Effector Repertoire of Phytophthora betacei : In Search of Possible Virulence Factors Responsible for Its Host Specificity.

Phytophthora betaceiis an oomycete plant pathogen closely related toPhytophthora infestans.It infects tree tomato (Solanum betaceum) in northern South America, but is, under natural conditions, unable to infect potatoes or tomatoes, the main hosts of its sister speciesP. infestans. We characterized, and compared the effector repertoires ofP. betaceiand otherPhytophthoraspecies. To this end, we usedin silicoapproaches to predict and describe the repertoire of secreted proteins inPhytophthoraspecies and determine unique and core effectors.P. betaceihas the largest proteome and secretome of allPhytophthoraspecies evaluated. We identified between 450 and 1933 candidate effector genes inPhytophthora ramorum, Phytophthora sojae, Phytophthora cactorum, Phytophthora parasitica, Phytophthora palmivora, P. infestans, andP. betaceigenomes. TheP. betaceipredicted secretome contains 5653 proteins, 1126 of which are apoplastic effectors and 807cytoplasmic effectors. Genes encoding cytoplasmic effectors include 791 genes with an RxLR domain (the largest number known so far in aPhytophthoraspecies) and 16 with a Crinkler (CRN) domain. We detected homologs of previously described avirulence gene (Avr) present inPhytophthoraspp., such as Avr1, Avr3b, Avr4, and Avrblb1, suggesting a high level of effector gene conservation amongPhytophthoraspecies. Nonetheless, fewer CRN effectors were obtained inP. betaceicompared to all otherPhytophthoraspecies analyzed. The comparison betweenP. infestansandP. betaceieffector profiles shows unique features inP. betaceithat might be involved in pathogenesis and host preference. Indeed, 402 unique predicted effector genes were detected inP. betacei, corresponding to 197 apoplastic effector genes, 203 RxLR cytoplasmic effector genes, and 2 effector genes with CRN domain. This is the first characterization of the effector profile ofP. betaceiand the broadest comparison of predicted effector repertoires in the genusPhytophthorafollowing a standardized prediction pipeline. The resultantP. betaceiputative effector repertoire provides a reasonable set of proteins whose experimental validation could lead to understand the specific virulence factors responsible for the host specificity of this species.