Arbuscular Mycorrhizae

Arbuscular mycorrhizae (AM) are the most common type of mycorrhizae on the planet. Ninety percent of all plant families contain AM species, and most mycorrhizasts believe that the vast majority of all land plants (from hornworts to grasses) are AM. These mycorrhizae are ubiquitous, living on at least six of the seven continents and almost all islands. The association is also old: Asteroxylon, a 400 million year old vascular plant fossil from the Rhynie chert, shows arbuscules in its rhizomes (Smith and Read 1997, Pirozynski 1981). Because of these fossils, AM are assumed to predate the evolution of roots, and it is hypothesized that these mycorrhizae played an integral role in the evolution of land plants. The available evidence indicates that arbuscular mycorrhizae are an ancestral characteristic of all vascular plants if not all land plants, and all other mycorrhizal types, as well as non-mycorrhizal states, are derived and more recent.

The nomenclature of arbuscular mycorrhizae has changed over the last two decades. Currently, many researchers refer to them as arbuscular mycorrhizae (AM). They are also commonly referred to as vesicular arbuscular mycorrhizae (VAM), and in the older literature, they may be referred to as endomycorrhizae.

All of these terms refer to mycorrhizal structures. In arbuscular mycorrhizae, the fungus penetrates the soil for a few centimeters (or less) around the fine roots. The fungi penetrate the roots, ramifying throughout the primary root cortex, growing between the walls of the plant cells. To exchange nutrients, the fungi penetrate the cell walls and form a branching, tree-like structure termed an arbuscule inside the cell lumen, and the plant invaginates its plasma membrane, matching the branching pattern of the fungus. The resulting dendritic structure has an large membrane surface area, and these membrane interfaces are where nutrient transfer between plant and fungus occurs. Because the fungi penetrate the cell wall, the structures were referred to as endomycorrhizae (in contrast to ectomycorrhizae, in which the fungi do not penetrate the cell wall). Since other mycorrhizal fungi (notably those in orchid and ericoid mycorrhizae) penetrate the cell wall as well, the term endomycorrhiza has been abandoned as a specific term for this group.

One group of arbuscular mycorrhizal fungi (suborder Glomineae) also forms vesicles, which are sac-like terminal swellings on hyphae inside the plant root. The function of vesicles is unclear, but they are thought to store lipids. These structures gave rise to the term vesicular-arbuscular mycorrhizae (VAM) for the whole group, and most species of arbuscular mycorrhizal fungi belong to the Glomineae. However the other AM suborder (Gigasporeae) does not form vesicles, leading many mycorrhizasts to use arbuscular mycorrhizae as the umbrella term for the whole group. VAM is a more distinctive (and searchable) acronym than AM, and the terminology remains unsettled.

The fungal partners in this relationship are a group of 200-300 species currently called the order Glomales. This group was classically placed in the Zygomycota, but the current evidence suggests that it is sister to the Ascomycota and Basidiomycota. Within the order, there are generally agreed to be two suborders (Glomineae and Gigasporineae), and thereafter the taxonomy becomes messy. Currently, the genera Glomus, Paraglomus, Archaeospora, Acaulospora, Entrophospora, Gigaspora, Scutellospora and Geosiphon are recognized. Gigaspora and Scutellospora form the Gigasporineae, and the other genera fall into the Glomineae. Glomus contains most of the Glomalean species, and the other genera contain roughly 1-20 species each. All but Geosiphon are exclusively mycorrhizal ( Morton and Redecker 2001, Redecker, Morton and Bruns 2000).

Geosiphon piriforme is a bizarre fungus that grows on a few muddy patches in German fields. Instead of forming mycorrhizae, Geosiphon is lichen-like, retaining cells of the cyanobacteria Nostoc inside enlarged, finger-like hyphal structures. The cyanobacteria provide carbon and nitrogen to the fungus, as in a lichen (Gehrig, Schubler and Kluge, 1996). Oddly, current molecular phylogenetic studies place Geosiphon inside the Glomalean clade, not in the outlying position that its wildly divergent anatomy and physiology would seem to indicate (Redecker, Kodner and Graham 2000).

The Glomales are among the most common fungi on the planet, with spores (living or dead) found on all continents and most islands. Fossil Glomalean spores have been found in 460 million year old rocks, along with fragmentary fossils of bryophytes (Redecker, Kodner and Graham 2000)(these spores predate the oldest evidence of mycorrhizae by about 60 million years, and the fossils were found in Wisconsin).

Biologically, the Glomales are unusual in a number of ways. As with most Zygomycetes, they are coenocytic and massively multinuclear. However, genetic studies have shown that they are multi-genomic, with genetic sequences from different Glomalean families found in single spores (Sanders 1999). Meiosis and genetic recombination have not been observed in the Glomales (Kuhn, Hijri and Sanders 2001), and they produce spores with up to 2,000 nuclei by simple structural differentiation of hyphal tips. Since these nuclei may represent multiple genomes, selected apparently at random, these fungi are capable of asexually producing genetically distinct spores through random selection of genetically distinct nuclei in different concentrations (Sanders 1999). Rather than having sex (undergoing syngamy), these fungi probably exchange nuclei by fusing hyphae, but this has not been definitively shown (Kuhn, Hijri and Sanders 2001). Given that these fungi have multiple nuclear genomes as well as mitochondria within their hyphae, they might metaphorically be considered as membrane-bound microbial communities, rather than individuals in any genetic sense. Despite their unique biology, there is evidence that spore morphology (used to differentiate species) is conservative and highly heritable from one generation to the next in culture (Morton, Franke and Bentivenga 1999). Conversely, there is also evidence that fungal species with identical spore morphologies have different physiologies, some being able to sequester heavy metals, for instance (Leyval, Turnau and Haselwandter 1997). The relationship among genetics, genomics, physiology and morphology in these fungi is a fascinating enigma and an area of active research. The problem is that researchers have to develop new conceptual and genetic tools to understand the Glomales; the conventional tools used to study monogenomic organisms such as humans, mice, flies or yeast are inadequate. Just to make things more interesting, the Glomales are all obligate biotrophs, in that they have little or no ability to take in carbohydrates from the environment, and depend on their symbionts (plant or cyanobacterial) to provide their energy. This means that, apart from Geosiphon, all Glomalean fungi have to be grown on a plant. In decades of attempts, no one has succeeded in growing any AM fungus in pure culture for more than a few weeks (Smith and Read 1997).

By all evidence, arbuscular mycorrhizae are the ancestral state for all vascular plants, if not all plants. Hornworts (Anthocerophyta) and liverworts (Hepatophyta) have been induced to form mycorrhizae-like structures with Glomales in their thalli (Read et al 2001), so it is likely that the genes for arbuscular mycorrhizal formation came into being very early in land plant evolution. Many plant clades have apparently independently lost the ability to form AM, and many plant clades have evolved new forms of mycorrhizae, sometimes with the concurrent loss of AM production, usually without. Arbuscular mycorrhizae are therefore not an adaptation to any particular environmental condition, but loss of these structures likely is adaptive or the result of an adaptation.

Among plants that are AM, the relationship between plant and fungus is murky. The fungi exhibit some degree of host specificity, but no one has been able to find a pattern. Apparently, most Glomalean fungi and most plants, when put together in a pot, will form some sort of mycorrhizae. In most cases, the growth rates of innoculated AM plants increase, but rates vary widely down to zero. The amount of root infected, arbuscules formed, and physiological changes vary as well, depending not only on the species and strains involved, but also on growing conditions.

Under controlled conditions, the relationship between plant and fungus is simple. The plant provides carbohydrates to the fungus, and the fungus provides nutrients to the plant. Phosphorus is the principal nutrient provided, but the fungi have also been shown to increase plant uptake of micronutrients and apparently nitrogen. The fungi possess much the same enzyme systems as do plants, so they do not benefit the plant by exploiting nutrient pools (such as detritus) that the plant cannot reach. Rather, fungal hyphae are much finer than root hairs, allowing them to more thoroughly penetrate the soil. Phosphorus is an extremely immobile element, and a root or hypha has to be at most a few micrometers from a phosphorus atom in order to take it up. This contrasts with elements such as nitrogen, which are water soluble and are carried through the soil by water. Arbuscular mycorrhizae contribute relatively little to nitrogen uptake in plants (Smith and Read 1997).

The plants seem to monitor levels of phosphorus uptake (or perhaps levels of P in their tissues), and above a species-specific level, the plant will eject the fungus from its root. In domestic plants, the critical P concentration seems to be around 100 ppm (Fitter and Merryweather 1992). In wild prairie grasses, the level drops as low as 20-40 ppm (Schultz et al 2001).

It is a truism among mycorrhizasts that it is much, much harder to show the nutritional benefits of AM on plants in the wild (Allen 1991), although it has been done (Fitter and Merryweather 1992). Glomalean fungi are ubiquitous in the soil, and while they can be surpressed with fungicides (Benomyl is commonly used), the fungicides also suppress or kill many other soil fungi, including pathogens (which would inhibit plant growth much as mycorrhizae enhance it), complicating the interpretation of any experiment. Moreover, Glomales seem to have benefits beyond simple nutrition. Several experiments have shown that Glomalean fungi can inhibit some pathogenic fungi, apparently by occupying the same space in the root. Other pathogens, however, are apparently favored by AM (Smith and Read 1997). In addtion, Glomalean hyphae in the soil may be food for grazers such as collembolans, which, by reducing soil hyphae, decrease the ability of the fungus to take up nutrients. Such multitrophic and multi-organismal interactions are only starting to be explored, not surprisingly, given the technical challenges that such research presents (Fitter and Merryweather 1992).

One intriguing area of research is the effect that AM have on plant diversity. Several greenhouse experiments have shown that, in mesocosms planted with many plant species, Glomalean fungi increase both biomass and diversity, usually by differentially favoring the growth of smaller plants (Grime 1988, van der Heijden et al 1998). The van der Heijden et al research also showed that there was a positive correlation between the number of fungi present in the microcosm and the diversity of plants growing in it. This is exciting to ecologists, because one of the classic problems in restoring plant communities is that the number of plant species in restorations is usually lower than in remnants of the same community, no matter how many plant species are seeded in. It is possible that increasing the number of fungal species will make restorations more succesful. Unfortunately, AM can also decrease diversity. In cases where a dominant plant species is AM (such as the C4 grasses in tallgrass prairies), or where an invasive weed is AM, the presence of the fungi can decrease plant community diversity by favoring the dominant species at the expense of others (Hartnett and Wilson 1999, Marler et al 1999).

Basically, the result of decades of research is that arbuscular mycorrhizae are much more complicated than researchers might have hoped. Given the ubiquity and antiquity of this relationship, this is perhaps not surprising, and it may be why this particular type of symbiosis has endured for so long. It is perhaps a cheering thought that something so commonplace as arbuscular mycorrhizae can be so enigmatic.