 colony forming units (CFU)/ml of wild-type (WT) Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) or the ΔalgU mutant (ΔalgU). Total RNA was isolated at 6, 12, 24, and 48 h post inoculation (hpi). Expression of a. AtPR1 and b. AtPR2 was determined using real-time quantitative reverse transcription-polymerase chain reaction (RT-qPCR) with gene-specific primer sets. Expression was normalized using AtUBQ1 (Supplementary Table S1). Vertical bars indicate the standard error for three biological replicates. Asterisks indicate a significant difference from WT in a t test (*P < 0.05; **P < 0.01). (PPTX 83 KB)

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Ishiga, T., Ishiga, Y., Betsuyaku, S. et al. AlgU contributes to the virulence of Pseudomonas syringae pv. tomato DC3000 by regulating production of the phytotoxin coronatine. J Gen Plant Pathol 84, 189–201 (2018). https://doi.org/10.1007/s10327-018-0775-6

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