Abstract
Pseudomonas cichorii harbors the hrp genes. hrp-mutants lose their virulence on eggplant but not on lettuce. A phosphinothricin N-acetyltransferase gene (pat) is located between hrpL and an aldehyde dehydrogenase gene (aldH) in the genome of P. cichorii. Comparison of nucleotide sequences and composition of the genes among pseudomonads suggests a common ancestor of hrp and pat between P. cichorii strains and P. viridiflava strains harboring the single hrp pathogenicity island. In contrast, phylogenetic diversification of aldH corresponded to species diversification amongst pseudomonads. In this study, the involvement of aldH and pat in P. cichorii virulence was analyzed. An aldH-deleted mutant (ΔaldH) and a pat-deleted mutant (Δpat) lost their virulence on eggplant but not on lettuce. P. cichorii expressed both genes in eggplant leaves, independent of HrpL, the transcriptional activator for the hrp. Inoculation into Asteraceae species susceptible to P. cichorii showed that the involvement of hrp, pat and aldH in P. cichorii virulence is independent of each other and has no relationship with the phylogeny of Asteraceae species based on the nucleotide sequences of ndhF and rbcL. It is thus thought that not only the hrp genes but also pat and aldH are implicated in the diversity of P. cichorii virulence on susceptible host plant species.
Highlights
Pseudomonas cichorii causes bacterial rot in lettuce, which is characterized by shiny, dark brown, firm necrotic spots on leaves underneath the second or the third outermost head-leaves [1,2,3].P. cichorii reportedly causes midrib rot of greenhouse-grown butterhead lettuce [4].In culture medium and on lettuce leaves, P. cichorii does not produce pectate lyase, the most important extracellular plant cell wall-degrading enzyme
To compare the in vitro growth ability of the aldehyde dehydrogenase gene (aldH)-deficient mutant (ΔaldH) and the pat-deficient mutant (Δpat) from SPC9018 with that of SPC9018, P. cichorii strains were incubated in PY-medium and the optical density at 600 nm (OD600) of the bacterial suspensions was measured
The ΔaldH and the Δpat mutants lost their virulence on eggplants (Figure 2A)
Summary
Pseudomonas cichorii causes bacterial rot in lettuce, which is characterized by shiny, dark brown, firm necrotic spots on leaves underneath the second or the third outermost head-leaves [1,2,3]. The hrp cluster encodes proteins of the type III secretion system (T3SS), which transports virulent proteins directly into the host cells. These proteins subsequently cause leakage of plant nutrients into the extracellular spaces of infected tissues and suppress host defenses. Hrp-deficient mutants of SPC9018 grow slowly, and the appearance of disease symptoms on infected lettuce leaves is delayed compared with the wild type strain, suggesting that the putative T3SS-dependent effector proteins may hinder or delay the plant defense response, giving the bacteria time to multiply before inducing PCD in lettuce leaves [6]. P. viridiflava strain Pv9504 (Pv9504) harboring the S-PAI These lines of evidence suggest a common ancestor for the hrp cluster between P. cichorii strains and the S-PAI of P. viridiflava. We analyzed the involvement of aldH and pat in the diversity of P. cichorii virulence on susceptible host plant species
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