Small secreted proteins as symbiotic effectors in the ectomycorrhizal basidiomycete Laccaria bicolour

J. Gibon, A. Kohler, M-P Oudot-le Secq, F. Martin

UMR 1136, INRA-Nancy Université, Interactions Arbres/Microorganismes, INRA-Nancy, 54280 Champenoux, France.

Fungi are microbial eukaryotes with major roles in terrestrial ecology, as soil saprobes, beneficial plant symbionts and devastating plant pathogens. As plant associated fungi have a common phylogenetic origin and a long history of co-evolution with plants, they likely share ancestral functions involved in interactions with host plants. Studies on genes/functions involved in pathogenicity and symbiosis have highlighted a particular class of effectors corresponding to secreted fungal proteins. Within the "FungEffector" ANR project we have developed a genome-wide analysis of small secreted protein (SSP) from Laccaria bicolor and we are investigating their possible role as effectors involved in symbiotic interactions with host trees.

As the corn smut Ustilago maydis, L. bicolor genome contains a large number of putative secreted proteins with unknown function. Of 2,931 proteins predicted to be secreted, 1,979 (67%) cannot be ascribed a function, and out of these 82% (1,633) are specific for L. bicolor. Approx. 440 are cystein-rich and have a size < 300 AA; several of these SSP being arranged in gene clusters. Genome-wide expression oligoarray profiling revealed that the expression of several SSP genes is induced in symbiotic tissues. Within the few symbiosis-specific transcripts (ectomycorrhizins) detected in both Populus and Douglas fir ectomycorrhizas, we found a 69 amino acid-SSP [Lacbi1:298595] and a 180 AA secreted protein with a CFEM domain [Lacbi1:332226]. The latter eight cysteine-containing domain was found in several fungal membrane proteins, such as Pth11 from Magnaporthe grisea, and proposed to have important roles in fungal pathogenesis. In addition, a dozen of SSP showed a significant similarity to effector proteins and haustoria expressed secreted proteins (HESP) of Magnaporthe oryzea, Melampsora lini, and Uromyces fabae involved in pathogenesis. Although a large proportion of SSP are likely involved in hyphal aggregation during fruit body and symbiotic mantle differentiation, it is tempting to speculate that this rich assortment of effector small proteins may interact with or manipulate host plants during infection and subsequent symbiosis as suggested for rust and Phythophtora species.

Acknowledgements:

JG wishes to acknowledge the financial support from the MENESR. This project was supported by grants from the Agence Nationale de la Recherche (project "FungEffector") and the Région Lorraine. We would like to thank the JGI Production Sequencing, Assembly & Annotation Staffs, and members of the Laccaria Genome Annotation Consortium. The genome sequencing of Laccaria bicolor H82 was funded by the U.S. Department of Energy’s Office of Science, Biological, and Environmental Research Program and by University of California, Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48; Lawrence Berkeley National Laboratory under contract no. DE-AC02-05CH11231; and Los Alamos National Laboratory under contract no. W-7405-ENG-36.