Characterization of an Insecticidal Toxin and Pathogenicity of <i>Pseudomonas taiwanensis</i> against Insects

<div><p><i>Pseudomonas taiwanensis</i> is a broad-host-range entomopathogenic bacterium that exhibits insecticidal activity toward agricultural pests <i>Plutella xylostella</i>, <i>Spodoptera exigua</i>, <i>Spodoptera litura</i>, <i>Trichoplusia ni</i> and <i>Drosophila melanogaster</i>. Oral infection with different concentrations (OD = 0.5 to 2) of wild-type <i>P. taiwanensis</i> resulted in insect mortality rates that were not significantly different (92.7%, 96.4% and 94.5%). The TccC protein, a component of the toxin complex (Tc), plays an essential role in the insecticidal activity of <i>P. taiwanensis</i>. The Δ<i>tccC</i> mutant strain of <i>P. taiwanensis</i>, which has a knockout mutation in the <i>tccC</i> gene, only induced 42.2% mortality in <i>P. xylostella</i>, even at a high bacterial dose (OD = 2.0). TccC protein was cleaved into two fragments, an N-terminal fragment containing an Rhs-like domain and a C-terminal fragment containing a Glt symporter domain and a TraT domain, which might contribute to antioxidative stress activity and defense against macrophagosis, respectively. Interestingly, the primary structure of the C-terminal region of TccC in <i>P. taiwanensis</i> is unique among pathogens. Membrane localization of the C-terminal fragment of TccC was proven by flow cytometry. Sonicated pellets of <i>P. taiwanensis</i> Δ<i>tccC</i> strain had lower toxicity against the Sf9 insect cell line and <i>P. xylostella</i> larvae than the wild type. We also found that infection of Sf9 and LD652Y-5d cell lines with <i>P. taiwanensis</i> induced apoptotic cell death. Further, natural oral infection by <i>P. taiwanensis</i> triggered expression of host programmed cell death-related genes JNK-2 and caspase-3.</p>