Determination of virulence contribution from Phytophthora infestans effector IPI-O4 in a resistant potato host containing the RB gene

Late blight of potato, caused by the oomycete pathogen Phytophthora infestans, is persistent and costly disease of this important crop plant. Breeders have continually struggled with the evolution and introduction of new pathogen genotypes that threaten to overcome major resistance genes incorporated into new varieties. Wild species relatives of potato have been a valuable resource for the identification of novel resistance genes including the RB gene, from the diploid wild potato species Solanum bulbocastanum. The RB gene confers partial resistance to most P. infestans genotypes through its recognition of members of the corresponding pathogen effector protein family IPI-O. Multiple alleles are present at the IPI-O locus and while some alleles are recognized by RB to elicit host resistance (e.g. IPI-O1, IPI-O2), others are able to elude detection (e.g. IPI-O4). In our previous research, we found that in planta expression of P. infestans effector IPI-O4 is able to suppress the HR elicited by IPI-O1 in the presence of RB. Additionally, we have observed that P. infestans lineages containing IPI-O4 are able to cause more disease on RB plants compared to those without IPI-O4. This led to a hypothesis that the presence of IPI-O4 results in suppression of RB. In this study, we provide evidence that in planta over-expression of IPI-O4 is able to suppress RB-mediated resistance, causing enlarged lesions in RB containing K41 potato lines. The results of this study indicate that even subtle effects from host or pathogen factors during the early stages of infection can heavily influence the ultimate outcome of the interaction.