FUNGAL GENE-EXPRESSION DURING ECTOMYCORRHIZA FORMATION

Ectomycorrhiza development involves the differentiation of structurall y specialized fungal tissues (e.g., mantle and Hartig net) and an inte rface between symbionts. Polypeptides presenting a preferential, up-, or down-regulated synthesis have been characterized in several develop ing ectomycorrhizal associations. Their spatial and temporal expressio ns have been characterized by cell fractionation, two-dimensional poly acrylamide gel electrophoresis, and immunochemical assays in the Eucal yptus spp. Pisolithus tinctorius mycorrhizas. These studies have empha sized the importance of fungal cell wall polypeptides during the early stages of the ectomycorrhizal interaction. The increased synthesis of 30- to 32-kDa acidic polypeptides, together with the decreased accumu lation of a prominent 95-kDa mannoprotein provided evidence for major alterations of Pisolithus tinctorius cell walls during mycorrhiza form ation. Differential cDNA library screening and shotgun cDNA sequencing were used to clone symbiosis-regulated fungal genes. Several abundant transcripts showed a significant amino acid sequence similarity to a family of secreted morphogenetic fungal proteins, the so-called hydrop hobins. In P. tinctorius, the content of hydrophobin transcripts is hi gh in aerial hyphae and during the ectomycorrhizal sheath formation. A lteration of cell walls and the extracellular matrix is therefore a ke y event in the ectomycorrhiza development. An understanding of the mol ecular mechanisms that underlies the temporal and spatial control of g enes and proteins involved in the development of the symbiotic interfa ce is now within reach, as more sophisticated techniques of molecular and genetic analysis are applied to the mycorrhizal interactions.