Source:  Mushroom    Tag:  plasmogamy

Definition of a Mushroom

Mushrooms with other fungi are something special in the living world, being neither plants nor
animals. They have been placed in a kingdom of their own called the kingdom of Myceteae. But
what are mushrooms? The word mushroom may mean different things to different people and
countries. It has emerged that specialised studies and the economic value of mushrooms and their
products had reached a point where a clear definition of the term “mushroom” was warranted.  In
a broad sense “Mushroom is a macrofungus with a distinctive fruiting body, which can be either
epigeous or hypogeous and large enough to be seen with naked eye and to be picked by hand”
(Chang and Miles, 1992). Thus, mushrooms need not be basidiomycetes, nor aerial, nor fleshy,
nor edible. Mushrooms can be ascomycetes, grow underground, have a non-fleshy texture and
need not be edible. This definition is not a perfect one but can be accepted as a workable term to
estimate the number of mushrooms on the earth. The  most common type of mushrooms is
umbrella shaped with a pileus (cap) and a stipe (stem) i.e.  Lentinula edodes.  Other species
additionally have a volva (cup) i.e.  Volvariella volvacea or an annulus (ring) i.e.  Agarius
campestris or with both of them i.e. Amanita muscaria. Furthermore, some mushrooms are in the
form of pliable cups; others round like golf balls. Some are in the shape of small clubs; some
resemble coral; others are yellow or orange jelly-like globs; and some even very much resembles
the human ear. In fact, there is a countless variety of forms.

The structure that we call a mushroom is in reality only the fruiting body of the fungus. The
vegetative part of the fungus, called the mycelium, comprises a system of branching threads and
cord-like strands that branch out through soil, compost, wood log or other lignocellulosic
material on which the fungus may be growing. After a period of growth and under favourable
conditions, the established (matured) mycelium could produce the fruit structure which we call
the mushroom. Accordingly mushrooms can be grouped into four categories:  those which are
fleshy and edible fall into the edible mushroom category, e.g.,  Agaricus bisporus;
mushrooms which are considered to have medicinal applications, are referred to as medicinal
mushrooms, e.g., Ganoderma lucidum;  those which are proven to be, or suspected of being
poisonous are named as poisonous mushrooms, e.g., Amanita phalloides; and a miscellaneous
category which includes a large number of mushrooms whose properties remain less well
defined, which may tentatively be grouped together  as ‘other mushrooms’. Certainly, this
approach of classifying of mushrooms is not absolute and not mutually exclusive. Many kinds of
mushrooms are not only edible, but also possess tonic and medicinal qualities.

Mushrooms are devoid of leaves, and of chlorophyll-containing tissues. This renders them
incapable of photosynthetic food production. Yet, they grow, and they produce new biomass.
How? For their survival, for their growth, and for their metabolism, they rely on organic matter
synthesized by the green plants around us, including organic products contained in agricultural   10
crop residues. The organic materials, on which mushrooms derive their nutrition, are referred to
as substrates. Mushrooms are a unique biota which assembles their food by secreting degrading
enzymes and decompose the complex food materials present in the biomass where they grow, to
generate simpler compounds, which they then absorb, and transform into their own peculiar
tissues. These substrate materials are usually by-products from industry, households and
agriculture and are usually considered as wastes. And these wastes, if carelessly disposed of in
the surrounding environment by dumping or burning, will lead to environmental pollution
and consequently cause health hazards. However, they are actually resources in the wrong
place at a particular time and mushroom cultivation can harness this waste/resource for its own
beneficial advantage.

Mushrooms lack true roots. How then are they anchored into the substrates where we find them?
This is affected by their tightly interwoven thread-like hyphae, which also colonise the
substrates, degrade their biochemical components, and siphon away the hydrolysed organic
compounds for their own nutrition.

The Concept of Mushroom Biology  
The biological science that is concerned with fungi is called mycology. Mushroom biology is the
branch of mycology that deals with mushrooms in many disciplines. When knowledge increases
and areas of specialisation develop within the discipline, it is convenient to indicate that area of
specialisation with a self-explanatory name. In biology, there are such specialisations as
neurobiology, bacteriology, plant pathology, pomology, molecular biology, virology, fungal
physiology, embryology, endocrinology, phycology, and entomology. These names indicate
either a group of organisms (e.g., bacteria, algae, and insects) and /or an approach to the study
(e.g., disease, development and physiology).

Although several terms for this important branch of mycology that deals with mushroom have
been used, and each of these has its merit, when we get down to the matter of definitions, it
seems that there is a place for a new term. The new term is mushroom biology.  Mushroom
biology is a new discipline concerned with any aspect of the scientific study of mushrooms, such
as: taxonomy; physiology; genetics; etc.

Sexuality in the Edible Mushrooms 
Although the process of sexuality is complicated by nutritional and physiological conditions,
genetic constitution is the most critical factor determining both the occurrence and the
morphology of the fruiting bodies in the edible mushrooms.
Sexuality in fungi consists of three important stages.  The first essential stage is plasmogamy
which is the fusion of cytoplasm of the two mating individuals. By plasmogamy the nuclei from
two strains are brought together in a common cytoplasm. The second essential stage in sexuality
is known as karyogamy or nuclear fusion. The third  essential stage is meiosis, the nuclear
division in which the chromosome number is reduced  from the diploid to the haploid number.
The product of meiosis is the formation of a tetrad. Through the process of sexuality, genetic
recombination and segregation subsequently occurs.

Life Cycle 
If a section of the gills is cut and examined under the microscope, spores will be observed on
their surface. The spores will start to fall as the cap fully expands, indicating maturity of the
mushroom. The spores are so minute that they float  in the air and are carried by the wind.
Eventually, they fall to the ground, usually with rain. If conditions are favourable (optimum
temperature and moisture), the spores will germinate to form a mass of mycelium. This is the
start of the vegetative phase of the mushroom. Given an unrestricted amount of nutrients and
favourable growing conditions, it is capable of unlimited growth. The mycelium developing from
the germinating spore is the so-called primary mycelium and is usually uninucleate and haploid.
This stage is short-lived because mycelia from different spores tend to ramify and fuse to form
the secondary mycelium with two compatible nuclei, which continues to grow vegetatively and
is able to form fruiting bodies.