Prediction, structure characterization, and evolutionary analysis of Erwinia psidii putative type III effectors

Erwinia psidii is a gram‐negative bacterium that threatens both guava and eucalypt plantations in several countries. Despite the economic importance of both crops, nothing is currently known about the molecular mechanisms underlying E. psidii pathogenicity and, consequently, how it evolved to infect Eucalyptus species besides its presumed native host Psidium guajava. In this study, we predicted putative type III secretion system effectors that may play important roles during plant–E. psidii interactions and conducted effector structure and phylogenetic analyses to gain important insights into their function and evolution. For that, the whole genomes of four E. psidii strains that exhibit differential aggressiveness towards eucalypt clones were sequenced and their effector repertoires predicted based on sequence identity with known effectors of the model phytopathogen Erwinia amylovora. Only proteins sharing significant sequence identity with the DspE and Eop1 effectors were found. Here, it is shown that these two E. psidii effectors retain all structural characteristics of their corresponding protein superfamilies, but exhibit allelic variations that are consistent with the observed aggressiveness differences between strains. Phylogenetic analyses revealed that whereas E. psidii housekeeping gene sequences are more closely related to those from Erwinia tracheiphila, the effector (either nucleotide or amino acid) sequences are more closely related to their Pantoea agglomerans counterparts, suggesting that dspE and eop1 were both acquired through horizontal gene transfer from the latter bacterial species. The results of this study provide important insights on E. psidii pathogenicity and set the stage for future effector functional studies. Erwinia psidii type III effectors DspE and Eop1 retain structural characteristics of their protein superfamily members and their encoding genes seem to have been acquired by horizontal transfer.