Unified Classification of the Type III Secreted Effectors of Bacterial Plant Pathogens to Advance Phytopathology Research

Many diverse bacterial phytopathogens deploy type III secreted effectors (T3SEs) to promote virulence by interrupting host immunity and other critical plant processes. However, the virulence of T3SEs has been countered on the host side through the evolution of a multitude of resistance genes (<i>R</i> genes) capable of recognizing the presence of T3SEs and eliciting a response termed effector-triggered immunity. This dynamic sets up an evolutionary arms race that has led to enormous diversification of both bacterial T3SEs and plant <i>R</i> genes. Over the past decade, efforts to document and characterize the pangenome T3SE profiles of individual pathogens have generated indispensable resources that have facilitated collaborative research progress on these focal pathogens. However, despite the deeply integrated evolutionary history of T3SEs, the lack of a concerted effort to synthesize T3SE conventions across diverse pathosystems has resulted in a lack of connectivity across the literature. Here, we catalog the distribution of T3SEs across six of the most globally significant genera of bacterial phytopathogens. We show that the number of T3SEs per genome varies dramatically within and between genera and that many T3SE families are present in multiple genera despite their sparse distributions across closely related strains. We also document all inter-genera evolutionary relationships for each T3SE family and propose integrated nomenclature conventions for all phytopathogen T3SEs. Ultimately, our expanded T3SE collection includes thousands of newly classified alleles, catalogs several previously unestablished homologies between distinct genera, and will enable more comprehensive studies on the implications of T3SE diversification for virulence and immunity.