RNA silencing in the ectomycorrhizal fungus Laccaria bicolor

KEMPPAINEN M & PARDO AG

Gene Silencing: Theory, Techniques and Applications

Nova Science Publishers, Inc.

Lugar: New York; Año: 2010; p. 43 - 80

Mycorrhiza is a mutualistic association between fungi and the roots of the vast majority of terrestrial plants. In natural ecosystems the plant nutrient uptake from the soil takes place via the extraradical mycelia of these mycosimbionts. While most herbaceous plants and tropical trees form endomycorrhiza-type interactions, trees of boreal and temperate ecosystems are typically ectomycorrhizal (ECM). These species include the majority of ecologically and economically important trees and the fungal symbionts are predominantly filamentous basidiomycetes. The symbiotic phase in the life cycle of ECM basidiomycetes is the dikaryon. Hence, studies on symbiotic relevant gene functions would require the inactivation of both gene copies in the dikaryotic mycelium. RNA silencing is a sequence homology-dependent degradation of target mRNAs based on an ancient cellular mechanism believed to have evolved as protection of eukaryotic cells against alien nucleic acids. In different eukaryotic organisms, including fungi, the RNA silencing pathway can be artificially triggered to target and degrade gene transcripts of interest, resulting in gene knock-down. Most importantly, RNA silencing can act at the cytosolic level affecting mRNAs originating from several gene copies and different nuclei, and it can thus offer an efficient way for altering gene expression in dikaryotic organisms. Laccaria bicolor, the first symbiotic fungus with its genome sequenced, has rapidly turned into a model fungus in ectomycorrhizal research. Laccaria possesses a complete set of genes known to be needed for RNA silencing in eukaryotic cells. We have demonstrated that RNA silencing is functional in L. bicolor and that it can be triggered via Agrobacterium-mediated transformation. Moreover, targeted gene knock-down in dikaryotic mycelium can result in functional phenotypes altered in the symbiotic capacity confirming that RNA silencing is a powerful way to study symbiosis- regulated genes. These findings have now initiated the RNA silencing era in mycorrhizal research, a fieldthat has been hindered by the lack of proper genetic tools.