In this article we will discuss about the life cycle of agaricus with the help of suitable diagrams.
Mycelium of Agaricus:
The primary mycelium produced by the germination of basidiospore is of short duration. It is haploid and either of plus strain or minus strain. The hyphae are septate. The cells contain oil globules, vacuoles and are short and uninucleate.
Soon as a result of hyphal fusions the primary mycelium becomes binucleate usually without clamp connections rarely with clamp connections. The mycelium with binucleate cells is called secondary or dikaryotie mycelium. It is long-lived and abundant. It produces mushrooms year after year. The secondary mycelium forms a branching network of hyaline hyphae. The hyphae are long, branched and short celled.
The cells are binucleate and communicate with one another by means of central pore in the septum. It is a typical Basidiomycete dolipore septum in which the opening is guarded on both sides with its parenthesomes.
The hyphae traverse the soil in all directions from a central point and form a circular colony covering an extensive area. It may remain in this condition for a considerable period before it forms the over ground fructifications.
Commonly the hyphae interlace and twist to form thick, white hyphal cords called the rhizomorphs which bear the fruiting bodies (Fig. 15.3A). In many mushrooms the fructifications develop at the tips of the hyphae of the circular colony forming a ring.
This process is repeated with further growth of the hyphae so that widening circles of basidiocarps are seen. The appearance of basidiocarps in a ring from the invisible mycelium is called a “fairy ring“ because of an old superstition that the mushrooms growing in a ring indicate the path of dancing fairies.
Mushroom Cultivation:
The edible mushrooms are cultivated on a large scale. They form the basis of a great industry in many foreign countries. The principal growth requirements of this fungus are a suitable supply of moisture, moderate temperature and abundant organic matter.
In India the mushrooms are cultivated in Himachal Pradesh, South India and other places. Himachal Pradesh offers ideal Agro-climatic conditions for growing edible mushrooms. A special mention may be made of Himachal University College of Agriculture, Solan in this connection.
It has achieved commendable success in mushroom cultivation. The Mushroom Research Laboratory at Solan has not only excelled in standarizing the technique for raising white button mushrooms and Oyster mushrooms but has also helped in establishing mushroom growing centres in different parts of the state such as Chail, Simla and Kasauli, and 116 other mushroom farms all over the country by supplying mushroom spawns to the growers. At Solan the mushrooms are cultivated in chopped paddy straw and compost.
Freguson (1978) reported that a mixture of horse dung, chopped wheat straw, gypsum and additives constitute the compost. It is the usual substratum for the commercial propagation of the mushrooms. Compost is prepared by a complicated process called composting. It takes about 12 days to prepare compost.
The beds for cultivation of mushrooms are specially prepared. They contain substratum of compost rich in nitrogenous compounds. These beds are then inoculated with crumbs of mushroom spawn.
The mushroom spawns are usually of the following types:
1. Blocks of richly manured soil containing the fungus mycelium and basidiospores in abundance.
2. Pure culture of the fungus mycelium on some nutritive medium.
Oyster mushrooms but has also helped in establishing mushroom growing centres in different parts of the state such as Chail, Simla and Kasauli, and 116 other mushroom farms all over the country by supplying mushroom spawns to the growers.
At Solan the mushrooms are cultivated in chopped paddy straw and compost. Freguson (1978) reported that a mixture of horse dung chopped wheat straw, gypsum and additives constitute the compost.
It is the usual substratum for the commercial propagation of the mushrooms. Compost is prepared by a complicated process called composting. It takes about 12 days to prepare compost.
The beds for cultivation of mushrooms are specially prepared. They contain substratum of compost rich in nitrogenous compounds. These beds are then inoculated with crumbs of mushroom spawn.
The mushroom spawns are usually of the following types:
1. Blocks of richly manured soil containing the fungus mycelium and basidiospores in abundance.
2. Pure culture of the fungus mycelium on some nutritive medium.
3. Grain spawn:
It is a sterilized cereal grain colonized by the mushroom mycelium. The mycelium will grow from the grains and spread throughout and on to the surface of the compost as white hyphae and white mycelial strands visible to the unaided eye.
As the mycelium grows it induces the compost to change from black to light reddish brown colour. A compost temperature of 25-28°C during spawn growth is optimum. If the temperature rises above 33°C, mushroom mycelium will not grow.
After mycelial colonization of compost sporophores are formed. Mander (1943) observed the presence of a volatile substance secreted by the mushroom mycelium which inhibits strand and fruit body formation if allowed to accumulate in mushroom beds.
A layer of soil or heat chalk mixture known as the “casing layer” is necessary to stimulate fruit body formation. It is suggested that bacteria in this layer are responsible for the induction of fruiting though the exact effect of bacteria remains obscure.
Recent evidence indicates that they may be responsible for removing self-inhabitors of fruiting located in the mushroom mycelium.
Reproduction in Agaricus:
Sexual Reproduction:
The sexual apparatus in the form of sex organs is completely lacking. Their function has been taken over by the somatic hyphae which are heterothallic. The fusion between two somatic hyphae of plus and minus strains represents the first phase.
Some mycologists, however, hold that it is homothallic and somatogamous copulation takes place between the somatic hyphae of the mycelium formed from a single spore.
Plasmogamy in all cases is accomplished by somatogamy or somatogamous copulation as follows:
1. Plasmogamy (Fig. 15.3):
Two vegetative hyphae with uninucleate haploid cells from mycelia of opposite strains or from the same mycelium come into contact (Fig. 15.2). The intervening wall at the point of contact dissolves.
The fusion cell comes to possess two nuclei which move towards each other to lie in a pair. This pair of nuclei in the fusion cell constitutes a dikaryon. The dikaryotised cell by successive divisions gives rise to the binucleate or dikaryotic mycelium.
At every division the two nuclei of the dikaryotic cell divides conjugately into four daughter nuclei. This cell then develops a clamp connection which ensures that the sister nuclei separate into two daughter cells.
The process is repeated several times resulting in the formation of a mycelium with binucleate cells. Some mycologists hold that clamp connections are absent in Psalliota.
The binucleate mycelium thus originates from the uninucleate mycelium as a result of plasmogamy by somatogamous copulation. Plasmogamy in fact initiates dikaryophase in the life cycle.
The binucleate or the dikaryotic mycelium is perennial. It forms the chief food absorbing phase of the fungus and at the appropriate time bears fructifications called the basidiocarps (Fig. 15.3).
The basidiocarps are formed when the mycelium has absorbed and accumulated abundant food supply. They are developed in groups from the mycelium under suitable temperature and sufficient moisture.
2. Karyogamy:
It means the fusion of the two nuclei of the dikaryon. It is considerably delayed. It takes place in the young basidium.
3. Meiosis:
Karyogamy is immediately followed by meiosis and takes place in the basidium prior to basidiospore formation. The basidiospores are thus haploid.
Development of the Basidiocarp (Fig. 15.3):
The basidiocarps arise as tiny white apical swellings on the branches of the subterranean mycelial strands which are known as the rhizomorphs. Each swelling consists of a dense knot of dikaryotic hyphae.
The tiny white knots enlarge into round or ovoid structures which break through the surface. These tiny balls represent the button stage of the basidiocarp (A).
A longitudinal section of a button shows differentiation into a bulbous basal part representing the pileus (B). At the junction of these is a ring-like cavity, the gill chamber.
It is formed by drawing of tissue radiating outwards from the stipe. These are the gills. They are already forming. The margin of the button is connected with the stalk by a membrane called the partial or inner veil or velum (C).
By further elongation of the stalk, the button projects above the soil and enlarges considerably in size. The growth proceeds more rapidly at the upper portion of the button. It is slow at the lower portion.
As a result the button opens into an umbrella-like cup or the pileus (D). At the same time the inner veil which covers the lower surface of the pileus ruptures exposing the hymenium on the gills.
The mushroom thus is hemiangiocarpic. Remnants of the inner veil remain attached to the stalk or the stipe in the form of a ring or annulus (B). The latter encircles the stipe close to the attachment of the pileus. The exposed young gills are white, at first, but later turn pink.
In some gill fungi such as Amanita the button when young is completely covered by a membrane called the universal veil.
As the button grows and enlarges into the pileus and the stalk the universal veil ruptures leaving a cup-like structure, the volva surrounding the base of the stipe. Volva is absent in Agaricus campestris.
In dry weather when the substratum is hard and dry the young basidiocarps (buttons) do not increase in size. They remain underground in a partially developed condition until the soil above become thoroughly wet by rain.
In a moist soil they enlarge rapidly due to the extension of existing cells rather than to cell division. This accounts for the sudden overnight appearance of numerous mushrooms (basidiocarps) above ground after a heavy shower in the rainy season. The phrase ‘mushroom growth’ refers to this phase of rapid expansion.
Mature Basidiocarp. (Fig. 15.3 E):
It is a massive structure which consists of a stalk-like portion, the stipe supporting at the top a broad umbrella-shaped cap, the pileus. The stipe is a thick fleshy, cylindrical structure, pinkish white in colour.
It is usually broader and swollen at the base and is centrally attached to the pileus. The remains of the strands of the mycelium are sometimes seen still sticking to the base of the stipe.
More than half way up, the stipe, bears a membranous ring called the annulus. It represents the remains of the ruptured inner veil. The stipe supports and raises the pileus into the correct position for spore dispersal.
It also transports nutrients and water from the mycelium to the pileus and gills for their development. The mature pileus is 2.4 inches in diameter. It is umbrella-shaped. The convex upper surface of the pileus is white, cream coloured or brown.
From the underside of the pileus hang numerous thin vertical strips or plates of tissue, the gills or lamellae (Fig. 15.4 B). They look like partitions which hang down in a vertical position.
The negative geotropism of the stipe, diageotropism of pileus and positive geotropice nature of gills help keep the latter in an accurate vertical position. The gills are flesh coloured or pink known as the hymenium or thecium.
The specialized fertile cells constituting the hymenium are the basidia which produce the basidiospores. The umbrella-shaped pileus affords protection to the gill tissue.
The function of the basidiocarp is to produce and disperse the numerous basidiospore produced by the basidia when young. The mature gills are dark brown or purplish black. The colour changes with the pigmentation of spores.
They radiate from the margin of the pileus towards the stipe. However, they are not joined to the stipe. All the gills are not of the same length.
Between the longer gills there are shorter gills. The surface of the gill is covered on both sides with a fertile layer of spore producing tissue. It is called the hvmenium or thecium.
Anatomy of Basidiocarp (Fig. 15.5):
The entire fructification, as mentioned above, is composed of a fundamental tissue of an interlacing mass of tertiary hyphae. The hyphal cells are multinucleate. The septa between the cells have one central pore each with a septal pore appartus.
(a) Stipe:
The hyphae run longitudinally parallel to each other in the stipe region. In the peripheral portion they are compactly arranged to form a pseudoparenchymatous tissue. It is called the cortex.
Towards the central region of the stipe these hyphae are loosely interwoven so that intercellular spaces are easily recognizable between them. The central region of loosely arranged hyphae is called medulla.
The cells constituting the hyphae are large and elongated. Within the cell wall is the thin electron dense plasma membrane. According to Manocha (1965) the cytoplasm contains a few mitochondria.
The ER is scanty and generally inflated and fragmented into vesicles. There are a large number of dense, granular glycogen particles. Interspersed between these large fundamental cells are the second type of cells known as the connective cells.
They are long and thread-like. Both the type of cells are multinucleate. According to some workers the rapid elongation of the upper portion of stipes during the later developmental stages of the basidiocarp is due to cell elongation alone.
However, Craig et al (1977) showed that both cell elongation and cell division are responsible for elongation of the upper part of the stipe region.
(b) Pileus:
The structure of the pileus is like that of the stipe. The two regions, cortex and medulla of the stipe extend right up to it. The hyphae of the stipe region extend into the pileus and fan outwards. Some of these pass down into the gills.
(c) Structure of a Gill (Fig. 15.6):
The gill exhibits a complex structure. It is a sheet of interwoven hyphae which are more closely compacted and denser in this region. They extend from the pileus to form the undersurface of the gill.
In a tangential section of the pileus, the gills are cut transversely.
In a transverse section of gill we can make out the following regions:
1. Trama:
It is the innermost or central part of the gill between the two hymenial surfaces. It is made up of extensions of the hyphae of the pileus. The hyphae in this region anastomose and are irregularly interwoven.
They run more or less longitudinally. The cells contain fewer nuclei than the cells of the hyphae constituting the stipe and the pileus.
2. Subhymenium or Hypothecium:
The hyphae constituting the trama give off short lateral branches. The latter curve outwards towards the two surfaces of the gill where they form a more closely compacted tissue of small inflated cells with still fewer nuclei (2 or 3 per cell). This tissue of small cells on each side of the trama is called the subhymenium.
3. Hymenium or Thecium:
On the surface of sub-hymenium covering both sides of the gill is the fertile layer called the hymenium. It consists of a closely packed palisade-like layer of club-shaped cells. They are called the basidia (A).
The basidia are, in fact, the terminal elongated cells of the same hyphae which constitute the trama and the subhymenium. The basidia are the spore producing elements. They are non-septate.
Interspersed between the basidia are sterile, more slender hyphhae called the paraphyses or cystidia. Each basidium bears four basidiospores at its free end. They are elevated on short stalks, the sterigmata.
The young basidium has a dikaryon (B1) and is packed with cytoplasm. As the basidium grows the two nuclei of the dikaryon fuse to form the synkaryon (B2). This is karyogamy. It ends the dikaryophase.
The basidium containing the synkaryon in the mature basidium undergoes meiosis to form four haploid nuclei (B3). Segregation of sex strains takes place at this time so that two of these nuclei are of plus strain and two of minus strain.
At this stage four delicate small peg-like outgrowths, the sterigmata, develop at the top of the basidium. The tip of each sterigma swells to become a basidiospore initial, which is small, and bead-like in appearance (B4).
Meanwhile a vacuole appears at the base of the developing basidium. It progressively enlarges pushing one of the 4 daughter nuclei along with some cytoplasm into each basidiospore initial (B5).
The basidiospore in A brunnescence is thus uninucleate. During its passage through the narrow neck of the basidiospore, the haploid nucleus assumes a vermiform shape (15.6 B5). It again becomes sphencal in the basidiospore.
The young basidiospore is subsequently separated from its respective sterical by a septum. The mature basidium has a large vacuole with the cytoplasm. When young the basidiospores are unpigmented but towards maturity the spore wall turns pinkish purple.
Each mature basidiospore is seated slightly obliquely at the top of the sterigma with the spore point which is a minute projection (hilar appendix), a little to one side near its attachment to the sterigma (B6).
Morphology of Basidiospore (Fig. 15.7):
The haploid basidiospore of A. campestris is a violet-brown, thick-walled oval or ellipsoid structure with a minute, eccentrically placed spine-like projection—the hilar appendix on the adaxial side of the basidiospore wall at the basal end called the hilum.
The latter denotes the former point of attachment of the spore to the sterigma. Because of its small size the morphology of hilar appendix has been under dispute.
Buller (1922) and Olive (1964) described it as the specialized part of the adaxial wall of the basidiospore which is the site of formation of the liquid or gaseous droplet associated with the violent “spore discharge”.
There are a few reports dealing with the electron microscope study of this structure. Olak and Resinger (1974) reported that the hilar appendix in the Agaricales contains two orifices.
One of these, the hilar orifice (orifice hilare) is a pore-like scar. It results from the rupture of the sterigma-spore interface. The second which is associated with droplet formation and its subsequent forcible discharge, is the hilare droplet orifice.
According to Rast and Hollenstein (1977), the adaxial supra hilar region of the spore surface shows on special differentiation of hilar appendix (B). It is an open pore type hilum. The innermost layer of the spore wall thickens to plug the latter.
Yoon and McLaughlin (1979) consider hilar appendix to be a specially differentiated part of adaxial spore wall at the junction of the sterigma and the basidiospore.
In the young basidiospore when it is less than half its normal size and the nucleus has not yet migrated into it, a change in the adaxial wall structure in the site of hilar appendix takes place.
It thickens and an eletron light zone develops. When the electron light zone is fully developed and the spore reaches maturity, the hilar appendix swells outwards and takes on a drop-like appearance.
No plug is seen at this stage between the sterigma and the spore. Prior to spore discharge, the cytoplasm adjacent to the hilar appendix is replaced with an eletron-light plugging material.
Fine structure of basidiospore (Fig. 15.8):
The basidiospore in the dormant state has a thick spore wall which lacks any ornamentation. Thus the spore has a smooth surface. Even the preformed germ pore is not detectable.
Ultrastructurally the spore wall is differentiated into three layers, the outer, middle and the inner. The former two are thicker than the innermost which is next to the cytoplasm.
The outer smooth layer is electron opaque. It is comparatively thicker than the middle layer and is chiefly granular amorphous. In addition it contains some fibrillar material. The middle layer is fibrillar.
The chitin with numerous evenly dispersed fibres are embedded in a B-glucan-protein matrix. According to Rast and Hollenstein (1977), it is relatively less electron-dense than the outer layer which stains deeply.
The thin inner layer is almost electron-transparent. It is amorphous and seems to be composed of a mucilaginous type of substance.
It thickens at the junction of the sterigma and the spore to plug the open pore type hilum. Lipid occurs throughout the spore wall.
Within the three-layered basidiospore wall is the spore protoplast. According to Manocha (1965) and O’Sullivan (1969), it is differentiated into a cell membrane and cytoplasm which contains a single nucleus and a few other cell organelles such as mitochondria, ribosomes and endoplasmic reticulum.
The reserve food is in the form of oil globules, Greuter and Rast (1975) reported that the basidiospore of A. bisporus is normally binucleate. The cytoplasm is closely covered by the plasma membrane.
Embedded in it are the mitochondria with plate-like cristae and numerous free ribosomes. The cytoplasm is packed with one large or several lipid droplets of varying sizes.
According to these investigators, the cytoplasm of A.bisporus lacks endoplasmic reticulum. Close to the lipid droplets occur clusters of granules probably glycogen. Mitochondria with microbodies also occur near the lipid bodies.
Spore discharge (Fig. 15.6 B6):
The mechanism of spore discharge is still uncertain. The common view is that when the basidiospore reaches maturity, hilum secretes a tiny drop of watery fluid. It gradually increases till it attains a certain size (B6).
At this stage basidiospore is suddenly shot off the sterigma perpendicularly to the gill surface with a force just enough to carry it about half way (1/10 to 1/8 mm.) into the narrow space between the two gills.
As regards the mechanism of discharge the latest view is that drop is contained in a limiting membrane. With its rupture there is release of pressure at the base of the basidiospore. Consequently the basidiospore is shot off from the sterigma by jet propulsion.
Spore dispersal:
The discharged basidiospores falling vertically downwards from the narrow space between the gills and out into the air below the cap are carried away by the currents of air.
The long stipe facilitates the process as it raised the pileus into correct position for spore dispersal above the soil surface providing sufficient space underneath, which allows air currents to carry away the falling spores.
The basidia mature in successive crops. The older ones which have discharged the basidiospores wither away. New ones grow up to take their place until all the basidial initials have been used up.
The basidiospores are thus discharged successively over short intervals for several days. Each basidium bears a single crop of basidiospores.
Agaricus bisporus, the mushroom which is cultivated commercially is distinguished from A. campestris the field mushroom by its darker and more firmer pileus, inferior flavour and two basidiospores per basidium.
The mature basidiospores are purplish black in colour. The number of spores produced by a single basidiocarp is enormous, about 1,800,000,000 spores.
They are discharged successively over short intervals for several days. The umbrella- shaped pileus affords protection to the fertile layer from rain.
Germination of basidiospores (Fig. 15.9):
The basidiospores (A) which chance to fall on a suitable substrate germinate. An adequate moisture supply and favourable temperature are essential for germination.
Mc Teague, Hutchinson and Read (1959) found that the presence of the vegetatively growing mycelium of A. campestris in the medium containing spores of this fungus stimulates spore germination.
The stimulating factor is a volatile metabolite secreted by the mycelium. The presence of olefin which is probably 2:3 dimethyl – I- pentene was shown to be present by analysis with spectrometer.
Each basidiospore puts out through some point on the spore wall a protoplasmic sac which elongates into a narrow germ tube (B). The latter develops into a primary mycelium which is either of plus or minus strain.
It consists of uninucleate cells. The binucleate condition is soon achieved by somatogamous copulation between hyphae of two primary mycelia of opposite strains growing in the vicinity (C). The binucleate mycelium starts producing the basidiocarps.
Asexual Reproduction:
The field mushroom is reported to multiply by the formation of chlamydospores and conidia.
Economic Importance of Agaricus:
(a) Positive:
All species of Agaricus are edible. The only two exceptions are A. silircola and A. xanthodermis which are supposed to be slightly poisonous at times but usually not lethal.
Several years ago, Tokyo Research Institute reported about the anticancerous properties of certain mushroom varieties and the Japanese doctors recommended mushrooms to patients to check the spread of the dreaded disease.
In food value the mushrooms range from excellent to fair. In spite of 88 percent of water content the mushrooms are a valuable source of nourishment being rich in high quality proteins (5-7 percent) and amino acids.
For this reason they are often known as the vegetarians meat. They also contain a digestive enzyme that aids digestion. The nutritional value of Oysters (dhingri) is reported to be more than button mushrooms.
Besides they contain folic acid, vitamin B-complex and C and several minerals like Calcium, Phosphorus, Iron and Copper. Their mineral content is higher than that of mutton.
Because of high folic acid content the mushrooms are extremely beneficial for blood formation and thus are good for anaemic patients.
It has been reported that glycogen content of A. bisporus sporophore is normally about 2 to 4 percent of the dry weight at the immature button stage. It increases to 5-8 percent at the mature flat stage.
The mushrooms (Oysters) being devoid of fat cholesterol and arch, are prescribed as excellent diet for diabetes patients and for those who suffer from high blood pressure.
Dr. Chandramani reported that mushrooms are rich in vitamin C, niacin and pantothenic acid (a constituent of vitamin B2). All these are considered beneficial or diabetes anaemic and heart patients.
The mushrooms thus being of considerable economic importance are grown commercially and form the basis of food industry. Agaricus bispora, the white spored mushroom, is cultivated to a greater or lesser extent all over the world.
The other commonly cultivated species are A. bitorquis and A. arvensis, Cortinellus berkeylyanus is cultivated in Japan. Another mushroom Volvariella is commercially grown in the tropics.
The two commonly cultivated species of this genus are V. volvacea and V. diplosia. They are commonly grown in China, Indonesia, India, Burma, Indo-China and Madagascar. Pholiota praecox is another edible mushroom but it has no flavour.
Purple-spored agarics such as Agaricus campestris, A. rodmani and A. placomyces are also edible. Agaricus xanthodermis and a few others contain bacteriostatic substances which inhibit development of bacteria.
The hallucigenic mushrooms of the genus Psilocybe, which are considered sacred by certain Mexican tribes, produce ecstatic effects. These sacred mushrooms are now used experimentally in medicine to assist in the treatment of disease by psychological methods.
(b) Negative:
A few members of the order are important wood rotters. Armillaria mellea is one of them. It is called the honey mushroom. It causes foot rot of apple and some forest trees.
Some species of this family are supposed to be of possible use in the production of antibiotics. Of course, no new and useful substance has so far been discovered.
A member of species form mycorrhizal associations with the trees. Many agarics are highly poisonous.
To this category belongs Amanita muscaria, the fly mushroom. Amanita verna is the deadliest of all known poisonous mushrooms. Some members of this order aid in the decomposition of dead bodies and their wastes. In this way they restore simple substances to the air and soil.
