P-199 - Phytoplasma Effectorome Diversity: Mechanisms of Innovation in Genome-Reduced Bacteria

Obligate host-associated bacteria with reduced genomes face evolutionary constraints, including limited horizontal gene transfer (HGT) and recurrent bottlenecks. While many function as mutualistic endosymbionts, others, such as phytoplasmas and mollicutes, act as parasites, manipulating host defence and development. Phytoplasmas are insect-transmitted plant pathogens that rely on effectors to hijack host cellular processes. Phytoplasma effectors have unique functions, with SAP05 and SAP54 mediating ubiquitin-independent degradation of plant transcription factors, triggering developmental changes that enhance pathogen transmission. How these parasites sustain and diversify their effector repertoires despite minimal genetic exchange remains unclear. To investigate, we analysed the effectorome of 239 phytoplasma genomes, identifying diverse effectors, which we termed phytoplasma-associated molecular effectors (PhAMEs). The effectorome is largely composed of novel folds, with gene duplication, variation in interacting surfaces, domain fusions, and tandem repeat duplications driving diversification. Comparative analysis with bacterial proteomes revealed both unique and shared pathogenic strategies among mollicutes and other bacterial pathogens and symbionts. These findings provide insights into how genome-reduced bacteria innovate new molecular functions, informing protein engineering strategies for the discovery and design of novel biomolecules with biotechnological applications.