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The Threads That Bind: Symbiotic Fungi in the Garden

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The Threads That Bind: Symbiotic Fungi in the Garden veryone knows that a plant they formed a symbiosis with fungi, has leaves and flowers, and whose growing network of threads (the below ground its roots mycelium) allowed them to explore branch and explore the soil. the soil and forage for phosphate. In Yet that picture misses an return the plant provided the fungus essential component as with organic compounds. EEvirtually all plants live with a fungus in Three lines of evidence support this a symbiosis called a mycorrhiza (from story: first, molecular evidence shows the Greek, meaning ‘fungus root’) that that this group of fungi originated at or is essential to both partners. The plants before that distant time; second, we get phosphate and some other nutrients find plants that form this type of mycor- from the fungus, and the fungus gets all rhiza in all branches of the evolutionary its sugars from the plant. Plants gain tree of land plants, showing that they The threads other benefits too from the symbiosis must all have shared a common mycor- and sustainable farming, forestry and rhizal ancestor; and most convincingly, gardening will all increasingly rely on there are fossils from Devonian rocks understanding how it behaves. nearly 400 million years old that contain the fungal structures (Fig. 1a) that bind: Meet the ancestors and older Ordovician spores that are To explain why they are mycorrhizal, unequivocally from the group. This we need to go back to when plants first ancestral type is known as an arbus- colonized the land more than 400 cular mycorrhiza, after the tiny struct- million years ago, in the Ordovician ures called arbuscules that develop symbiotic fungi and Devonian periods. The first land inside the cells of the root and branch plants had no roots; their coarse, like trees (Fig. 1b). The arbuscule is branching rhizomes lay on or near the where the plant obtains phosphate from surface of the ground. The obvious the fungus. Surprisingly, we still do not problem for these plants was getting know where the fungus gets sugars in the garden water, but in practice they probably from the plant, but it may be in the grew in wet places where water may intercellular spaces where the fungal not have been the challenge. A greater hyphae proliferate inside the root. The difficulty was getting phosphate. plant is the only source of organic The aquatic ancestors absorbed it from carbon for the fungus, which appar- the water, where it was present, ently cannot grow solo; no-one has yet although very dilute. On land, a new cultured these fungi without a plant. problem emerged: phosphate ions form extremely insoluble compounds with Thoroughly modern elements such as iron, aluminium mycorrhizas and calcium (think of bone), all of The reasons for the origin of this which are abundant in rocks and soil; symbiosis do not, however, explain hence phosphate ions diffuse extremely why about two-thirds of modern plant slowly into soils, taking days to move species form arbuscular mycorrhizas. a few millimetres. Phosphate would Most modern plants have well develop- not therefore move to the plants, and ed root systems and can obtain phos- with no roots, the plants could not phate themselves: they can grow in pots forage for phosphate in the soil. Instead of sterilized soil. However, the root ᭡ Fruiting body of the ectomycorrhizal (a) (b) ᭡ Hyphae (white) of a fungus (phylum fungus Thelephora terrestris. Robert L. Most people think of fungi and plants in Basidiomycetes) growing on the roots Anderson, USDA Forest Service (brown) of the strawberry tree (Arbutus (www.forestryimages.org) terms of disease, but as Alastair Fitter unedo). Dr Jeremy Burgess / Science Photo Library explains, certain fungi are essential to plant ᭣ Fig. 1a. Arbuscule in a fossil rhizome of Aglaophyton major. The arrow points to the dichotomously branching hypha that gives growth. They form mycorrhizas – mutually rise to the arbuscule, as also seen in the modern root. Reprinted with permission beneficial associations with plant roots. from Taylor et al. (1995) Mycologia 87, 567 ᭣ Fig. 1b. Arbuscules of an arbuscular mycorrhizal fungus (Glomeromycota) in a Without this symbiosis our gardens would modern root of Hyacinthoides non-scripta. J. Merryweather not be so green. 56 microbiology today may 05 microbiology today may 05 57 Ornithogallum umbellatum Plantago major Sagina procumbens Hor. cm Hor. cm Hor. cm A A A 40 10 5 A/(B) (B) (B)/G 80 Lolium multiflorum Mercurialis annua Aphanes arvensis Hor. cm Hor. cm Hor. cm A1 20 A ᭣ Fig. 2. Variation of root system architecture displayed by root tracings 20 A2 of a range of species. Reproduced with permission from L. Kutschera A 10 (1960) Wurzelatlas mitteleuropäischer Ackerunkräuter und 40 Kulturpflanzen (Frankfurt: DLG) A/Ca ᭡ Fig. 3. An ectomycorrhizal root of pine, colonized by Lactarius sp. Ca 40 The roots are stunted and branch dichotomously; the sheath of fungal 60 C hyphae surrounding the root is clearly visible. F. Martin ᭤ Fig. 4. The achlorophyllous orchid Epipogium aphyllum growing at (B) 80 one of its last British sites in 1966; it is now apparently extinct in Britain. The plant has no leaves and virtually no roots either, and is wholly reliant on a mycorrhizal fungal partner for all its nutritional requirements. A. Fitter systems of plants display enormous variation in form: grasses mycelium may be the genetic equivalent of a population of rhizas: the fungus explores soil and whether in natural communities or several distinct types with unique have densely branched root systems with very fine roots (Fig. individuals (or even possibly a community of species!). We obtains nutrients that it exchanges for in managed systems, such as farms, ecological properties. In natural eco- 2, bottom row), whereas plants with bulbs and corms (snow- are only just beginning to understand the basic biology of sugars with the plant. However, ecto- forestry and gardens. However, there systems mycorrhizal fungi appear to drops, crocuses, onions) have coarse, unbranched root sys- these organisms, even though they are among the most mycorrhizal fungi can be grown in pure are many other root–fungus symbioses, control the diversity of the plant tems (Fig. 2, top row). Plants with dense, fine roots rely on the abundant creatures on earth. culture, without a plant, because they generally confined to particular groups community and the speed of the carbon fungus for phosphate only in very infertile soils, whereas the can decompose and obtain nutrients – of plants, which suggests that they have cycle; in managed systems, except in bulbs and other thick-rooted species may be wholly depend- Ectomycorrhizas especially nitrogen – directly from evolved recently. One is with heathers some types of forestry, their potential ent on the symbiosis except in the most heavily fertilized soils. There is no way to tell whether a plant is symbiotic with an organic materials. Without this direct and their allies (Ericales). This sym- has yet to be exploited and will not be In modern agriculture, biological solutions to problems that arbuscular mycorrhizal fungus just by looking at it. You need access to nutrients, plants rely on other biosis is with a group of fungi that until we understand better the biology could be solved technologically have been unfashionable, to stain the roots to reveal the fungal structures, though under fungi and bacteria to break down are especially effective at breaking and ecology of the organisms involved. and little attention has been paid to the potential benefits of a microscope you can also see hyphae around the root. the materials and release them, for down organic materials for nitrogen. this symbiosis. However, phosphate fertilizers are all mined However, this is not the only form of mycorrhiza. Later in which they must compete with other Heathers grow typically in soils where Alastair Fitter unsustainably, and farmers and gardeners may therefore need evolution, a new mycorrhizal symbiosis emerged: this one is microbes. The ectomycorrhizal sym- peat forms, which shows that decom- Department of Biology (Area 2), to rely more on their fungal partners in future. visible to the naked eye, because the fungus changes the way biosis seems to have evolved as an position has almost ceased and that University of York, YO10 5YW, UK Modern plants with fine root systems still form mycor- the roots develop, and the resulting stubby, often branched adaptation to growing in ecosystems the nitrogen cycle has also ground to a (t 01904 32 8549; f 01904 32 8564; rhizas, even though they can often acquire phosphate root tips, coated with fungal hyphae, are very distinctive, where plant litter accumulates because halt. Again, the symbiosis works as an e [email protected]) themselves, because the fungi do more than forage for phos- hence the name ectomycorrhiza (Fig. 3). The ectomycorrhizal decomposition (and hence the nitrogen exchange: the heather gets nitrogen phate: they can acquire other nutrients from soil, especially symbiosis seems to have evolved at least twice, in each case cycle) is slow, leading to nitrogen and the fungus gets sugars. the less soluble and hence less mobile ones such as zinc; they with woody plants – first with pines and their relatives, and deficiency. As far as we know, arbu- Orchids too form their own mycor- Further reading can protect roots from other fungi that may be parasitic or later with woody flowering plants, such as oaks, beeches scular mycorrhizal fungi cannot direct- rhiza, but a very odd one: the orchid is Helgason, T. & others (2002). pathogenic; and they can bind roots to soil so improving their and birches (Fagales). The same fungi are involved in each ly breakdown organic matter, and are parasitic on the fungus, shown most Selectivity and functional diversity in drought resistance.
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