Orchidoid mycorrhizae, as the name suggests, are characteristic of roughly the 20,000 species in the orchid family, and are not known in any other group. Orchids are unusual in that they all pass through an obligate mycoparasitic stage in the course of normal development. Like other mycoparasitic plants, they have a number of structural specializations to this way of life, including their own distinctive form of mycorrhiza.

Orchid seeds are dustlike, consisting of a tiny spherical embryo with no endosperm and a thin seed coat. The seed may or may not germinate in the presence of a suitable mycorrhizal fungus, but it cannot grow until a fungus has infected it. Once fungal contact has been made, the orchid seedling grows into a callus-like lump of tissue termed a protocorm (which corresponds to the hypocotyl and radicle of other plants). The protocorm is achlorophyllous, deriving all of its nutrients and energy from its fungal host. Once the protocorm has grown to a sufficient size, the plant shoot starts to grow, producing a structure termed the mycorrhizome (the earliest and most heavily infected part of the rhizome), which produces the first true roots. The first above-ground leaves develop from the rhizome, and the plant starts photosynthesizing. In many species, the next step is the development of a non-mycorrhizal storage tuber, followed by continued plant growth and maturation of the plant.

Orchids parasitize a number of fungi, including species (such as Rhizoctonea solani and Armillaria mellea) which are well-known plant pathogens, and lab tests have shown that an embryo paired with the wrong fungus can be quickly killed by the fungus. Thus it is not surprising that orchids have specialized cells and structures for mycorrhizae. In adult plants, much of the plant body contains antifungal phytoalexins, and this is presumably true of embryos and protocorms as well.

In orchid embryos, cells near the suspensor end enlarge and undergo nuclear endoreplication (the chromosomes in the nucleus replicate repeatedly, until there are as many as 128 copies of the genome inside the nucleus). These cells are infected by the fungus, which forms a tightly coiled hyphal structure termed a peloton. As with all endocellular mycorrhizae, the fungus does not actually penetrate the cell membrane. The pelotons remain stable for a varying length of time, and then are apparently digested by the plant. The empty cells may be reinfected, or the plant may enlarge new cells, so that the band of mycorrhizae moves progressively down the plant as it grows. As in other mycorrhizal types, infection is a dynamic process. Some orchids are infected seasonally, and adults may be non-mycorrhizal for part of the year. Others are continuously infected, but the infection is confined to enlarged cells in specific parts of the plants, such as in the rhizome, in rhizome scale leaves, or in roots. In orchids, the roots are few and coarse with few root hairs, and they are frequently shed and regrown. The roots often become mycorrhizal, and in some species, mycorrhizal root fragments can grow into new plants if they are separated from the parent rhizome. There is enormous variation in patterns of organ growth and mycorrhizal across orchid genera, and the variation across the family is likely one reason why orchids have been able to live in so many different habitats throughout the world.

There are a number of achlorophyllous orchids that remain mycoparasites throughout their lives, but this can be seen as retention of juvenile traits (perhaps akin to neoteny). The achlorophyllous lifestyle has apparently evolved independently a number of times in the orchid family, and there are some intermediate genera (Corallorhiza is a local example) which apparently have enough chlorophyll to photosynthesize above compensation point (they are net producers of oxygen), but which can only grow as mycoparasites.

A large number of the fungi found in orchid mycorrhizas belong to the anamorphic genus Rhizoctonia. They include teleomorphic genera Thanetophorus, Ypisilonidium, Sebacina, and Tulasnella. Achlorophyllous orchids often seem to parasitize basidiomycetes, including members of Marasmium, Xerotus, Hymenochaete, Armillaria, Fomes, Favolaschia, Coriolus, Thelephorus and Tomentella. All of these fungi obtain carbohydrates from outside the orchid, from a variety of sources. Many of these fungi are saprophytic, able to break down dead organic matter in the soil. Others (such as Rhizoctonia solani/Thanetophorus cucumeris and Armillaria mellea) are better known as plant pathogens. Still others are ectomycorrhizal.

Orchid species vary in their fungal specificity, both among genera and over the course of their life cycles. The embryos of photosynthetic orchids appear to form mycorrhizas with fewer fungal species than the adults do. It is relatively common to find many endophytic fungal species in adult orchid mycorrhizae, and many fungal isolates from adults have failed to produce mycorrhizae with embryos of the same species. In the achlorophyllous orchids, all available evidence suggests that the plants are highly host specific, parasitizing only one or a few fungal species. Often, these plants parasitize ectomycorrhizal fungi, so that the carbohydrates the orchid gains come from the trees surrounding it.

Ecologically, orchid mycorrhizae are important in that they allow the plant to have two possible energy sources, and the plant may use fungal carbohydrates to supplement, replace or alternate with its own photosynthetic activity. This ability allows some orchid species to colonize microhabitats that are too shady for other vascular plants, and it allows the rhizomes of some species to remain leafless for seasons to years, until the aboveground environment is suitable for leaf production. Since the fungal partners are generally able to break down complex organic materials, the orchids that grow with them are able to tap unusual substrates for nutrients, including the bog peat, highly calcareous soils, and the dust and debris on tree branches. Indeed, orchid's tiny, highly dispersable seeds and mycotrophic habit are undoubtedly why they are so successful as epiphytes. Some seventy percent of orchid species are epiphytic, and they comprise perhaps two-thirds of epiphytic vascular plant species.

One final question: do the fungi get anything from this mycorrhizal relationship? In terms of nutrition, the answer appears to be no. Tests of Goodyera repens (a photosynthetic orchid) have shown that carbon flow is apparently one way, from fungus to plant. The same appears to be true for other nutrients. The only benefit that any researcher has suggested accrues to fungal species, such as Rhizoctonia solani, which do not form resistant spores. In these cases, the orchid rhizome may act as a safe site in seasonally dry soils where the hyphae would normally shrivel and die. Beyond this, it appears that the orchid's relationships with their mycorrhizal fungi are parasitic ones.

Sources: Rasmussen (1995), Arditti (1992), Smith and Read (1997)