Mycorrhizal Sulfur Tufts?
What sets a mycorrhizal species apart from a species with a saprotrophic lifestyle? Is it truly an “either/or” situation, or are some or all species capable of doing both? Could it be that species that get their carbs (sugar components) from a tree on whose roots they form mycorrhizae are also able to break down wood and litter?
When ectomycorrhizal species gained the ability to grow with trees, did they lose the capability to use wood and other dead plant material as their energy source? Th e jump from an exclusively saprotrophic existence to a mycorrhizal way of life has been made repeatedly in evolutionary time, by all kinds of mushroom groups: Boletus, Russula, Cortinarius, and Amanita, to name a few, which have close non-mycorrhizal relatives. Did they all abandon their former lifestyle?
In fact, ectomycorrhizal species do make some of the enzymes involved in the break-down of lignin. One example is the class of enzymes known as laccases, which are secreted outside the fungal cell. But, laccases have other functions too, so their presence does not by itself prove that the species is able to break down lignin. A second class of enzymes, lignin peroxydases, is, as far as we know, not present in ectomycorrhizal fungi (though there is a paper claiming to fi nd genes for these enzymes in a broad range of fungi; unfortunately, or fortunately, the data on which this conclusion was based are not to be trusted). Th e third class of lignin-decomposing enzymes, manganese peroxydases, have been found in all kinds of mushroom-producing fungi. In other words there seems to be evidence that, to some degree, ectomycorrhizal fungi have lost the ability to degrade lignin.
These questions were also approached from the saprophytic side of the divide by a group of researchers who found wood-decay fungi growing in or on the roots of perfectly happy seedlings. In a study of around 15,000 root tips, three were colonized by mushroom species that were supposed to grow on dead wood only, namely sulfur tufts (Hypholoma fasciculare), and two crusts: Phlebiopsis gigantea and Phlebia centrifuga. To figure out what these three wood decayers were doing on the roots, this symbiosis was synthesized from scratch with seedlings of Picea abies and the local pine, Pinus sylvestris. Only one combination (Picea abies with Phlebiopsis gigantea) really produced the same structure as an ectomycorrhizal fungus, with a fungal mantle around the root tip, and the hyphae of the fungus inside the root. Although they didn’t produce mantles, the other two fungi were also found within the roots. In all cases, the seedlings looked healthy after growing for half a year with these fungi. They were not parasitized, as you might expect. This is the first time that wood-decay fungi have been found on live, healthy tree roots, however another example has been reported. The earth tongue, Geoglossum nigritum, a litter decomposer, has been found in roots of tanbark oak in Humboldt County. What saprotrophic fungi are doing in living roots is the next question, and it has still to be answered. Whether this is a widespread phenomenon among saprotrophic species is also under investigation.
An interesting piece of information to ponder is that foresters in boreal forests spread Phlebiopsis spores on dead stumps to prevent the growth of root pathogens. If this species can colonize the roots of the trees, what are the implications for the ectomycorrhizal community and the health of the forest in general?
The complete story can be found at:
- Vasiliauskas, R., A. Menkis, R.D. Finlay & J. Stenlid, 2007. Wood-decay Fungi in Fine Living Roots of Conifer Seedlings. New Phytologist 174: 441-446. (PDF)