Mycorrhizal fungi: growing on roots
Mycorrhizal fungi have a close and beneficial association with the roots of plants. Most plants co-exist with these fungi, which help them absorb nutrients from the surrounding soil.
The body of a fungus consists of many hyphae (fine threads or filaments). In mycorrhizal fungi these extend from the host plant’s roots into the soil, greatly increasing the surface area for the absorption of nutrients by both the fungus and the plant. In return the fungus, which cannot make food from photosynthesis as plants do, receives carbohydrates produced by the plant’s leaves.
There are two main groups of mycorrhizal fungi: ectomycorrhizal and endomycorrhizal.
The hyphae of these fungi form a covering around the root tips of host trees, grow between the cells of the root cortex, and cause the root tips to become branched. They usually co-exist with one or only a few plant species.
Several hundred New Zealand ectomycorrhizal species are endemic (they are found nowhere else). They are most obvious in autumn, when they send up mushrooms under trees. Cortinarius elaiochrous, for example, grows with beech (Nothofagus species), and Amanita nothofagi with beech, mānuka and kānuka (Leptospermum and Kunzea species).
Other ectomycorrhizal fungi include:
- brown and purple Phellodon species, with a toothed lower surface
- robust branched coral fungi (e.g. Ramaria species)
- boletes with a central stalk and lower surface of pores (e.g. Tylopilus species)
- many truffle-like fungi.
Introduced ectomycorrhizal species, though less varied, are common under exotic trees such as oaks and pines. They include fly agaric (Amanita muscaria) and pine bolete (Suillus granulatus). Fly agaric has recently been found growing under native beech trees, where, as an invasive species, it may be competing with native fungi.
Endomycorrhizal fungi live on a wide range of host trees and other plants. Unlike ectomycorrhizas they do not form large, visible fruiting bodies. Instead, their hyphae form a closely woven cover around plant roots, and nutrients are exchanged through fine branches (arbuscules) that push into plant cells without breaking the cell membrane. They are mostly only visible to the naked eye as packets of spores (often yellow) among soil particles.
Truffles, an expensive fungal delicacy, grow underground and are sniffed out by trained dogs. The introduced black truffle Tuber melanosporum is now cultivated in New Zealand as an ectomycorrhizal associate on the roots of oak and hazel trees. The native truffle-like fungi are small and sometimes colourful. They mostly live on the roots of beech or mānuka/kānuka trees, and are probably spread when animals such as ground-feeding birds eat them.
Fungi and orchids
Another kind of fungus–plant association exists with non-photosynthetic orchids. In New Zealand one example is a three-way relationship involving harore (the native bootlace mushroom), the parasitic orchid Gastrodia cunninghamii, and beech trees. Harore acts as a go-between, passing sugars from the host beech tree to the orchid, which cannot produce food by photosynthesis. In this case the orchid is acting as a parasite of the fungus.