The obligate biotrophic rust fungus Melampsora larici-populina is the most devastating and widespread pathogen of poplars. Studies over recent years have identified various small secreted proteins (SSPs) from plant biotrophic filamentous pathogens and have highlighted their role as effectors in host-pathogen interactions. The recent analysis of the M. larici-populina genome sequence has revealed the presence of 1,184 SSP-encoding genes in this rust fungus. In the present study, the expression and evolutionary dynamics of these SSPs were investigated to pinpoint the arsenal of putative effectors that could be involved in the interaction between the rust fungus and poplar. Similarity with effectors previously described in Pucciniales, richness in cysteines and organization in large families were extensively detailed and discussed. Positive selection analyses conducted over clusters of paralogous genes revealed fast evolving candidate effectors. Transcript profiling of selected M. larici-populina SSPs showed a timely coordinated expression during leaf infection and the accumulation of four candidate effectors in distinct rust infection structures was demonstrated by immunolocalization. This integrated and multifaceted approach helps prioritizing candidate effector genes for functional studies.
Via Kamoun Lab @ TSL