Decoding the Mycorrhizal Symbiosis:
Why Plants Like Fungi So Much

© Peter Kennedy

Original publication: Mycena News, May 2005

Many of the mushrooms that we enjoy collecting so much are symbiotic with trees. These symbioses are referred to as mycorrhizas, literally "fungus-root." In the traditional view of this symbiosis, the plants photosynthesize and provide carbon to the fungi in return for nutrients that the fungi take up from the soil (I say traditional because mycorrhizal fungi can also provide other benefits to their hosts such as helping them deal with summer water stress and protecting them from pathogens that attack their roots). We all know that plants are able to take up nutrients without fungi, so why is it that so many of them "pay" to get nutrients from the fungi?

Mycorrhizal fungi help plants acquire nutrients in many ways. The first is by greatly extending plant root systems. Nutrients are often quickly depleted in areas directly around plant roots and the fungal hyphae are able to grow out beyond low nutrient zones into places where more nutrients are available. Fungi also often build special hyphal structures known as rhizomorphs that allow for very efficient transfer of nutrients from the areas where the nutrients are being taken up back to the plant root. In addition to extending the root system, fungal hyphae are often much smaller in diameter than roots, which allows them to access nutrients and water in smaller soil pores. This latter mechanism effectively increases the soil volume exploited by the plant. Finally, fungi have higher surface-to-volume ratios than roots, which increases the rate at which nutrients are absorbed.

It has also been suggested that mycorrhizal fungi may be more effective nutrient competitors against free-living soil microbes than roots, or that mycorrhizal fungi may alter the bacterial community in the rhizosphere (the area of soil influenced by a plant root) in ways that help plants acquire more nutrients (e.g. attract nitrogen fixing bacteria). Although there are few studies providing evidence to support either one of these mechanisms, both are certainly possible ways that fungi help plants acquire nutrients. However, there is strong evidence showing that fungi produce a large diversity of enzymes and chelating compounds that allow them to capture nutrients from the soil that are not normally accessible to plants (chelating compounds bind metals into different forms in the soil to prevent their interference with uptake of other nutrients). Some of the best evidence for this mechanism involves mycorrhizal fungi that are able to take up nitrogen in an organic form. Because this maybe particularly important where nitrogen is believed to be the most limiting nutrient to plants (e.g. forests such as those here in California), let me elaborate.

In soil, nitrogen comes in two forms, either organic (attached to carbon) or inorganic (without carbon). Plants take up inorganic nitrogen directly (e.g. ammonium or nitrate), but they lack the enzymes necessary to take up complex forms of organic nitrogen. In temperate forest soils, there is often an abundance of organic N and much less inorganic N available for plants to utilize. Fungi were long known to take up amino acids (the building blocks of protein), but until the mid 1980s, their ability to utilize protein (a more complex form of organic nitrogen) was less clear. Using a set of lab experiments, researchers grew fungi on media containing different forms of nitrogen and documented that a number of mycorrhizal species did very well with protein as their only source of N. Interestingly, the fungi used in these experiments fell in one of two groups; those that could grow with protein and those that could not. The 'protein fungi' included Amanita muscaria, Cenococcum geophilum, Paxillus involutus, Rhizopogon roseolus, Suillus bovinus, and Hebeloma crustuliniforme, while the 'non-protein' fungi included Laccaria laccata and Lactarius rufus. Not surprisingly, plants growing with 'protein' fungi had a higher N content than those plants growing with 'non-protein' fungi.

Not everything, however, is lost for 'non-protein' fungi. Laccaria bicolor, a putatively 'non-protein' fungus, has figured out a way to get extra nitrogen. In a recent study, J. Klironomos and M. Hart found that L. bicolor can be a very effective predator of springtails, an abundant fungal-feeding soil insect. They noticed that when they added springtails to pots containing L. bicolor, springtail survival was very low (~5%), while in other pots without L. bicolor springtail survival was very high. Looking closer, they observed that the springtails were internally infected with L. bicolor hyphae and they wondered if L. bicolor could be preying on the springtails for their N. So they set up a second experiment examining whether N in the springtails ended up in the leaves of plants growing with L. bicolor. Plants growing with L. bicolor contained significant amounts of N derived directly from the springtails, while plants grown without L. bicolor showed no similar N enrichment, suggesting that L. bicolor was indeed preying on the springtails for their nitrogen! Interestingly, the researchers did to the same experiment with another mycorrhizal fungus, C. geophilum, and that species had no negative effect of springtail survival and no N enrichment for their plant partners.

Although we do not know exactly how many mycorrhizal fungi are insect predators or protein eaters, their unique abilities to access different nutrient sources makes them an essential symbiant of most plants. So the next time you are out in the woods looking for mushrooms, take a break for a minute, and marvel at the amazing symbiotic role that these fungi play in keeping our planet green.

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