Date of this Version Mycorrhizal symbioses—the union of roots
and soil fungi—are universal in terrestrial ecosystems and may
have been fundamental to land colonization by plants. Boreal,
temperate and montane forests all depend on ectomycorrhizae.
Identification of the primary factors that regulate symbiotic
development and metabolic activity will therefore open the door
to understanding the role of ectomycorrhizae in plant
development and physiology, allowing the full ecological
significance of this symbiosis to be explored. Here we report
the genome sequence of the ectomycorrhizal basidiomycete
Laccaria bicolor (Fig. 1) and highlight gene sets involved in
rhizosphere colonization and symbiosis. This 65-megabase genome
assembly contains ~20,000 predicted protein-encoding genes and a
very large number of transposons and repeated sequences. We
detected unexpected genomic features, most notably a battery of
effector-type small secreted proteins (SSPs) with unknown
function, several of which are only expressed i