Orchid mycorrhiza

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. 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. == 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. 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. 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. 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, 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 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. 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. 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. 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. but orchid mycorrhizal nutrient transfer is less specific (but no less regulated) and there is often bidirectional flow of carbon between the fungus and plant, as well as flow of nitrogen and phosphorus from the fungus to plant. In around 400 species of plants there is no flow of carbon from plant and all of the nutrients of the plant are supplied by the fungus. Nitrogen is significantly easier to obtain than phosphorus and far more abundant, but mycorrhizal interactions still provide a significant benefit in the allocation of nitrogen. Bioavailable nitrogen as nitrate and ammonium are absorbed from the soil media into the extraradical mycelium of the mycorrhizal fungi and assimilated into amino acids. Amino acids contain a significant amount of carbon as well and the transport of carbon may be the primary driving cause of the observed upregulation of the amino acid transporter genes, nonetheless nitrogen can still be transported to the plant via this pathway. The transport of inorganic nitrogen in the form of ammonium and the transport of organic nitrogen as amino acids most likely will occur simultaneously in the same species and or organism, depending on the abundance of different nitrogenous species in the soil, These reactions are often instigated and progressed by the activity of the mycorrhizal fungus, being part of the basidiomycete phyla, orchid associated fungi have an extensive metabolic arsenal with which to pull from, and are readily able to digest cellulose and lignin to obtain the carbon. Further more the fungi which mycoheterotrophically interact with orchid plants are often also found in mycorrhizal association with beech trees, and translocation of photosynthate from tree to fungus and then to orchid has been proposed, but a thorough study is still lacking. Once acquired by the fungus the carbon is either converted into sugars, trehalose being extremely common for most mycorrhizal fungus, amino acids, or simply assimilated into the fungal organism. The transfer of carbon from fungi to plant happens in one of two forms either as carbohydrates primarily trehalose, but glucose and sucrose may also be involved, or as an amino acids primarily as arginine but glycine and glutamine can also be transferred. Once the fate of the peloton is decided, and degradation and digestion are to occur, a secondary membrane forms around the fungal peloton which is essentially a large vacuole which will allow the isolated degradation of the peloton. Micro-nutrient transfer Micro-nutrient transfer is thought, for the most part, to occur by passive transport across cellular membranes, both during absorption, from soil by fungi, and transfer from fungi to host plants. The upregulation of cation transporters is observed in orchid D. officinale symbioses, suggesting fungi may assisted in the transfer of nutrients from fungi to plant. Cations, especially iron, are often bound tightly to organic and clay substrates keeping them out of reach of plants, fungi and bacteria, but compounds such as siderophores are often secreted into the soil by fungi and bacteria to aid in the acquisition of these cations. These compounds are released into the soil surrounding the hyphal web and strip iron from mineral compounds in the soil, the siderophore can then be reabsorbed into the fungal hyphae where the iron can be dissociated from the spiderohore and used. Haselwandter investigated the presence of siderophores in orchid associated mycorrhizal fungi within the genus Rhizoctonia which utilize the siderophore basidiochrome as the major iron-chelating compound. Other vital nutrients may be transferred between mycorrhizal fungi and orchid plants via specialized methods, such as chelating molecules, but more research on this subject is needed. == Symbiont specificity ==

Current molecular analysis has allowed for the identification of specific taxa forming symbiotic relationships which are of interest in the study, cultivation, and conservation of orchids. This is especially important in the trade and preservation endangered species or orchids of commercial value like the vanilla bean. There have been seen trends in the type of symbioses found in orchids, depending primarily on the life-style of the orchid, as the symbiosis is primarily of benefit to the plant. Terrestrial orchids have been found to commonly associate with Tulasnellaceae, however some autotrophic and non-autotrophic orchids do associate with several ectomycorrhizal fungi. Epiphytic fungi, however, may associate more commonly with limited clades of rhizoctonia, a polyphyletic grouping. These fungi may form significant symbioses with either an epiphytic or terrestrial orchid, but rarely do they associate with both. as they do not depend as heavily on the fungus for carbon. Some orchids can be very specific in their symbionts, preferring a single class or genus of fungi. Genotypes of Corallorhiza maculata, a myco-heterotrophic orchid, have been found to closely associate with Russulaceae regardless of geological location or the presence of other orchids. The terrestrial Chilean orchids Chloraea collicensis and C. gavilu have been found to have only one key symbiont in the genus Rhizoctonia. Research suggests that orchids considered to be generalists will associate with other generalist fungi, often with several species, however orchids with high degrees of specialization use a more limited range of fungal partners. Conversely, the photosynthetic orchid Goodyera pubescens was found to associate with only one dominate fungus, unless subjected to changes in the environment, like drought, in which case the orchid was able to change symbionts in response to stresses. There is a tremendous amount of diversity in orchid fungal specificity. Interestingly, most arbuscular and ectomycorrhizal symbioses form associations with several fungi at the same time. == References ==