A fungal conserved gene from the basidiomycete Hebeloma cylindrosporum is essential for efficient ectomycorrhiza formation
Résumé
We used Agrobacterium-mediate insertional mutagenesis to identify genes in the ectomycorrhizal fungus Hebeloma cylindrosporum that are essential for efficient mycorrhiza formation. One of the mutants presented a dramatically reduced ability to form ectomycorrhizas when grown in the presence of Pinus pinaster. It failed to form mycorrhizas in the presence of 0.5 g l-1 glucose, a condition favourable for mycorrhiza formation by the wild-type strain. It however formed few mycorrhizas when glucose was replaced by fructose or when glucose concentration was increased to 1 g l-1. Scanning electron microscopy examination of these mycorrhizas revealed that this mutant was unable to differentiate true fungal sheath and Hartig net. Molecular analyses showed that the single-copy disrupting T-DNA was integrated 6884 bp downstream the start codon, of an ORF potentially encoding a 3096 amino acid-long protein. This gene, which we named HcMycE1 has orthologs in numerous fungi as well as different other eukaryotic microorganisms. RNAi inactivation of HcMycE1 in wild-type strain also led to a mycorrhizal defect, demonstrating that the non-mycorrhizal phenotype of the mutant was due to mutagenic T-DNA integration in HcMycE1. In the wild-type strain colonizing P. pinaster roots, HcMycE1 was transiently up-regulated before symbiotic structures differentiation. Together with the inability of the mutant to differentiate these structures, this suggests that HcMycE1 plays a crucial role upstream fungal sheath and Hartig net differentiation. This study provides the first characterization of a fungal mutant altered in mycorrhizal ability.v