Laser microdissection and microarray analysis of Tuber melanosporum ectomycorrhizas reveal functional heterogeneity between mantle and Hartig net compartments

The ectomycorrhizal (ECM) symbiosis, a mutualistic plant-fungus association, plays a fundamental role in forest ecosystems by enhancing plant growth and by providing host protection from root diseases. The cellular complexity of the symbiotic organ, characterized by the differentiation of structurally specialized tissues (i.e. the fungal mantle and the Hartig net), is the major limitation to study fungal gene expression in such specific compartments. We investigated the transcriptional landscape of the ECM fungus Tuber melanosporum during the major stages of its life cycle and we particularly focused on the complex symbiotic stage by combining the use of laser capture microdissection and microarray gene expression analysis. We isolated the fungal/soil (i.e. the mantle) and the fungal/plant (i.e. the Hartig net) interfaces from transverse sections of T.melanosporum/Corylus avellana ectomycorrhizas and identified the distinct genetic programmes associated with each compartment. Particularly, nitrogen and water acquisition from soil, synthesis of secondary metabolites and detoxification mechanisms appear to be important processes in the fungal mantle. In contrast, transport activity is enhanced in the Hartig net and we identified carbohydrate and nitrogen-derived transporters that might play a key role in the reciprocal resources' transfer between the host and the symbiont.