Biology of cultivated mushrooms

Author: András Geösel

Role and specifications of fungi

Fungi are heterotrophic (mostly osmotrophic or chilotrophic), aerobic or facultative anaerobic, eukaryotic organisms with well defined nucleus. Fungi can either be single-celled or multicellular, mostly with filamentous structures. Their cell walls contain chitin and they reproduce via spores.

Fungi are considered to be a separate group amongst all living things for a number of reasons. What sets them apart from plants is that fungi do not contain any chlorophyll, which is responsible for photosynthesis, thus the absence of chlorophyll is the reason why fungi do not photosynthesize. In the course of evolution, fungi, plants and animals developed parallel. The discipline devoted to the study of fungi is mycology and the mushroom cultivation is part of the horticultural discipline.

The biological significance of fungi is quite diverse given their role in the ecosystem and their vigorous enzyme system. Fungi are saprotrophic (feed on decomposing organic matter), symbionts (mutually beneficial association of two or more organisms), parasites (the parasite lives and feeds on the body of another organism) or even predators (the fungi feed on other fungi) species.

By the help of cultivated mushrooms, agricultural wastes and byproducts can be transformed into food with high biological value. Medicinal mushrooms are proved to be effective against certain human illnesses. Many fungi have antifungal, antibacterial properties. Moreover, numerous species have antitumor and immune boosting effects; help reducing blood sugar and cholesterol levels and blood pressure as well.

Fungi have an important role in improving soil quality (mycoremediation) and soil life, which characteristic can be used for establishing new forests or plantations.

Fungi are able to break down organic or inorganic molecules (even carcinogenic ones) and to accumulate heavy metals (mycofiltration). Due to these characteristics of fungi, they can be used in waste and sewage treatment and disposal and in soil remediation processes. In the Far East, fungi are also used to neutralize toxic cloth dyes.

Some species are effective biological pesticides. Pesticides containing Cordyceps or Metarhizium are used against ants and termites. Fungicides are available to treat Botrytis, Fusarium and Phytophtora.

Taxonomy

Whittaker created the five kingdom taxonomic classification of the world’s biota. He divided living things into the following kingdoms: Monera, Protista, Plantae, Animalia and Fungi.

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Nowadays it is obvious that fungi is a distinctive class, which is considered to be close to animals, with whom they likely to have a common ancestor. Fungi are a polyphyletic group, their taxonomy changes constantly. The current system is shown in next Table.

Regnum

Phylum

Classis

Protozoa

Acrasiomycota

Acrasiomycetes

Dictyosteliomycota

Dictyosteliomycetes

Myxomycota

Myxomycetes

 

Protosteliomycetes

 

Plasmodiophoromycota

Plasmodiophoromycetes

Chromista

Hyphochytridiomycota

Hyphochytridiomycetes

 

Labyrinthulomycota

Labyrinthulomycetes

Oomycota

Oomycetes

Fungi

Chytridiomycota

Chytridiomycetes

Zygomycota

Trichomycetes

 

Zygomycetes

 

Ascomycota

Ascomycetes

 

Basidiomycota

Basidiomycetes

 

Teliomycetes

Ustomycetes

Fungi Imperfecti

 

Morphology of cultivated mushrooms

Cultivated mushrooms produce a mass of hyphae, called mycelium in their vegetative stage. Hyphae are long, tubular, thread-like structures 5-10 mm in diameter. Only a few fungi groups do not form any hyphae (e. g. slime molds and yeasts). With the help of these structures fungi are able to grow into (colonize) different materials (substrates). Mycelium connects to the substrate on a huge surface. It breaks down some of the materials in the substrate and in the meantime it absorbs them. Certain ambient conditions cause the mycelium to “thicken” and become dense, which ultimately leads to primordia, then fruitbody formation. When the fruitbodies are mature enough, they produce basidiospores, which are part of the sexual development phase.

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Not every mushroom have all parts shown on that figure. The most important cultivated species only have stems and caps. The role of the stem is to transport water and nutrients from the substrate to the cap; and to aid spore disposal by holding the cap (and the spore bearing gills) up high. Scales on the cap surface are not common, especially on oyster mushrooms. On button mushrooms, they mostly only form in case of inadequate ambient conditions (e.g. low humidity).

There are various types of hymenial tramas: lamellar (e.g. button and oyster mushrooms), tubular (e.g. Ganoderma spp.) and toothed (e.g. hedgehog mushroom). The basidiospores are formed by the basidia on the gills.

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Lamellar trama

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Tubular trama of reishi

Spores are not used in cultivation; in fact, spore spread should be avoided (see oyster cultivation). The color and size of the stem is characteristic in case of each mushroom species. Oysters have shorter, while button mushrooms have 10-15 cm long stems. Not every species have volvas or rings (remains of the partial veils) or their stems, but these features have an important role in the identification of wild grown species.

Lifestyles and life cycles of mushrooms

Based on their lifestyles mushrooms can either be saprotrophic, parasite or symbiotic species.

Saprotrophic species feed on decaying organic materials, which is an important characteristic in mushroom cultivation. These types of mushrooms have extracellular enzyme systems, which breaks down the cellulose content of the substrate. Wood-destroying mushrooms have the strongest enzymes. One group of these species are the white-decay (or white-rot) fungi (e.g. shiitake and oyster mushrooms), which, by breaking down cellulose and lignin, cause the substrate to lose color and eventually turn whitish. The other group is the brown-decay fungi, which cannot break down lignin, only cellulose, thus the substrate turns darker by the passing of time.

Parasite species grow on organisms (mostly on animals or plants, but in some cases even on other fungi) that are still alive. They feed in/on the host, taking the necessary nutrients from it. The two species can live together for several years before the host dies. Many fungi belong to this group. They cause health-related problems, but there is a cultivated species amongst parasite fungi as well (reishi).

A symbiotic connection between two species (fungi and plants or animals) is beneficial for both parties, and it is quite common in the nature. There are many important species amongst symbiotic fungi: e.g. truffles, chanterelles and bolete. There is no assured cultivation method for these species yet.

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Lifecycle of basidiomycetes

In the nature, the primary generative hyphae (developed from the basidiospores) fuse. This process is called anastomosis, cytogamy, somatogamy or plasmogamy. The fusion of the two, genetically different nuclei happens only after plasmogamy is completed and the basidia are formed. Secondary hyphae are the result of the fusion of two primary hyphae. Secondary hyphae are generally dicaryotic. The two nuclei divide simultaneously by clamp connection.

Clampsa replay

Clamp connection (black and white circles represent the genetically different nuclei)

In mushroom cultivation and in spawn production only the secondary hyphae are used. This intensively growing type of hyphae colonizes the substrate. This is called the vegetative stage of a mushroom’s life-cycle. Various effects (climatic, enzymatic etc.) and the expression of certain genes initiate fruitbody formation. Primordia, or otherwise known pinheads starts to form. The pinheads grow, evolve and differentiate, which leads to the development of fruitbodies.

The hymenium (spore-bearing tissue layer), where the basidium forms, is found in the fruitbodies. Nuclei fuse in the basidium, than start to divide. The first division of the nuclei is meiotic, which results in four haploid nuclei in the form of four basidiospores on the basidia. These spores are the source of the following vegetative cycle.

Besides spores, fungi have another way of reproduction. The vegetative mycelia can break into smaller pieces and spread. The slightly modified “life-cycle” of cultivated mushrooms is shown here.

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Development of cultivated basidiomycetes

The vegetative mycelia of the cultivated mushroom get into the compost or substrate in the form of spawn (LINK). The mycelia originating from the spawn grow into and colonize the material. Then the growers change the ambient conditions, which results in the initialization of the generative stage and the forming of fruitbodies. In cultivation, it is important to provide optimal environment (water content, humidity, CO2 level, temperature, light etc.) for the mushrooms. Cultivated species require different ambient conditions in their vegetative (colonization) and generative (initialization and development of fruitbodies) stage, thus it is equally important to have both the expertise and properly conditioned mushroom houses.

The main actions that lead to the initialization of the generative stage and fruitbody formation are the following: lowering the temperature, modifying humidity, lowering CO2 level, increasing light intensity and altering the water and nutrient content of the substrate. A change in single one of these conditions could be sufficient for fruitbody initialization, but in mushroom production it is more common to modify more than one. A common “combination” is to lower the temperature and at the same time to intensify air flow in order to ensure lower CO2 level.

Taxonomy of cultivated mushrooms

A number of species with various characteristics belong to the kingdom of fungi. Only a small group of these species are edible and therefore significant to us. The estimated number of fungi is around 500.000-1.000.000, of which as few as 10-15% (about 90.000 species) is currently known to us. This includes micro- and macroscopic fungi as well.

Edible mushrooms belong to the kingdom of fungi (Regnum Fungi), in which two phyla have species that can potentially be cultivated. Ascomycota or sac fungi have their spores (usually eight together) in a special sac-like structure called ascus. This phylum includes single-celled and filamentous fungi, which have glucan, mannan and chitin in their cell walls. One of the most important subjects of biotechnology and food industry researches is Saccharomyces cerevisiae (a yeast species) is also a member of the Ascomycota phylum.

Most of the cultivated species belong to the group (phylum) of Basidiomycota fungi. Their cell walls contain chitin; they have septate hyphae and basidia that bear basidiospores.

The most important Basidiomycota species are the button mushrooms (Agaricus spp.), the oysters (Pleurotus spp.) and the shiitake (Lentinula edodes). Other medicinal mushrooms, such as shaggy mane, sheathed woodtuft, enoki, black poplar mushroom, reishi and Jew’s ear also belong to this phylum.

Check yourself!

  1. What lifestyles do mushrooms have?
  2. What is the definition of “fungi”?
  3. What are the characteristics of plant, animal and fungal cells?
  4. What are the 10 most important cultivated species (with Latin names and taxonomy)?
  5. What causes (which conditions) the vegetative stage to switch into generative?
  6. Characterize the saprotrophic lifestyle!

References

Chang, S.T., Miles, P.G. (2004): Mushroom Cultivation, Nutritional Value, Medicinal Effect, and Environmental Impact. CRC Press Boca Raton, London, New York, Washington.

Győrfi, J. (szerk)(2012): Gombabiológia, gombatermesztés. Mezőgazda Kiadó, Budapest.

Jakucs, E., Vajna, L. (2003): Mikológia. Agroinform Kiadó, Budapest.

Lelley, J. (1997): Die Heilkraft der Pilze. Econ Verlag GmbH, Düsseldorf und München.

Oei, P. (2003): Mushroom Cultivation, Backhuys Publishers, Leiden, The Netherlands.

Stamets, P. (2000): Growing Gourmet and Medicinal Mushrooms. Ten Speed Press, Toronto.

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Utolsó frissítés: 2014 11. 13.