Allotropa virgata, or candystick, is a perennial species of western North America. It is a mycoheterotroph, meaning that it forms a part of a complex threesome, obtaining all of its carbohydrates and at least some of its minerals from a fungus, which in turn obtains its carbohydrates from a tree or shrub. The fungus provides an important service to the tree or shrub (helping it to absorb water and minerals), but it does not appear that Allotropa virgata has anything to offer to its partners.
A 2006 article in Mycena News (PDF) explains that mycoheterotrophy represents a type of “hacking” into the mycorrhizal network (PDF) (a network that moves carbon between all plants that share a fungal symbiont). Mycoheterotrophs such as Allotropa virgata use similar methods to carbon-producing members to tap into this system, and seem to remain undetected by the other members of the network. For this reason, mycoheterotrophs are sometimes referred to as mycorrhizal cheaters. Allotropa virgata was featured on Botany Photo of the Day in 2005, and a related species, Monotropa uniflora, was written about in the same year. Those articles add to this explanation of mycoheterotrophy.
Candystick belongs to a subfamily of the Ericaceae, Monotropoideae. Most members of the Monotropoideae are very specific in their choice of fungal partners; candystick is associated with only one species of fungus, Tricholoma magnivelare, or American matsutake (see Fine-level mycorrhizal specificity in the Monotropoideae (Ericaceae): specificity for fungal species groups (Bidartondo and Bruns 2002)). Until recently, little was known about the structural form of the root-fungus connection between these species, but an article by Massicotte et. al (PDF) provides some clarity. Through microscopic examination, Masicotte et. al. determined that the mycorrhizal structure of this species places it well within the general characteristics of monotropoid-type mycorrhizas (described below).
The roots of Allotropa virgata are slender and brittle, consisting of elongated rhizomes with mycorrhizal adventitious roots. Like the other monotropoid mycorrhizas, only the epidermis, or outside layer of the roots, is colonized by the fungus. Surrounding the colonized portion of the roots is a mantle composed of layers of fungal hyphae (the long, filamentous structure of fungus). A Hartig net–a hyphal structure that connects to the roots of its host–is joined to Allotropa virgata via fungal pegs. The fungal pegs are composed of individual hyphae that grow out of the Hartig net and protrude into the plant cell wall. The cell walls invaginate to enclose these hyphae in finger-like wall projections. There is still debate about the function of the fungal pegs, but it seems likely that these are a means to transfer nutrients between the fungus and its host. David Moore (see monotropoid mycorrhizas) explains that fungal peg formation increases in the spring, during the period when Monotropa species are forming their flowering scapes and energy needs are high. The pegs open in July and August when the host species is releasing its seeds, likely supplying a final burst of energy at this critical time.
For those of you who are fans of the complex and spicy (yet slightly foul) taste of American matsutake mushrooms, take note of any candystick that you notice blooming in the spring. Since Allotropa virgata is associated only with this fungus, it is a reliable indicator of where you may find American matsutake fruiting in the autumn.