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Orchid mycorrhiza

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Orchid mycorrhizae are symbiotic relationships between the roots of plants of the family Orchidaceae and a variety of fungi. 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.[1][2]

The symbiosis starts with a structure called a protocorm.[3] During the symbiosis, the fungus develops structures, called pelotons, within the root cortex of the orchid.[4]

Many adult orchids retain their fungal symbionts, although the benefits to the adult photosynthetic orchid and the fungus remain largely unexplored.

Seed germination

Orchid protocorm

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.[5] Each seed has an embryo that is undifferentiated and lacks root and shoot meristems.[3] An orchid seed does not have enough nutritional support to grow on its own.[2] Instead, it gets nutrients needed for germination from outside sources. In nature, the nutritional support comes from a fungal partner.[4] When the orchid seeds geminate, they form seedling structures called protocorms which consist of parenchyma cells.[6][3] Infected protocorms tend to develop an active meristem within a few days.[7] 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.[8]

Orchids lacking chlorophyll, called achlorophyllous MHPs, will retain their fungal symbionts their entire lives, relying on the fungus for carbon.[9] The debate over whether fungal symbiosis is necessary for the orchid is an old one, as Noel 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 the actual fungus.[10]

Although epiphytic orchids grow on other plants they may produce chlorophyll in their leaves, stems, and roots. As with all orchids, germination and the develop into protocorms, young plants which have germinated but lack leaves, is reliant upon fungal symbionts, which decrease the time of germination and increase the vigor of the protocorm.[10] 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.[11]

Fungal entry into orchid

The fungus can enter at various orchid life stages. Fungal hyphae can penetrate the parenchyma cells of geminated orchid seeds, protocorms, late-staged seedlings, or adult plant roots.[12] The fungal hyphae that enter the orchid have many mitochondria and few vacuoles.[13] In the protocorm stage, hyphae enter the chalazal (top) end of the embryo.[14] In terrestrial orchids, fungal entry into adult plant roots happens mainly through root hair tips, which then take on a distorted shape.[2] Typically, the partnership is maintained throughout the lifetime of the orchid because they depend on the fungus for nutrients including minerals.[15] However, some orchids have been found to switch fungal partners during extreme conditions.[16]

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.[4] The formation of pelotons in root cortical cells is a defining anatomical structure in orchid mycorrhiza that differentiate it from other forms of mycorrhiza.[17] The pelotons can range in size and in the loose or tight packaging of their hyphae.[7] Pelotons of live fungal hyphae are eventually disintegrated, or lysed, and become brown or yellow clumps in the orchid cells.[6] 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.[18] The cortical cells of older roots tend to have more lysed pelotons than young pelotons.[2] Although pelotons are lysed, new pelotons continue to be formed, which indicates a high amount of entering hyphal activity.[8] Pelotons are separated from the orchid's cytoplasm by an interfacial matrix and the orchid's plasma membrane.[18] The material that makes up the interfacial matrix can differ depending on the orchid mychorrizal stage of interaction.[19] 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.[13] 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.[20] The increased DNA content has been correlated with the differentiation of parenchyma cells suggesting its role in orchid growth.[21]

Fungi forming orchid mycorrhizae

The fungi that form orchid mycorrhizae are typically basidiomycetes. These fungi 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,[22] partially due to the difficulty in culturing fungi from protocorms and identification of fungal samples,[22] as well as changes in evolving rDNA.[23] However it has become clearer that different fungi may associate with orchids at specific stages, whether at germination or protocorm development, or throughout their life.[24] 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.[25]

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.[26] 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.[27][28] Epiphytic fungi, however, may associate more commonly with limited clades of rhizoctonia, a polyphyletic grouping.[29] These fungi may form significant symbioses with either an epiphytic or terrestrial orchid, but rarely do they associate with both.[29] Using seed-baiting techniques researchers have been able to isolate specific species and strains of symbiotic orchid mycorrhizae. Using this technique seeds of the epiphytic orchid Dendrobium aphyllum were found to germinate to seedlings when paired with fungal strains of Tulasnella, however, seeds of this species did not germinate when treated with Trichoderma fungi taken from adult orchids, indicating there are stage specific symbionts. These fungal symbioses, as well as their affinity towards specific symbionts, vary based on the stage of development and age of the host roots[30][9][24] As an orchid ages the fungal associations become more complex.

Mixotrophic orchids like Cephalanthera longibracteata may associate generally with several fungi, most notably from Russulaceae,Tricholomataceae, Sebacinales, Thelephoraceae.,[31] 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.[32] The terrestrial Chilean orchids Chloraea collicensis and C. gavilu have been found to have only one key symbiont in the genus Rhizoctonia.[33] Research suggests that orchids which are considered to be generalists will associate with other generalist fungi, often with several species, however orchids with high degrees of specialization will have less fungal associations.[23]

The environment may also affect the fungal symbiosis, with differences in tropical versus temperate orchids. 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.[16] This is unique in that it conflicts with the trends seen in most arbuscular and ectomycorrhizal symbioses which form associations with several fungi at the same time.[16]

The knowledge of species-specific fungal symbionts is crucial given the endangered status of many orchids around the world. The identification of their fungal symbionts could prove useful in the preservation and reintroduction of threatened orchids.[26] Researchers in India have used samples from adult orchids to germinate seeds from an endangered orchid Dactylorhiza hatagirea which were found to associate closely with Ceratobasidium.[34]

References

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  30. ^ Zi, Xiao-Meng; Sheng, Chun-Ling; Goodale, Uromi Manage; Shao, Shi-Cheng; Gao, Jiang-Yun (2014-10-01). "In situ seed baiting to isolate germination-enhancing fungi for an epiphytic orchid, Dendrobium aphyllum (Orchidaceae)". Mycorrhiza. 24 (7): 487–499. doi:10.1007/s00572-014-0565-8. ISSN 0940-6360. PMID 24563211.
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  32. ^ Taylor, D. Lee; Bruns, Thomas D.; Hodges, Scott A. (2004-01-07). "Evidence for mycorrhizal races in a cheating orchid". Proceedings of the Royal Society of London B: Biological Sciences. 271 (1534): 35–43. doi:10.1098/rspb.2003.2557. ISSN 0962-8452. PMC 1691555. PMID 15002769.
  33. ^ Pereira, Guillermo; Romero, Christian; Suz, Laura M.; Atala, Cristian (2014). "Essential mycorrhizal partners of the endemic Chilean orchids Chloraea collicensis and C. gavilu". Flora - Morphology, Distribution, Functional Ecology of Plants. 209 (2): 95–99. doi:10.1016/j.flora.2013.12.001.
  34. ^ Aggarwal, Simmi, Zettler, Lawrence W. (2010). "Reintroduction of an endangered terrestrial orchid, Dactylorhiza hatagirea (D. Don) Soo, assisted by symbiotic seed germination-First report from the Indian subcontinent". Nature and Science. 8 (10): 139–145 – via ResearchGate.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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