Let It Rot (Or Not)

© Else Vellinga
Lab
Original publication: Mycena News, September 2012

The wooden gate beside our house is slowly disintegrating into square bits and pieces. It’s definitely a sign of fungi at work, though they are invisible, and do their job without making fruitbodies. The kind of rot the fungi in my gate produce is called brown-rot, in which the lignin in the wood is left behind in brown chunks, and the cellulose is attacked by the fungus’ enzymes and broken down for food. A walk in the California woods will give you lots of examples of brown-rot, as brown-rot fungi love conifer trees. The red-belted conk, Fomitopsis pinicola, is a good example of a very common brown-rotter; Serpula the dry-rot fungus is another well-known, and feared, example.

Brown Rot

There is a second type of common rot, white-rot, so called because of the white cellulose that is left behind, after the fungi have attacked the lignin. The material left behind is white and stringy and can be torn as if it were a cooked chicken breast. Degrading lignin is a difficult job, and a specific set of enzymes is needed for it. Nevertheless, there are many different white-rot fungi, Pluteus and the turkey tail Trametes versicolor are just two of many examples.

White Rot

Brown-rot fungi differ from white-rot fungi in their toolkit—they have a different set of tools (enzymes) to break down the wood components. Not only are the enzymes different, but also the genes regulating the enzyme production. That is the area of research that has received much attention lately. It is easier nowadays to look at the genes than at the various enzymes!

The few examples I just mentioned belong to very different systematic groups: Fomitopsis and Trametes are polypores, but Pluteus is a gilled mushroom, and Serpula is a relative of the boletes. Species are assigned to a certain genus based on the kind of rot they produce. The brown-rotting Neolentinus species, such as N. ponderosus of the Sierra Nevada, were removed from the genus Lentinus and put in their own genus, leaving the white-rot causing Lentinus species behind. The phylogenetic tree of the basidiomycetes shows a mosaic of brown-rot and white-rot fungi. At the base of the tree are brown-rotters such as Dacrymyces, followed later by the Agaricomycetes which are thought to be the first white-rot fungi. The rise of the white-rot, lignin and wood decomposing fungi coincides with the end of the big coal deposits. Coal is of course a form of fossil wood, and wood comes from trees. Could it be, that with the explosion of white-rotters, all the dead trees were decomposed leaving no wood behind to form coal?

After that first flurry of white-rotting species, brown-rotters evolved again and again.

The fungal phylogenetic tree also harbours species that are not decomposing wood to satisfy their energy needs: parasitic fungi living off live trees are scattered throughout and so are mutualistic ectomycorrhizal fungi—those that get their carbon directly from the tree in exchange for nitrogen and other nutrients. The latter also occur on various branches of the phylogenetic tree: chanterelles, boletes, coccoras and shrimp russulas to name a few. They often are close relatives to saprotrophic species and again often placed in their own genus—Paxillus involutus is ectomycorrhizal, but P. atrotomentosus is now placed in the genus Tapinella as it is a wood-rotter. There is always the question how these different modes came about, did the wood-decayers gain the enzymes to tackle the various components of wood, or did the ectomycorrhizal species lose the ability to degrade plant material?

The set of questions raised above was tackled in a neat research project for the genus Amanita. A small number of Amanita species is found in grasslands or in forests without ectomycorrhizal tree hosts; these species are decomposers. The majority of Amanita species, however, lives in a mutualistic relationship with trees. The saprotrophic Amanita species form a small group at the base of the Amanita phylogenetic tree, with the rest of the species forming a single-stemmed huge branching tree with many many species. In other words, a single event caused the change from saprotrophism to ectomycorrhizal nutrition, and this was followed by the rise of many different ectomycorrhizal species. They took off and formed many species as soon as they discovered that living trees, rather than dead trees, are a really good source for carbs.

So what was this single event that caused the fungi to switch from eating dead trees to living happily with live ones? The switch, as it turned out, was the loss of two important genes active in cellulose degradation: none of the ectomycorrhizal Amanita species has these two genes, but the genes are in the DNA of the saprotrophic species. Of course now we know the differences, we can investigate what came first, the discovery of living tree roots as a source of food, or the loss of enzymes and the forced change in nutritional mode (or lifestyle if you like).

Other fungal groups might show a different approach, with the loss of different enzymes, or the gain of others. Nature is inventive, and the same path is not necessarily taken twice. Time will show, and in the meantime, our garden gate is slowly but surely consumed, by fungi.

Further reading

Else Vellinga, Ph.D., is interested in mushroom taxonomy and has been studying mushrooms in California and beyond for years. A frequent contributor to Mycena News, she is also fascinated by interactions between fungi and other organisms. In her free time she knits, and knits, and knits!