Identification of toxic effector proteins of the Type VI Secretion System produced by bacteria of the plant microbiome using novel bioinformatic approaches

The plant microbiome is a complex, heterogenous community that is formed, among other major factors, by microbe-microbe interactions. A recent study has shown that a novel toxic effector family of the Type VI Secretion System (T6SS) is enriched in plant-associated bacteria, which is likely used in antagonism between microbial competitors (Levy et al., 2018). To expand the search for effectors of the T6SS in the plant environment, we employed a novel search algorithm that systematically identified a variety of putative candidate effectors. Heterologous expression assays using IPTG induction in Escherichia coli showed that a representative sample of the algorithmically-predicted effectors inhibit cell growth as compared to an uninduced control in both solid in liquid media. We are working towards testing the toxins in eukaryotic cells, and plan to show a molecular interaction with core T6SS components using co-immunoprecipitation, among other future experiments. An understanding of these toxins and their presence and absence in specific members of the plant microbiome will lead to a deeper understanding of the microbial ecology of plants, and therefore to the factors that lead to plant growth promotion or inhibition by probiotic and pathogenic bacteria, respectively.