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 on ectomycorrhizae1. 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 in symbiotic tissues. The
most highly expressed SSP accumulates in the proliferating hyphae
colonizing the host root. The ectomycorrhizae-specific SSPs probably
have 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 wall polysaccharides,
reveals the dual saprotrophic and biotrophic lifestyle
of the mycorrhizal fungus that 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 to perform vital functions in
the carbon and nitrogen cycles that are fundamental to sustainable
plant productivity.
