Orchid mycorrhizae are endomycorrhizal fungi which develop symbiotic relationships with the roots and seeds of plants of the family Orchidaceae. Nearly all orchids are myco-heterotrophic at some point in their life cycle. Orchid mycorrhizae are critically important during orchid germination, as an orchid seed has virtually no energy reserve and obtains its carbon from the fungal symbiont.
The symbiosis starts with a structure called a protocorm. During the symbiosis, the fungus develops structures called pelotons within the root cortex of the orchid.
Many adult orchids retain their fungal symbionts throughout their lives, although the benefits to the adult photosynthetic orchid and the fungus remain largely unexplained.
Seed germination
Orchids have several life stages. The first stage is the non-germinated orchid seed, the next stage is the protocorm, and the following stage is the adult orchid. Orchid seeds are very small (0.35mm to 1.50mm long), spindle-shaped, and have an opening at the pointed end. Each seed has an embryo that is undifferentiated and lacks root and shoot meristems. An orchid seed does not have enough nutritional support to grow on its own, and lacks endosperm. Instead, it gets nutrients needed for germination from fungal symbionts in natural habitats.
When an orchid seed germinates it forms an intermediate structure called protocorm, young plants which have germinated but lack leaves and which consist mainly of parenchyma cells. Infected protocorms tend to develop an active meristem within a few days. In the adult stage, many orchids have a small amount of thick unbranched roots which results in a root system with a small surface area that is favorable to potentially mycotrophic tissue.
Orchids lacking chlorophyll, called achlorophyllous mycoheterotrophs, will retain their fungal symbionts their entire lives, relying on the fungus for carbon.
The debate over whether fungal symbiosis is necessary for the orchid is an old one, as Noël Bernard first proposed orchid symbiosis in 1899. In 1922 the American botanist Lewis Knudson discovered that orchid seeds could be grown on agar and fungal sugars without mycorrhizae, however modern research has found that the germination of orchids may be more successful with specific fungi.
Although epiphytic orchids grow on other plants they may produce chlorophyll in their leaves, stems, and roots. As with all orchids, germination and the development into protocorms is reliant upon fungal symbionts, which decrease the time of germination and increase the vigor of the protocorm. The reliance of orchids on specific fungi has been widely studied, and the populations of certain fungi which are present in the soil have proved to be of greater importance in seed germination than the orchid's proximity to older plants or their geographical location, as previously assumed.
Fungal entry into orchid
Fungi can enter at various orchid life stages. Fungal hyphae can penetrate the parenchyma cells of germinated orchid seeds, protocorms, late-staged seedlings, or adult plant roots. The fungal hyphae that enter the orchid have many mitochondria and few vacuoles., thus increasing their metabolic capacity when paired with an accepting symbiote. In the protocorm stage hyphae enter the chalazal (top) end of the embryo, however in terrestrial orchids fungal entry into adult plant roots happens mainly through root hair tips which then take on a distorted shape. The symbiosis is typically maintained throughout the lifetime of the orchid because they depend on the fungus for nutrients, sugars and minerals. However, some orchids have been found to switch fungal partners during extreme conditions.
Fungal pelotons and orchid root cortex Shortly after the fungus enters an orchid, the fungus produces intracellular hyphal coils called pelotons in the embryos of developing seedlings and the roots of adult plants. The formation of pelotons in root cortical cells is a defining anatomical structure in orchid mycorrhiza that differentiate it from other forms of fungi. The pelotons can range in size and in the arrangement and density packaging of their hyphae. Pelotons are separated from the orchid's cytoplasm by an interfacial matrix and the orchid's plasma membrane. The material that makes up the interfacial matrix can differ depending on the orchid mychorrizal stage of interaction. Orchid cells with degenerating pelotons lack starch grains, whereas the newly invaded orchid cells contain large starch grains, suggesting the hydrolysis of starch resulting from the fungal colonization. There is an enlargement of the nucleus in infected orchid cortical cells and in non-infected cortical cells near an infected area as a result of increased DNA content. The increased DNA content has been correlated with the differentiation of parenchymal cells suggesting its role in orchid growth. As pelotons of live fungal hyphae age and are eventually disintegrated, or lysed, they appear as brown or yellow clumps in the orchid cells. The disintegrated pelotons are an area of considerable interest in current research. The disintegrated pelotons first experience a collapse where orchid microtubules surround the pelotons, which may be the mechanism behind the peloton collapse by producing physiological and structural changes of the hyphae. The cortical cells of older roots tend to have more lysed pelotons than young pelotons. Although pelotons are lysed, new pelotons continue to be formed, which indicates a high amount of new hyphal activity.
Fungi Basidiomycetous Rhizoctonia Fungi
The fungi that form orchid mycorrhizae are typically basidiomycetes. These come from a range of taxa including Ceratobasidium (Rhizoctonia), Sebacina, Tulasnella and Russula species. Most orchids associate with saprotrophic or pathogenic fungi, while a few associate with ectomycorrhizal fungal species. These latter associations are often called tripartite associations as they involve the orchid, the ectomycorrhizal fungus and its photosynthetic host plant. Some of the challenges in determining host-specificity in orchid mycorrhizae have been the methods of identifying the orchid-specific fungi from other free living fungal species in wild-sourced samples. Even with modern molecular analysis and genomic databases, this can still prove difficult, partially due to the difficulty in culturing fungi from protocorms and identification of fungal samples, as well as changes in evolving rDNA. However it has become clearer that different fungi may associate with orchids at specific stages, whether at germination, protocorm development, or throughout the orchid's life. The types of orchids and their symbiotic fungi also vary depending on the environmental niches they occupy, whether terrestrial or growing on other plants as an epiphyte.
Ectomycorrhizal Basidiomycota
A number of other basidiomycetous fungi have been documented in various orchids that do not fall under the Rhizoctonia umbrella. In particular, many fully mycoheterotrophic orchids associate with ectomycorrhizal basidiomycetes belonging to genera such as Thelephora, Tomentella and Russula.
Basidiomycetes of the Atractiellales
Ascomycota
Though rare in orchids, ascomycete associations have been documented in several orchid species. The European terrestrial orchid Epipactis helleborine has a specific association with ectomycorrhizal ascomycetes in the Tuberaceae.
Nutrient transport
Orchids mycorrhiza (OM) are found in approximately 10% of the botanical diversity of earth and have unique and specialized mycorrhizal nutrient transfer interactions which define the fitness and diversity of the orchid family. Orchid mycorrhizal associations involve a plethora of unique nutrient transport systems, structures and phenomena which have only been observed in the family Orchidaceae. These interactions are formed between basidiomycete fungi and all Orchidaceae species. The basidiomycete fungi most commonly found associated with orchids, rhizoctonia, are known for their saprophytic abilities making this fungi orchid association anomalous, allowing both the plant and fungi to access a source of carbon within the mycorrhizal association not available in arbuscular mycorrhiza.
The way and degree to which different orchid species exploit these interactions varies. Orchid mycorrhizal interactions can range from wholly parasitic on the fungal partner, to a mutualistic interaction involving bidirectional nutrient transfer between the plant and mycorrhizal fungus. Orchid plants have an obligatory parasitic life stage at germination where all of their nutrients must be supplied by a fungus. After germination, the orchid mycorrhizal interactions will become specialized to utilize the carbon and nutrients available in the environment surrounding the interaction. These associations are often thought to be dictated by the plant. It has been indicated in past studies that orchid plant individuals which inhabit dense highly shaded forests may depend significantly more on their fungal partner for carbon, varying within and between species, demonstrating the variable and reactive nature of these interactions.
Nutrient transfer interfaces and mechanisms At infection of an orchid by a mycorrhizal fungus both partners are altered considerably to allow for nutrient transfer and symbiosis. Nutrient transfer mechanisms and the symbiotic mycorrhizal peloton organs start to appear only shortly after infection around 20–36 hours after initial contact. There is significant genetic upregulation and downregulation of many different genes to facilitate the creation of the symbiotic organ, and the pathways with which nutrients travel. As the fungus enters the parenchyma cells of the orchid the plasma membrane invaginates to facilitate fungal infection and growth. This newly invaginated plasma membrane surrounds the growing pelotons and creates a huge surface area from which nutrients can be exchanged. The pelotons of orchid mycorrhiza are intensely coiled dense fungal hyphae that are often more extensive in comparison to endomycorrhizal structures of arbuscular mycorrhiza. The surrounding plant membrane essentially becomes rough endoplasmic reticulum with high amounts of ribosomes and a plethora of transporter proteins, and aquaporins. Additionally there is evidence from electron microscopy that indicates the occurrence of exocytosis from the plant membrane. This highly convoluted and transporter rich membrane expertly performs the duties of nutrient exchange between the plant and fungus and allows for molecular manipulation by ribosomes and excreted enzymes within the interfacial apoplast. Pelotons are not permanent structures and are readily degraded and digested within 30 to 40 hours of their formation in orchid mycorrhiza. This happens in all endomycorrhizal associations but orchid plants readily digest fungal pelotons sooner after formation and more often than is seen in arbuscular mycorrhizal interactions. It is proposed that the occurrence of this more extreme digestive pattern may have something to do with necrotorphic nutrient transfer which is the absorption of nutrients form dead cells. The key nutrients involved in the majority of the transfer between fungi and orchid plants are carbon, nitrogen and phosphorus.