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, a mutualistic
interaction between a group of soil basidiomycetes and ascomycetes
and tree roots. Identification of the primary factors that regulate
development and metabolic activity of the symbiosis will therefore
open the door to understanding the role of fungal symbioses in
plant biology and ecology, allowing the full environmental
significance of this symbiosis to be explored.
Laccaria is a cosmopolitan genus of mushrooms
(Agaricales) collected frequently throughout North America and
Eurasia. Its taxa make up a sizeable part of the
basidiomycetous ectomycorrhizal species and have been reported from
every continent. The analysis of the genome sequence of the
ectomycorrhizal basidiomycete Laccaria bicolor (Maire)
P.D. Orton (common name: bicoloured deceiver) highlighted gene
networks involved in rhizosphere colonization and symbiosis
development and functioning. This 65-megabase genome assembly
contains ~20,000 predicted protein-encoding genes and a very large
number of transposons and repeated sequences. Upon ectomycorrhizae
development, L. bicolor expresses effector-type small
secreted proteins with unknown function, which probably have a
decisive role in the establishment of the symbiosis. Symbiosis
induces an increased expression of carbohydrate, oligopeptide and
amino acid transporters, suggesting increased fluxes of metabolites
at the symbiotic interface. 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, revealed
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
forest productivity. This project is relevant to the DOE mission
with regard to plant health.
To fully apply comparative, evolutionary genomics to this
symbiotic clade, JGI is currently resequencing the genome of
several Laccaria species (L. laccata, L. proxima, L.
tortilis) and geographic strains of L. bicolor and
L. amethystina. These sequences will be mapped to
the present reference L. bicolor genome.
Genome Reference(s)
Martin F, Aerts A, Ahrén D, Brun A, Danchin EG, Duchaussoy F, Gibon J, Kohler A, Lindquist E, Pereda V, Salamov A, Shapiro HJ, Wuyts J, Blaudez D, Buée M, Brokstein P, Canbäck B, Cohen D, Courty PE, Coutinho PM, Delaruelle C, Detter JC, Deveau A, DiFazio S, Duplessis S, Fraissinet-Tachet L, Lucic E, Frey-Klett P, Fourrey C, Feussner I, Gay G, Grimwood J, Hoegger PJ, Jain P, Kilaru S, Labbé J, Lin YC, Legué V, Le Tacon F, Marmeisse R, Melayah D, Montanini B, Muratet M, Nehls U, Niculita-Hirzel H, Oudot-Le Secq MP, Peter M, Quesneville H, Rajashekar B, Reich M, Rouhier N, Schmutz J, Yin T, Chalot M, Henrissat B, Kües U, Lucas S, Van de Peer Y, Podila GK, Polle A, Pukkila PJ, Richardson PM, Rouzé P, Sanders IR, Stajich JE, Tunlid A, Tuskan G, Grigoriev IV
The genome of Laccaria bicolor provides insights into mycorrhizal symbiosis.
Nature. 2008 Mar 6;452(7183):88-92. doi: 10.1038/nature06556