Genome of the Ectomycorrhizal Fungus Laccaria bicolor
Martin et al. published a paper, “The genome of Laccaria bicolor provides insights into mycorrhizal symbiosis”, in the 06 March 2008 issue of Nature. The paper (PubMed record) is accompanied by a News and Views article, “Fungal symbiosis unearthed”, by Dan Cullen of the USDA Forest Products Laboratory in Madison, Wisconsin. (A subscription to Nature is required to obtain electronic copies of the paper and the article.)
Fungi and plants enter a symbiotic relationship in which the microrrhizae of fungi join with the roots of plants, allowing an exchange of nutrients between the organisms. Plants transfer simple carbohydrates, the products of photosynthesis, to the fungi, and they receive in return water and minerals, including ammonia and phosphate ions.
The researchers identified approximately 21,000 genes in the 65 million base pair genome; about 70% of the predicted genes have significant similarity to genes in the sequence databases. As might be expected, the number of predicted membrane-bound transporter proteins is large.
The authors of the paper and Dr. Cullen in his commentary noted that it was expected that the L. bicolor genome would contain genes encoding enzymes for breaking down cellulose and lignin. It was surprising, therefore, that only one gene encoding an endoglucanase with a cellulose-binding domain was identified, and no genes encoding enzymes for breaking down cellulose were found. In contrast, a large number of genes predicted to encode lipases and proteinases were identified, suggesting that L. bicolor derives nutrients from the degradation of bacteria and invertebrates. Martin et al. write:
These observations suggest that the inventory of L. bicolor plant cell wall-degrading enzymes underwent massive gene loss as a result of its adaptation to a symbiotic lifestyle, and that this species is now unable to use many plant cell wall polysaccharides as a carbon source, including those found in soil and leaf litter.
Having now in hand this fungal genome and the genome of the black cottonwood, Populus trichocarpa, which enters into ectomycorrhizal symbiosis, the authors predict it will be much easier to identify the genes that mediate the symbiotic relationship.
March 18 2008 10:07 pm | Biology
