PRIMARY KNOWLEDGE ON AGARICUS BISPORUS

Agaricus bisporus

Agaricus is a genus of mushrooms containing both edible and poisonous species, with possibly over 300 members worldwide. The genus includes the common ("button") mushroom (Agaricus bisporus) and the field mushroom (Agaricus campestris), the dominant cultivated mushrooms of the West. Members of Agaricus are characterized by having a fleshy cap or pileus, from the underside of which grow a number of radiating plates or gills on which are produced the naked spores. They are distinguished

from other members of their family, Agaricaceae, by their chocolate-brown spores. Members of Agaricus also have a stem or stipe, which elevates it above the object on which the mushroom grows, or substrate, and a partial veil, which protects the developing gills and later forms a ring or annulus on the stalk. The cap is dry, smooth, Brown in color, with broad, flat scales on a paler background and fading toward the margins. It is first hemispherical in shape before flattening out with maturity, and 5–10 cm (2–4 in) in diameter. The narrow, crowded gills are free and initially pink, then red-brown and finally a dark brown with a whitish edge from the cheilocystidia. The cylindrical stipe is up to 6 cm (2⅓ in) tall by 1–2 cm wide and bears a thick and narrow ring, which may be streaked on the upperside. The firm flesh is white though stains a pale pinkish-red on bruising. The spore print is dark brown. The spores are oval to round and measure around 4.5–5.5 x 5–7.5 μm, and the basidia usually two-spored, although two tetrasporic varieties have been described from the Mojave desert and the Mediterranean with predominantly heterothallic and homothallic lifestyles, respectively. Commonly found in fields and grassy areas after rain from late spring through to autumn worldwide, especially in association with manure. It is widely collected and eaten, even by those who would not normally experiment with mushroom hunting.

 

OTHER NAMES OF AGARICUS BISPORUS

Agaric, Agaric Bispore, Agaricus, Agaricus Bisporus Mushroom, Agaricus blazei, Agarikusutake, Brazil Mushroom, Brazilian Mushroom, Brazilian Sun-Mushroom, Callampa Agaricus, Champignon Agaric, Champignon Brésilien, Champignon du Brésil,

Among English speakers, Agaricus bisporus is known by many names. A young specimen with a closed cap and either pale white or light brown flesh is known as a button mushroom or white mushroom. In strains with darker flesh, the immature mushroom is variously marketed as a cremini mushroom, baby portobello, baby bella, mini bella, portabellini, Roman mushroom, Italian mushroom, or brown mushroom. At this stage of maturation, the cap may also begin to open slightly. In maturity, it is called a portobello. The French name is champignon de Paris ("Paris mushroom").

The spellings "portobello", "portabella", and "portabello" are all used, but the first of these spellings is the most common.

TAXONOMICAL STUDIES OF AGARICUS BISPORUS

Phylum	Basidiomycota
Class	Agaricomycetes
Order	Agaricales
Family	Agaricaceae

Agaricus bisporus is not particularly common in the wild, where like the Field Mushroom and the Horse Mushroom it springs up in fields and appears after rain. The specific name bisporus refers to the fact that each of the basidia bear just two spores - most Agaricus species have four-spored basidia. (You will need a powerful microscope if you want to see these features on the gill faces of agaricoid fungi.)

This species was originally noted and named in 1753 by Carolus Linnaeus as Agaricus campestris. For many years, members of the genus Agaricus were given the generic name Psalliota, and this can still be seen in older books on mushrooms. All proposals to conserve Agaricus against Psalliota or vice versa have so far been considered superfluous. Several origins of Agaricus have been proposed; it possibly derives "from Agarica of Sarmatica, a district of Russia". Note also Greek ἀγαρικ όν, "a sort of tree fungus" (There has been an Agaricon Adans. genus, treated by Donk in Persoonia 1:180.)

Donk reports Linnaeus' name is devalidated (so the proper author citation apparently is "L. per Fr., 1821") because Agaricus was not linked to Tournefort's name. Linnaeus places both Agaricus Dill. and Amanita Dill. in synonymy, but truly a replacement for Amanita Dill., which would require A. quercinus, not A. campestris be the type. This question is compounded because Fries himself used Agaricus roughly in Linnaeus' sense (which leads to issues with Amanita), and A. campestris was eventually excluded from Agaricus by Karsten and was apparently in Lepiota at the time Donk wrote this, commenting that a type conservation might become necessary.

The alternate name for the genus, Psalliota, derived from the Greek psalion/ψάλιον, "ring", was first published by Fries (1821) as trib. Psalliota. The type is Agaricus campestris (widely accepted, except by Earle, who proposed A. cretaceus). Paul Kummer (not Quélet, who merely excluded Stropharia) was the first to elevate the tribe to a genus. Psalliota was the tribe containing the type of Agaricus, so when separated, it should have caused the rest of the genus to be renamed, but this is not what happened.

Some variants have been isolated over the years, a few of which now have species status, for example, Agaricus bernardii Quel. (1878), Agaricus bisporus (J.E. Lange) Imbach (1946), Agaricus bitorquis (Quel.) Sacc. (1887), Agaricus cappellianus Hlavacek (1987), and Agaricus silvicola (Vittad.) Peck (1872).
Some were so similar they did not warrant even variant status, others have retained it e.g. Agaricus campestris var. equestris (F.H. Moller) Pilat (1951) is still valid, and presumably favors pasture where horses have been kept. Agaricus campestris var isabellinus (F.H. Moller) Pilat (1951), and Agaricus campestris var.radicatus, are possibly still valid too.
The specific epithet campestris is derived from the Latin campus "field".

VEGETATIVE BODY

Vegetative body mycelia and consist of septate much branched hyphae. Spore on germination develop into monokaryotic or primary mycelium, either + or- typ. The primary mycelium is short lived and it soon transform into diakaryotic or secondary mycelium by the fusion of two cell of different monokaryotic mycelium following clamp connection. The hyphae of the diakaryotic mycelia interlace twist together to form thick hyphal cord, called rhizomorph which bear the fruit bodies.  

ETYMOLOGY

The specific epithet bisporus means having two spores - there are two spores on each basidium, whereas other Agaricus species (and indeed the majority of the Basidiomycota) have four-spored basidia.

IDENTIFICATIONAL  FEATURE

 

Cap

Initially hemispherical, becoming convex and eventually flattening. At maturity, the cap diameter is usually between 5 and 10cm.

Gills 

The crowded gills are narrow and free; initially pink they turn reddish and then chocolate brown as the fruitbody matures.

Stem

At 3 to 6cm tall and 1.5 to 2cm. Below the insubstantial membranous double ring, the surface of the stem is flaky.

Spores

Oviod to subglobose, smooth, 4-7.5 x 4-5.5µm.

Spore print

Deep chocolate brown.

Basidia

Two-spored - hence the specific name bisporus.

ECOLOGY
Agaricus bisporus is a terricolous secondary decomposer fungus, which plays an ecologically significant role in the degradation of leaf and needle litter in temperate forests. Soils contain humic compounds derived from modified lignin and other recalcitrant aromatic compounds, presenting a different catabolic challenge to intact woody resources. Thus, A. bisporus forms an important model for carbon sequestration studies to understand the persistence of mycelial material in humus and to determine the role of fungi in bioconversion of plant materials to humic acids. The homobasidiomycete species is arguably the most well studied member of the family Agaricaceae; a large, diverse and economically important group of fungi. Comprehending the carbon cycling role of the Agaricaceae in forests and other ecosystems is a prerequisite to model and optimise carbon management.
Agaricus genome sequence will provide new opportunities to compare the role of a secondary decomposer in various environmental and developmental fungal processes with other basidiomycete and ascomycetes. Comparisons with the sequenced Polyporales (Postia placenta -brown rot, and P. chrysosporium -white rot) and the mycorrhizal basidiomycetes will provide insight into transitions between different ecological modes (e.g. decomposer, mycorrhizal, white- and brown-rot fungi).
Fortunately, as the premier cultivated mushroom species, a substantial scientific knowledge base exists for A. bisporus, which forms a solid foundation from which launch comparative analyses of basidiomycota genome sequences. Physiology, metabolism, and development have all been investigated at the molecular level, including: substrate utilization, reproductive development, senescence and morphogenesis expression profiles, control of offspring ploidy at the meiosporangium level, and mechanisms of resistance to bacterial and fungal diseases.
Significantly, several Agaricus species have potential for bioremediation of substrates contaminated with heavy metals, and these decomposer fungi are more able to hyperaccumulate toxic metals than some mycorrhizal fungi. Full potential of this area will require more extensive research and all these areas will be expedited by availability of genome sequence.

PHYLOGENETICS

The use of phylogenetic analysis to determine evolutionary relationships amongst Agaricus species has increased the understanding of this taxonomically difficult genus, although much work remains to be done to fully delineate infrageneric relationships. Prior to these analyses, the genus Agaricus, as circumscribed by Rolf Singer, was divided into 42 species grouped into five sections based on reactions of mushroom tissue to air or various chemical reagents, as well as subtle differences in mushroom morphology. Restriction fragment length polymorphism analysis demonstrated this classification scheme needed revision.

SECTION

This genus is divided into several sections:

1. Section Agaricus
2. Section Arvense Konrad & Maubl.

Contains 19 species in six subgroups similar to the horse mushroom, A. arvensis, it has versatile heterothallic life cycles.

     3.  Section Xanthodermatei

Outlined by Singer in 1948, this section includes species with various characteristics similar to the type species A. xanthodermus. The section forms a single clade based on analysis of ITS1+2.

     4.  Section Chitonioides 
     5.  Section Sanguinoletti 
     6.  Section Spissicaules (Hainem.) Kerrigan 
     7.  Section Duploannulatae

Based on DNA analysis of ITS1, ITS2, and 5.8S sequences, this section (also known as section Hortenses) may be divided into six distinct clades, five of which correspond to well-known species from the temperate Northern Hemisphere: A. bisporus, A. subfloccosus, A. bitorquis, A. vaporarius and A. cupressicola. The sixth clade comprises the species complex A. devoniensis.

CULTIVATION OF A. bisporus

A. bitorquis, first cultivated commercially in 1968, has several growth characteristics that have piqued the interest of mushroom cultivators looking for an alternative to the standard button mushroom, A. bisporus. For example, A. bitorquis is more resistant to various viral diseases,can grow at higher temperatures and CO₂ concentrations, and has better resistance to bruising. Furthermore, high temperature-resistant strains have recently been developed which may help cultivators overcome problems associated with cooling production rooms during hot summer months. 

BIOACTIVE PROPERTIES

Water extracts of A. campestris have been shown to enhance the secretion of insulin, and to have insulin-like effects on glucose metabolism in vitro, although the mechanism is not understood.