Mutually beneficial mycorrhizal symbiosis
The laccaria fungus, sometimes known as the "deceiver", is a forest fungus that, like boletes and truffles, can establish symbiotic partnerships with different tree species such as beech, oak and pine. The fungus forms an underground network that ensheathes tree roots, creating a nutrient exchange structure called a mycorrhiza. In this symbiosis, the fungus transfers minerals - phosphorus, nitrates, ammonium - and amino acids to the plant. In return, it has preferred access to the simple sugars produced by plant photosynthesis. For Francis Martin, director of the INRA-Nancy University Joint Research Unit coordinating this particular genomics programme, "mychorrhizas are necessary to terrestrial ecosystems, because most plants depend on this sort of interaction to thrive".
To understand the underlying genetics of mycorrhizal symbiosis, an international consortium of researchers sequenced the 65 million base pairs of the Laccaria bicolor fungus. The genome, the largest ever sequenced to date, contains some 20,000 genes, of which several hundred have never been identified. They may play a fundamental role in establishing symbiotic life.
Laccaria bicolor, the first mycorrhizal fungus to be sequenced, was chosen as a model owing to its ecological and agricultural significance. It is widely distributed in forest ecosystems and is used in plantations the world over using controlled mycorrhization. The fungus is marketed in France under an INRA licence to promote growth of high-quality conifer seedlings.
An enzyme arsenal fit for Laccaria's double life as symbiont and saprotroph
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© INRA, Dominique Vairelles, ref : 9049-IMG0018
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As they analysed the genes of Laccaria, researchers were surprised to discover that the fungus did not have the enzymes needed to degrade polysaccharides like cellulose and pectins, complex sugars that form plant cell walls. These degrading enzymes are generally present in fungi that live on plants or in the ground. The observation explains why the fungus can live in its host's roots without attacking them, in a mutually beneficial relationship.
However, Laccaria is capable of using protein from decaying leaf litter. This ability to break down organic matter enables Laccaria to subsist temporarily in a free state, before symbiosis is established, and thus play a crucial role in the forest's nitrogen cycle. A wide range of nitrogen transporters allows Laccaria to use these different sources of nitrogen.
Special proteins that may play a role in symbiosis
The Laccaria genome codes for hundreds of proteins that have never been seen in other species. These proteins may play a role in establishing symbiosis. Some of them are secreted at the level of the host-fungus interface and appear to take part in the complex dialogue between the two partners.
The comparison of Laccaria's complex genome with that of other symbiotic fungi like the Perigord black truffle - currently being sequenced at Génoscope (the French National Sequencing Centre) - will help confirm these initial observations and further explain how plant-fungi symbiosis emerged over the course of evolution.
Ultimately, identifying the key factors in symbiosis will lead to a better understanding of the formation of fruiting bodies in edible fungi, and encourage optimal forest production through a greater command of mycorrhization. Such knowledge will also help provide deeper insight of how trees and their associated beneficial micro-organisms adapt to change and the many constraints that come with it.
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>> To learn more: http://www.jgi.doe.gov/laccaria
>> Source : "The genome of Laccaria bicolor provides insights into mycorrhizal symbiosis", NATURE, 06-03-2008
F. Martin1, A. Aerts2, D. Ahre´n3, A. Brun1, E. G. J. Danchin4, F. Duchaussoy1, J. Gibon1, A. Kohler1, E. Lindquist2,
V. Pereda1, A. Salamov2, H. J. Shapiro2, J. Wuyts1,5, D. Blaudez1, M. Bue´e1, P. Brokstein2, B. Canba¨ck3, D. Cohen1,
P. E. Courty1, P. M. Coutinho4, C. Delaruelle1, J. C. Detter2, A. Deveau1, S. DiFazio6, S. Duplessis1,
L. Fraissinet-Tachet8, E. Lucic1, P. Frey-Klett1, C. Fourrey1, I. Feussner7, G. Gay8, J. Grimwood9, P. J. Hoegger10,
P. Jain11, S. Kilaru10, J. Labbe´1, Y. C. Lin5, V. Legue´1, F. Le Tacon1, R. Marmeisse8, D. Melayah8, B. Montanini1,
M. Muratet11, U. Nehls12, H. Niculita-Hirzel13, M. P. Oudot-Le Secq1, M. Peter1,14, H. Quesneville15, B. Rajashekar3,
M. Reich1,10, N. Rouhier1, J. Schmutz9, T. Yin16, M. Chalot1, B. Henrissat4, U. Ku¨es10, S. Lucas2, Y. Van de Peer5,
G. K. Podila11, A. Polle10, P. J. Pukkila17, P. M. Richardson2, P. Rouze´5,18, I. R. Sanders13, J. E. Stajich19, A. Tunlid3,
G. Tuskan16 & I. V. Grigoriev2
1 UMR 1136, INRA-Nancy-Université, Interactions Arbres/Microorganismes, INRA-Nancy, 54280 Champenoux, France.
2 US DOE Joint Genome Institute, Walnut Creek, California 94598, USA.
3 Microbial Ecology, Lund University, SE-223 62 Lund, Sweden.
4 Architecture et Fonction des Macromolécules Biologiques, UMR 6098 CNRS-Universités Aix-Marseille I& II, 13288 Marseille Cedex 9, France.
5 Department of Plant Systems Biology, Flanders Interuniversity Institute for Biotechnology (VIB), Ghent University, B-9052 Ghent, Belgium.
6 Department of Biology, West Virginia University, Morgantown, West Virginia 26506, USA.
7 Department for Plant Biochemistry, Georg-August-Universita¨t Go¨ttingen, 37077
Go¨ttingen, Germany.
8 Université Lyon 1, UMR CNRS - USC INRA d’Ecologie Microbienne, 69622 Villeurbanne, France.
9 Stanford Human Genome Center, Department of Genetics,
Stanford University School of Medicine, 975 California Avenue, Palo Alto, California 94304, USA.
10 Institute of Forest Botany, Georg-August-Universita¨t, 37077 Go¨ttingen, Germany.
11 Department of Biological Sciences, University of Alabama, Huntsville, Alabama 35899, USA.
12 Eberhard-Karls-Universita¨t, Physiologische Oekologie der Pflanzen, 72076 Tu¨bingen,
Germany.
13 Department of Ecology & Evolution, University of Lausanne, 1015 Lausanne, Switzerland. 14Swiss Federal Research Institute WSL, 8903 Birmensdorf, Switzerland.
15 Unité de Recherches en Génomique-Info, INRA-Evry, 91034 Evry Cedex, France.
16 Environmental Science Division, Oak Ridge National Laboratory, Oak Ridge, TEN 37831, USA.
17 Department of Biology, The University of North Carolina, Chapel Hill, North Carolina 27599-3280, USA.
18 Laboratoire Associé de l’INRA, Ghent University, B-9052 Gent, Belgium.
19 Department of Plant and Microbial Biology, University of California, Berkeley, California 94720-3102, USA.
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