Mutants: New Weapons To Unravel Ascospore Germination Regulation in the Model Fungus .

In Podospora anserina as in many other , ascospore germination is a regulated process that requires the breaking of dormancy. Despite its importance in survival and dispersal, ascospore germination in filamentous fungi has been poorly investigated, and little is known about its regulation and genetic control. We have designed a positive genetic screen that led to the isolation of mutants showing uncontrolled germination, the () mutants. Here, we report on the characterization of the () mutant. We show that is mutated in , the ortholog of Magnaporthe oryzae , which encodes a carnitine-acetyltransferase (CAT) involved in the shuttling of acetyl coenzyme A between peroxisomes and mitochondria and which is required for appressorium development. Bioinformatic analysis revealed that the mutated residue (I441) is highly conserved among and that the mutation has a deleterious impact on the protein function. We show that GUN1 is essential for ascospore germination and that the protein is localized both in mitochondria and in peroxisomes. Finally, epistasis studies allowed us to place together with the MAPK pathway upstream of the / complex in the regulation of ascospore germination. In addition, we show that plays a role in appressorium functioning. The pivotal role of , the ortholog of , in ascospore germination and in appressorium functioning reinforces the idea of a common genetic regulation governing both appressorium development and melanized ascospore germination. Furthermore, we characterize the second CAT encoded in genome, /, and we show that the function of both CATs is conserved . The regulation of ascospore germination in filamentous fungi has been poorly investigated so far. To unravel new genes involved in this regulation pathway, we conducted a genetic screen in , and we isolated 57 mutants affected in ascospore germination. Here, we describe the () mutant, and we characterize the gene affected. GUN1 is a peroxisomal/mitochondrial carnitine-acetyltransferase required for acetyl coenzyme A shuttling between both organelles, and we show that is a pleiotropic gene also involved in appressorium functioning similarly to its ortholog, the pathogenesis factor , in the plant pathogen Magnaporthe oryzae. Given the similarities in the regulation of appressorium development and ascospore germination, we speculate that discovering new genes controlling ascospore germination in may lead to the discovery of new pathogenesis factors in pathogenic fungi. The characterization of , the ortholog of M. oryzae , represents a proof of concept.