The rust fungus Melampsora larici-populina expresses a conserved genetic program and distinct sets of secreted protein genes during infection of its two host plants,…C Lorrain, C Marchal, S Hacquard, C Delaruelle, J Pétrowski, B Petre, …Molecular Plant-Microbe Interactions
Mechanisms required for broad spectrum or specific host colonization of plant parasites are poorly understood. As a perfect illustration, heteroecious rust fungi require two alternate host plants to complete their life cycles. Melampsora larici populina infects two taxonomically unrelated plants, larch on which sexual reproduction is achieved and poplar on which clonal multiplication occurs leading to severe epidemics in plantations. We applied deep RNA sequencing to three key developmental stages of M. larici-populina infection on larch: basidia, pycnia and aecia; and we performed comparative transcriptomics of infection on poplar and larch hosts using available expression data. Secreted protein was the only significantly over-represented category among differentially expressed M. larici-populina genes between the basidial, the pycnial and the aecial stages, highlighting their probable involvement in the infection process. Comparison of fungal transcriptomes in larch and poplar revealed a majority of rust genes commonly expressed on the two hosts and a fraction exhibiting host-specific expression. More particularly, gene families encoding small secreted proteins presented striking expression profiles that highlight probable candidate effectors specialized on each host. Our results bring valuable new information about the biological cycle of rust fungi and identify genes that may contribute to host specificity.