Genes that let the humongous fungus get so humongous

The genomes of four species of Armillaria fungi, which include Earth’s largest terrestrial organism and some of the most devastating forest pathogens worldwide, are reported this week in Nature Ecology & Evolution. This study provides insight into the genetic basis of how these fungi spread and infect plants, and is a valuable resource for devising strategies to control the damage they cause in forests.

Armillaria are fungal pathogens that colonize more than 500 plant species in forests and parks worldwide. After killing the root of their host, these fungi decompose the root tissue to cause root-rot disease. Armillaria produces fruiting bodies (honey mushrooms) that appear in large clumps around infected plants, and also generates exploratory organs called rhizomorphs - 1-4 mm wide strings that grow underground in search of new roots. These rhizomorphs can be enormous: one Armillaria individual known as the ‘humongous fungus’ is one of the largest terrestrial organisms on Earth, measuring up to 965 hectares and weighing 544 tonnes.

Gyorgy Sipos and colleagues sequence genomes of four Armillaria species (A. ostoyae, A. cepistipes, A. gallica and A. solidipes). Comparing the Armillaria genomes with 22 related fungi, they find that the Armillaria species have expanded gene families associated with several pathogenicity-related genes and enzymes that degrade plant tissue. The authors also identify a wide array of genes in rizhomorphs associated with their role of dispersal and colonization of new plants. Both rizhomorphs and fruiting bodies show similar gene expression of genes associated with complex multicellularity, which the authors suggest is evidence for common developmental origins of the two structures.

The work provides a basis for devising strategies to control the spread of Armillaria and to fight root-rot disease.