Download Fungi represent a group of heterotrophic living organisms which are

Fungi represent a group of heterotrophic living
organisms which are distinct from all others, and have
been assigned to their own kingdom, Fungi. At present,
about 100,000 species of fungi are scientifically known,
and around 200,000 more species are estimated to still
undiscovered.
Fungi are causative agents for many plant diseases.
The plant pathogen that caused Irish Famine (1845-47)
was the fungus Phytophthora infestans. In humans, fungi
are responsible for the ringworm of the body and
athlete’s foot diseases. However, all fungi are not
detrimental. The yeasts are of commercial importance in
the production of bread, beer and wine. Other fungi are
used by pharmaceutical companies to produce drugs and
antibiotics such as penicillin. Together with heterotrophic
bacteria, fungi are the principal decomposers of the
biosphere. Decomposers are as essential to the existence
of life on earth as the producers. Many fungi form
important symbiotic relationships, such as mycorrhizae
and lichens.
General characters
Cellular organisation of Fungi
All fungi are filamentous, except the unicellular ones. The
individual thread-like filaments are called as hyphae
(singular: hypha). A
profuse
branching
network
of
hyphae is known as a mycelium (plural: mycelia) (Fig.
1.). The word ‘mycelium’ is derived from Greek word
mykes, meaning ‘fungus. It is because of this reason, the
discipline that deals with the scientific study of fungi is
known as mycology.
The hyphae can be long and multinucleate cells
known as coenocytes (e.g., Water molds), or can be
septate and cellular (e.g., Sac fungi).
A typical fungal cell is eukaryotic in character, i.e., it
has a membrane bound nucleus, a nucleolus, and
membranous organelles in the cytoplasm. Mitochondria
are generally abundant. The endoplasmic reticulum is
sparse and of smooth type, i.e., without attached
ribosomes. The ribosomes are of 80 S type. Small
vacuoles
are
present,
dictyosomes
plastids are totally absent.
are
rare,
while
Like roots, the hyphae grow only at tips. The cell wall
of all fungi, except Oomycota, particularly the innermost
wall
layer,
is
rich
in
chitin:
a
polymer
of
N-
acetylglucosamine arranged in microfibrils similar to
cellulose. The chitin layer is followed by a predominantly
proteinaceous layer. External to this layer, and outermost
to it, is a thick layer of polymerised sugars of complex
structure.
Septa
Another striking feature of hyphae is the nature of
the cross walls, called as septa (singular: septum). Lower
fungi mostly lack the septa. For example, hyphae in
Myxomycota are coenocytes with no septa; hyphae in
Oomycota and
Zygomycota are
usually
nonseptate,
comprising of long, tubular cells with hundreds of nuclei.
On the other hand, hyphae in Ascomycota and
Basidiomycota are septate. In Ascomycota, the septum
has a small and simple pore in its center that allows cellto-cell
cytoplasmic
connection
(Fig.
2.a.).
In
Basidiomycota, the pore in the septum has a complex
structure, with a collar or hemispherical cap on each side
(Fig. 2.b.).
Fruiting body
In Ascomycota and Basiodiomycota, some part of
mycelium forms a large, compact and highly organised
structure
called
as
a
fruiting
body,
e.g.
morel,
mushroom, and puffball. The fruiting body is the chief
means of spore production. Within large fruiting bodies of
some fungi (e.g., bracket fungi), three types of hyphae
occur:
(i)
Generative hyphae, which are thin-walled and
produce spores.
(ii)
Skeletal hyphae, which are thick-walled and
unbranched.
(iii) Binding hyphae, which are thick-walled but show
profuse branching. These adhere to the other
types and bind them into a solid structure.
Modes of nutrition
Like animals, all fungi show heterotrophic mode of
nutrition. However, unlike animals, fungi are unable to
engulf food materials. Therefore, fungi derive nutrients
from the environment, from living, dying or dead
organisms. Based on this, fungi are divided into three
categories:
i)
Biotrophs: These obtain nutrients from the living
hosts, without killing them. Many biotrophs
secrete
chemicals
that
make
the
plasma
membrane permeable to sugars. As they leak
from the host cell, the fungus absorbs them.
Whereas majority of biotrophic fungi remain
confined to intercellular spaces, a few are able
to pierce through the cell wall by creating a hole
and inserting a portion of its filamentous cell,
called as haustorium. The haustorium brings it
close to the plasma membrane and makes it
easier to absorb nutrients.
ii)
Necrotrophs: These first attack and kill the living
hosts and then absorb the released nutrients.
Many necrotrophs secrete toxins that kills host
cell.
Fungal
toxins
damage
the
plasma
membrane due to loss of nutrient equilibrium
that kills host cell.
iii) Saprotrophs: Absorb nutrients from the dead
organisms, after they have died of other causes.
Saprotrophs
extracellular
predominantly
digestion,
depend
whereby
on
the
digestive
enzymes are secreted to breakdown the host
polymers.
Modes of reproduction
Spores
The spore formation is a characteristic feature of
fungi. The spores are resistant resting stages, which are
the
primary
means
of
reproduction,
dispersal
and
survival. These are produced either asexually or sexually.
Asexual spores
In the Mastigomycota and Zygomycota, asexual
spores produced are called sporangiospores; the latter
are produced inside the large swollen tip of a hypha. At
the tip of hypha, cytoplasm and many nuclei accumulate.
Then a septum seals the region off from the rest of the
hypha and each nucleus organises the cytoplasm around
it to form a spore. When the hyphal wall breaks down,
the spores are released. When the sporangiospores
germinate, they are non-motile except in Mastigomycota.
In
the
Ascomycota,
Basidiomycota
and
Deuteromycota, asexual spores are formed in conidia
(singular: conidium). In other words, spores do not form
inside a sporangium. The simplest case of conidia
formation is where the tip of a hypha forms septa,
cutting off several uninucleate cells, each forming an
individual spore. In more complex forms, special flaskshaped cells push out a large bud of material, which
forms a septum and becomes conidium. Subsequently,
more and more material is budded off from the basal cell
and a chain of conidia is produced – forming a long and
upright conidiophore (Fig. 3). On germination, conidia
are always non-motile and grow out as a hypha that
develops a new mycelium.
In fungi, the spores are small in size and light in
weight, thus being able to get dispersed easily to great
distances. Many fungi have long-lived spores that can
withstand environmental fluctuations. The most common
type of resistant spore is a chlamydospore, especially in
soil fungi. Chlamydospores are formed, when a mass of
protoplasm
accumulates
rich
in
within
a
reserves
of
short
length
oil
of
or
glycogen
hypha
with
thickened walls. As the conditions become unfavourable,
the rest of the hypha dies. Only the chlamydospore
survives, until the return of favourable conditions.
Another type of resistant structures that develop
generally in biotrophic fungi are called as sclerotia
(singular: sclerotium). It develops from a portion of
branching mycelium in which the hyphae form an
aggregate structure. The outermost hyphae are swollen,
globose and thick-walled. The inner mass of hyphae is
thin-walled
and
filled
with
nutrients.
The
sclerotia
germinate depending upon the conditions favourable to
the host.
Heterokaryosis
Almost in majority of the fungi, sexual reproduction
does not involve the production of distinct gametes.
Instead, hyphae of one mycelium fuse with those of the
other. These two mating hyphae are morphologically
similar but physiologically different and are designated as
plus (+) and minus (-).
The fusion of two hyphae, called plasmogamy, is
not immediately followed by the nuclear fusion, called
karyogamy. The fused hypha that contains two types of
nuclei (+ and -) forms a dikaryon. It grows for
sometime, a condition called as heterokaryosis. The
latter is similar to being diploid as the + nuclei may carry
different alleles than – nuclei. Ultimately, the karyogamy
occurs, wherein the two nuclei of compatible mating
types fuse to form a diploid nucleus. This is followed
immediately by meiosis, during which synapsis and
crossing over takes place. As a result of this, + and –
types of spores are produced that have recombination of
alleles. The spores, numbering millions, blow away, and
each germinates into a new + or – mycelium.
Parasexuality
In
some
fungi,
the
compatible
nuclei
fuse
prematurely to form a diploid nucleus followed by
meiosis, a process termed as parasexual cycle. Then
the hypha returns to a heterokaryotic phase, but with
haploid nuclei. When these nuclei participate in the
formation of conidia or sporangiospores, the spores in
that case have different genotype than the original
parental nuclei. In the Deuteromycota, the parasexual
cycle is the only means of genetic recombination.
Classification of fungi
In the two-kingdom classification, fungi along with
bacteria were
grouped under plants, because
they
produce spores and have cell walls. More recently, it has
become now clear that the living organisms grouped
together as fungi are obviously different from plants
because:
(i)
They lack plastids.
(ii) Their walls are generally devoid of cellulose.
(iii) The
main
body
is
filamentous
rather
parenchymatous.
(iv) They differ in many biochemical pathways.
than
Presently, the classification of fungi is in a state of
flux. More recently, with molecular evidences available,
the classification of fungi is witnessing a continuous
splitting and regrouping of different divisions under the
kingdom Fungi. The Slime molds and Water molds
traditionally grouped together under fungi, are now
excluded from the true fungi, and are included under the
kingdom Protista. Still, for the purpose of simplicity, all
the organisms that were previously known as fungi,
including true fungi can are recognised under the
following seven divisions (or phyla):
Kingdom: Mycota (Fungi)
Division-I – Myxomycota
Division-II – Oomycota
Division-III – Chytridomycota
Division-IV – Zygomycota
Division-V – Ascomycota
Division-VI – Basidiomycota
Division-VII – Deuteromycota
Diagnostic characters of Divisions
Myxomycota (Slime molds)
Slime molds are quite distinct from the true fungi.
These are heterotrophic and form spores, but lack cell
walls and show a unique body organisation. The body is a
large
mass
of
protoplasm containing
thousands
or
millions of nuclei, all in the same cytoplasm and covered
by only a plasma membrane. This mass of protoplasm,
called as a plasmodium, is capable of migrating over a
substrate just like amoeba.
Oomycota (Water molds)
Members of this division are aquatic for part of their
life cycle. They live in water, soil, and a few are parasitic
on plants and animals. The great potato blight and
famine in Ireland in 1846-47 was caused due to
Phytophthora infestans, which belongs to this group.
The members belonging to Oomycota range from
unicellular
to
highly
branched,
coenocytic
and
filamentous forms. The latter somewhat resemble the
hyphae that are characteristic of true fungi. For this
reason, they were grouped under the fungi until recently.
They reproduce both sexually and asexually. The asexual
reproduction is by means of motile zoospores, with two
flagella, one tinsel and another whiplash. Unlike true
fungi, their cell wall is composed of cellulose, and not the
chitin. The sexual reproduction is of oogamous type.
Chytridomycota (Chytrids)
Chytrids are predominantly aquatic fungi. They also
occur in soils from ditches, ponds and streams. Several
chytrids live as saprophytes on substrates, such as dead
insects and a few are plant pathogens.
Chytrids are either unicellular throughout their life
cycle or form a small non-septate mycelium, and are thus
coenocytic. Like other fungi, their cell wall contains chitin,
and store glycogen. They are distinguished from other
fungi primarily by their motile zoospores and gametes,
most
of
which
have
a
single,
flagellum.
Zygomycota (Conjugation fungi)
posterior
whiplash
These fungi are mostly terrestrial. Most of these
fungi live on decaying plant and animal matter in soil,
occurring mostly as saprophytes. Some of these are
parasitic on plants and animals; and a few form symbiotic
association with plants (endomycorhizae). These fungi
have
simple
mycelia
consisting
of
branched
and
coenocytic hyphae. Unlike water molds, the asexual
spores and gametes are non-motile.
The
phylum
is
named
for
its
characteristic
zygospores, a highly resistant diploid cell formed at the
time of conjugation, when two gametangia unite, prior to
fertilisation. The zygospores develop within thick-walled
structures called zygosporangia, that remain dormant for
longer
periods.
stolonifer)
is
The
the
black
most
Zygomycota.
Ascomycota (Sac fungi)
bread-mold
familiar
example
(Rhizopus
of
the
All the members of the phylum show filamentous
growth forms, except the unicellular yeasts. The hyphae
have perforated septa. The characteristic feature of this
phylum is the ascus, a sac-like structure containing
haploid sexual spores termed as ascospores. The ascus
formation occurs within a dense compact mass of
interwoven hyphae, forming a fruiting body, called
ascocarp. Asexual reproduction usually occurs by the
formation of multinucleate conidia, which are borne at
the tips of modified hyphae called conidiophores. Unlike
Zygomycota, which produce spores internally within a
sporangium,
the
Ascomycota
produce
the
spores
externally as conidia.
Ascomycota include a number of economically and
ecologically important fungi. Perhaps the commercially
most important fungi are the yeasts. The morels and
truffles are highly prized delicacies. Most of the red and
brown molds that cause food spoilage belong to this
phylum. Fungi that cause Powdery mildew of grapes,
Dutch elm disease, chestnut blight, etc. belong to this
group.
Basidiomycota (Club fungi)
Club fungi include the mushrooms, puffballs, shelf
fungi, and important plant pathogens – the rusts and
smuts. Members of Basidiomycota play a central role in
the decomposition of plant litter, often contributing twothirds of the living biomass in the soil.
The mycelium of these fungi is always septate but
perforated. The pore of the septum has an inflated
barrel-shaped margin called a dolipore. The spores
produced by these fungi, following meiosis, are called
basidiospores,
and
are
borne
on
a
club-shaped
structure called basidium.
Deuteromycota (Fungi Imperfecti)
These are an artificial assemblage of fungi for which
only the asexually reproducing state is known. This may
be because the sexual phase has not been discovered as
yet, or has been lost in the course of evolution. The
economically important fungi, such as Penicillium and
Aspergillus belong to Deuteromycota. Some of these
fungi produce aflatoxins that are highly carcinogenic.
They have often been called as ‘Fungi Imperfecti’
because they were believed to be ‘imperfect’ members
among the sexually reproducing ‘perfect’ organisms.
When the sexual (i. e. ‘perfect’) phase in the life cycle of
these forms is discovered, most of them seem to belong
to Ascomycota, and a few to Basidiomycota or
Zygomycota. Like Ascomycota, they reproduce asexually
by means of conidia.