Orchids attract considerable attention largely because of their extraordinary diversity. Orchids are distributed throughout all moist habitats, and a few are found in deserts. Species are found in soil, in litter on the ground, or attached to plant surfaces as epiphytes. All species form very small seeds, and require a fungus to germinate seed and nourish seedlings in the wild. This latter interaction is the subject of this page.
Some orchids regenerate from tubers or modified shoots each year, others have perennial leaves. None are known to be annuals. Some are completely subterranean while most have much of their body above ground for much of the growing season. Most are commonly chlorophyllous, and a few are parasitic on their fungus.
Seeds of orchids are undifferentiated, and lack significant reserves of nutrients. Germination depends on colonisation by a specific mycorrhizal fungus. Germination follows a similar pattern in most cases. The seed imbibes water. The fungus penetrates the testa of the seed and enters either through epidermal hairs or the suspensor of the undifferentiated embryo. After invagination of the plasmamenbrane the fungus forms a tight coil or peleton in the cell. The peleton remains active for some time, but then collapses. The fungus colonises further cells, and the mycorrhiza spreads.
Initial contact between fungus and imbibed seed can have one of three results: the fungus and plant form a functional mycorrhiza, the fungus can parasitise the seed, or the fungus remains outside the seed. In any one batch of seed, all three processes seem to take place. That is, some seed are potentially germinable, some are parasitised and die, and others remain ungerminated, perhaps because of a lack of recognition between symbionts. The issue of specificity will be covered later.
The process of plant tissue differentiation follows initiation of colonisation. The imbibed seed starts to expand and cells multiply. The fungus colonises further cells of the basal region. Cells of the apex start to differentiate to eventually become the shoot tissue. The tip becomes slightly pointed. From this time, the plant is called a protocorm. The uncolonized tip continues to grow and becomes the shoot. The basal region remains the absorbing region, eventually differentiating into structures that resemble roots.
Orchid mycorrhizas are different from other types of mycorrhizas in the nature of the nutrient exchange. After establishment of a mycorrhiza, organic carbon and minerals are passed from the fungus to the seed. Because asymbiotic germination requires sugars, amino acids and vitamins, it is assumed that these are also obtained from the fungus. The fungus continues to supply the protocorm with all its organic energy until such times as the plant starts to photosynthesise. Even then, it appears that the fungus does not gain significant supplies of carbon from the photosynthetic partner.
The organic materials surrounding the plant may also provide nutrients to mycorrhizal fungi. Mycorrhizal fungi have the potential to solubilise complex carbohydrates, including cellulose. The fungi translocate trehalose in the hyphae and hexose is made available at the interface. Thus the fungus acquires, translocates and transfers organic energy to and from the plant. This role is significant even in adult plants, especially epiphytes that exist in shadows of the canopy.
The exception is for achlorophyllous orchids. In the few cases where they have been examined, the fungus is also an ectomycorrhiza associated with adjacent plants. Thus transfer of photosynthates from the photosynthetic host to orchid via the interconnected fungus is most likely.
Recent studies have provided evidence for increased mineral uptake. Concentrations of P and N increase in orchids, and because roots are limited in extent, it is assumed that the fungus contributes most of the plants needs. This aspect requires much more research. A few studies have demonstrated solubilisation of organic sources of P and N, and these would be the most likely plant and fungal source of minerals.
Any benefit the fungus gains from the association is unclear. Yet the fungus appears to be only associated with its host in the wild. Again, research to elucidate fungal benefit from orchid mycorrhiza is needed.
The interaction between plant and fungus is highly regulated by the plant. The plant releases orchinol, a phytoalexin that causes the peletons to collapse. The degree of colonisation changes over the season, indicating that the orchid is controlling uptake of nutrients while preventing parasitism by the fungus.
Some terrestrial orchids have an annual cycle, whereby a period of growth is followed by loss of leaves and/or roots. The orchid then is maintained below ground until conditions become suitable for further growth. The fungus is commonly excluded from the orchid during these periods of dormancy. It can be isolated from the surface of the tuber or root, but turgid peletons are absent inside the tissues of the orchid.
The roots of epiphytic orchids are colonised only in zones that are in contact with organic substrate. The root is rarely colonised at the tip, and long sections of roots in the air remain potentially availale for initiation of mycorrhizas. Like terrestrail orchids, epiphytic orchids may have periods where they are poorly colonised. In the absence of mycorrhiza, the fungus may be retained in the velamen of the root, or the organic substrate.
The orchid is colonised in “roots” or stems. Where the terrestrial plants have a "root", the fungus usually penetrates the cortex of the root close to the root tip. In some epiphytes, penetration takes place some distance from the tip. In all cases, entry to the cortex is via a passage cell of the exodermis.
The fungus then spreads rapidly within the cortex, forming peletons in each of the cells. The fungus does not penetrate between the cells, and in this is similar to Paris – type arbuscular mycorrhizas, and the coils of ectendomycorrhizas.
Other fungi penetrate the root. While they need not form peletons, they do appear to be recognised by the host plant. These fungi can be isolated from mature roots, and can be mistaken for mycorrhizal fungi. However, they cannot germinate seed, in isolation they are unlikely to enable plant growth and are therefore considered saprotrophs. The root may be colonised by more than one mycorrhizal fungus. It is common to see coils with fine hyphae alongside coils of much coarser hyphae.
The symbionts in roots of an orchid in the field are considered to be the ecological symbionts of the plant. A greater diversity of fungi can germinate seed in the laboratory than is found in the field. This larger group has been referred to as the physiological symbionts.
Physiological symbionts can include fungi from Rhizoctonia sensu stricto that are pathogens on other plants. It is also possible to isolate physiological symbionts from soil adjacent to functional plants in the field, but they are not always inside the roots. In the field, adjacent orchids of different species may be colonised by different ecologically specific fungi, differences that disappear under laboratory conditions.
The mechanisms that determine specificity appear to be fluid, but remain unclear. Of interest are the minerals available to the host plant, the carbon available to the fungus and the environmental conditions associated with the mycorrhiza.
Orchids have a unique association with a group of fungi. They use their symbiont to gain access to organic and mineral nutrients. The plant controls the degree of development of the mycorrhiza, both in space and time. The fungus appears to gain nothing from the association. Yet because the fungus recolonises the plant each year, and continues to benefit the host, it might be assumed that the plant exudes a powerful attraction. These and many other issues remain to be resolved.
Smith SE and Read DJ (2008) Mycorrhizal Symbiosis. Academic Press, London
Masuhara G and Katsuya K. (1994) In situ and in vivo specificity between Rhizoctonia spp and Spiranthes sinensis (Persoon) Ames var amoena (Orchidaceae). New Phytologist 127, 711 – 718.