Génomique fongique et évolution de la pathogénie pour les plantes

 

M.-H. Lebrun

 

UMR2847 CNRS-Bayer Cropscience, Fungal and Plant Physiology, 14-20 rue P Baizet, 69263 Lyon

 

 

Fungi have developed during their co-evolution with plants very different strategies to attack plants. Although biologically diverse, these infectious strategies may rely on common molecular mechanisms. Magnaporthe grisea, the rice blast fungus is the first fungal plant pathogen to be sequenced. Several other fungi have been or are currently being sequenced including the grape pathogen Botrytis cinerea. Comparative analysis of these genome sequences should help finding conserved pathogenicity genes as well as genes involved in the adaptation to a specific host. In many fungi, the early stage of infection relies on the formation of a specialised cell called the appressorium that breaches host cuticle and cell wall, mediating penetration of the fungus into host plant tissues. This process requires specific developmental programs and pathways identified through the study of non-pathogenic mutants obtained either by insertional mutagenesis or reverse genetics. In M. grisea, 40 genes required for infection have been characterized. They mainly encode proteins involved in signalling from receptors (MPG1, PTH11) to enzymes (MAP kinases, cAMP pathway enzymes) and effectors (transcription factors). Most other M. grisea pathogeniticy genes encode enzymes involved in metabolic pathways important for appressorium-mediated penetration. These genes are conserved among fungi including saprobes in which they are frequently involved in non-essential cellular functions. This situation suggests that these genes have been recruited for new tasks by plant pathogenic fungi becoming absolutely required for infection. This is also true for genes with unknown function such as PLS1 that is required for appressorium-mediated penetration in unrelated fungi such as M. grisea, B. cinerea and Colletotrichum lindemuthianum. PLS1 orthologues were also detected in saprobic fungi although their cellular function is still unknown. Another striking difference between saprobic and pathogenic fungi is the number of genes families. M. grisea has 8 PKS-NRPS involved in the production of secondary metabolites that include the avirulence gene ACE1, while saprobes like N. crassa or Aspergillus species have only one PKS-NRPS. Similar trends were observed for genes encoding other enzymes of the secondary metabolism, secreted proteins and receptors (GPCRs) suggesting an evolution toward pathogenicity through the amplification and diversification of genes families involved in its interaction with host plants.