The genome of Laccaria bicolor provides insights into mycorrhizal symbiosis

Abstract:
Mycorrhizal symbioses -- the union of roots and soil fungi -- are universal in terrestrial ecosystems and

may have been fundamental to land colonization by plants1,2. Boreal, temperate, and montane forests all

depend upon ectomycorrhizae1. Identification of the primary factors that regulate symbiotic

development and metabolic activity will therefore open the door to understanding the role of

2

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-million-base 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 (SSP) with unknown function, several of which are only

expressed in symbiotic tissues. The most highly expressed SSP accumulates in the proliferating hyphae

colonizing the host root. The ectomycorrhizae-specific proteins likely play a decisive role in the

establishment of the symbiosis. The unexpected observation that the genome of L. bicolor lacks

carbohydrate-active enzymes involved in degradation of plant cell walls, but maintains the ability to

degrade non-plant cell walls, reveals the dual saprotrophic and biotrophic lifestyle of the mycorrhizal

fungus which enables it to grow within both soil and living plant roots. The predicted gene inventory of

the L. bicolor genome, therefore, points to previously unknown mechanisms of symbiosis operating in

biotrophic mycorrhizal fungi. The availability of this genome provides an unparalleled opportunity to

develop a deeper understanding of the processes by which symbionts interact with plants within their

ecosystem in order to perform vital functions in the carbon and nitrogen cycles that are fundamental to

sustainable plant productivity.