GENERAL CHARACTERISTICS OF FUNGI - HABITAT, NUTRITION, VEGETATIVE STRUCTURE, FUNGAL CELL STRUCTURE, REPRODUCTION

GENERAL CHARACTERISTICS OF FUNGI - HABITAT, NUTRITION, VEGETATIVE STRUCTURE, FUNGAL CELL STRUCTURE, REPRODUCTION

On the basis of mode of nutrition, fungi are divided into two types – saprophytes and parasites.

1. Saprophytes:- The fungi which live on dead or decaying organic plant and animal remains, and other rotten organic food matters are known as saprobes or saprophytes. As per example, species of Mucor, Rhizopus, Agaricus, etc. The vegetative body i.e., hyphae of these fungi come in intimate contact with the substratum and obtain food by direct diffusion through the hyphal walls, causing thereby disintegration of the organic matter which is utilized by those fungi.

2. Parasites:- The fungi which live on other living organisms (plants and animals) and obtain their food i.e., the nutrients from the living tissues of the organisms are known as parasites. The organisms, upon which they live, are called hosts. Among the parasitic fungi, three degree of parasitism can be seen –

a) Obligate parasites – They can grow only upon suitable living host tissues. They cannot be grown in artificial media. Example: Downy and powdery mildews.

b) Facultative saprophytes – Normally they live as parasites and obtain their best development as such. However, they can also live as saprophytes under emergent circumstances. They can also be cultured in an artificial media. Example: Phytophthora infestans, a parasite on potato and Taphrina deformans, a leaf curl fungi.

c) Facultative parasites – Normally they live as saprophytes on soil, but when suitable host plants are shown in such a soil, they attack the plant and start living as parasites. Example: Fusarium species, Botrytis cinerea, etc.

Fungi are unable to manufacture their own food material due to the lack of green pigments in their body. They had to obtain their food either from living organism as parasites or from dead organic substances as saprophytes.

Alexopoulos and Mims (1979), mentioned that usually all fungi require – C, O, H, N, P, K, Mg, S, P, Mn, Cu, Mo, Ca, Fe and Zn. These elements are classified as major and minor elements according to the quantity required by fungi –

Macro or major elements:- Some of these elements are required by fungi in large amounts and due to the lack of which, the growth of fungi is adversely affected. Such elements are called macro elements. Example: C, N, H, O, S, P, K and Mg.

Minor or micro elements:- Some elements are required by fungi in small amounts or in trace. These are called trace or micro elements. Example: Ca (Calcium).

Fungi also synthesize some compounds which are functioning as vitamins. Glucose, ammonium and nitrogenous compounds as nitrates form the best food for fungi. Excess of food is stored by fungi in the form of glycogen and lipids.

C. VEGETATIVE STRUCTURE:

1. Thallus:- The vegetative phase of a fungus is a thallus. It may be unicellular or filamentous.

a) Unicellular thallus:- In some lower fungi such as chytrids, Plasmodiophora, yeast, etc. the thallus is made of single cell. The cells in chytrids are more or less spherical in structure. In Plasmodiophora, it is a naked, multinucleate, amoeboid mass of protoplasm.

b) Filamentous thallus:- Majority of fungi have filamentous thallus. It originates through the germination of spore. The spore germinates on falling in a suitable substratum give rise to a mass of fluffy thallus consisting of a cottony mass of fine, branched filaments. These filaments are called hyphae. Collectively, the hyphae comprise the vegetative body of a fungus called the mycelium.

2. Mycellium:- The fungal vegetative body is called the mycelium. The fungal mycelium, in fact is a multinucleate structure lacking complete internal cell boundaries. The fungal mycelium is of two types – aseptate and septate mycelium.

a) Aseptate mycelium:- In algal fungi (Class – Phycomycetes), the mycelium in the vegetative phase usually lacks internal partitions. The hyphae are thus multinucleate and aseptate. The mycelium is a continuous mass. It grows terminally by the apical elongation of the hyphae accompanied by the increase in the number of nuclei by nuclear divisions. The aseptate multinucleate mycelium is called coenocytic.

b) Septate mycelium:- The members of other class of fungi (Class – Ascomycetes and Basidiomycetes) develop internal cross walls called the septa which divide the hyphae into segments. The segments may be uninucleate or multinucleate. In a septate mycelium, the septa are in transverse position. Each septa has a pore through which cytoplasmic contents and even the nuclei can regularly pass from one cell to another.

D. FUNGAL CELL STRUCTURE:

A typical fungal cell shows the following parts –

1. Cell wall:- Cell wall is the outermost covering of the fungal cell. The composition of the cell wall is variable between different species of fungi. But in majority of fungi, the cell wall lacks cellulose and contains chitin, known as fungus cellulose. Chitin occurs as elongated variously oriented microfibrillar units which are laid down in layers to form the basic structure of the cell wall. Along with the chitin, the chief constituents of the cell wall are polysaccharides, proteins, lipids and other substances are also seen.

2. Protoplast:- The living substance of the cell inside the cell wall is the protoplast. It lacks chloroplast but is associated with other cell constituents like – cell membrane, cytoplasm, cell organelles and one or more nuclei.

a) Cell or plasma membrane – It is delicate, extremely thin, permeable, living membrane which encloses the protoplast. The cell membrane is pressed against the cell wall except the occasional invaginations in some regions. The invagination is in the form of folded convoluted pockets enclosing granular or vesicular material.

b) Cytoplasm – Within the plasma membrane lie the colourless cytoplasm in which sap filled vacuoles may occur. Immersed in the cytoplasm are structures known as organelles and inclusions. The organelles are living structures, each with a specific function. The inclusions are dead and have no specific function to the cell. The different cell organelles present in the cytoplasm of fungal cells are – ER, mitochondria, ribosomes, golgi apparatus and vacuoles.

i) Endoplasmic reticulum (ER): The ER of fungal cell is highly vesicular. Usually it is loose and more irregular than the green plants. ER of fungal cell is composed of a system of membranes or microtubular structures usually beset with small granules.

ii) Mitocondria: The mitochondria of a fungal cell is a small spherical body. Each mitochondrion is enveloped by a double membrane. The inner membrane is infolded to form a cristae, filled with fluid. The mitochondria functions as the power house.

iii) Golgi apparatus: Golgi apparatus occurs in fungi Oomycetes. It is a flattened sac surrounded by bubble like structures.

iv) Vacuoles: The vacuoles appears in older fungal cells. They are colourless sap-filled which has a tendency to enlarge and reduce the the cytoplasm to thin lining layer within the cell wall.

v) Nucleus: The cytoplasm of fungal cell contains one, two or more globose or ellipsoidal nuclei. Structurally nucleus consist of - central dense body with a clear area around it, chromatin strands and the nuclear membrane.

vi) Inclusions: The cytoplasm contains various type of inclusions. Some of the inclusions embedded are lipid globules, granules of glycogen, oils, carbohydrates, proteinaceous materials and volutin. It also contains carotenoids, enzymes and organic acids.

Three types of reproduction are observed in fungi. These are – vegetative, asexual and sexual method of reproduction.

1. Vegetative Reproduction:- Vegetative reproduction takes place by the following methods –

a) Fragmentation - It involves the breaking up of the entire thallus or some portion i.e., hypal filaments into one or more pieces. Each piece grows into a new plant.

b) Fission - This is noted in some unicellular fungi such as yeast. It involves the splitting up of the vegetative cell into two daughter cells by constriction or by transverse wall.

c) Budding - In this case a vegetative cell gives rise to small outgrowths called buds. The buds, after detachment from the parent cell grow into a new individual.

2. Asexual Reproduction:- Asexual reproduction takes place by means of one or more specialized reproductive units called spores. The different types of spores which bring about asexual reproduction are as follows –

a) Oidia – These are also called arthrospores. They are vegetatively formed rectangular or rounded fragments which arise directly from breaking up of hyphae. Example – Trichosporon biegeli.

b) Conidia – These are non-motile spores cut off externally either singly (Phytophthora) or in chains (Penicillium) from the tip of the specialized aerial hyphae called conidiophores.

c) Zoospores – These are endogenously produced unicellular, naked, motile spores having one or two flagella. Zoospores are produced within a sac-like structure called zoosporangium which is formed at the tip of zoosporangiophore. Example – Phytium, Phytophthora, etc.

d) Sporangiospores – These are unicellular, endogenously produced non-motile and non-flagellate spores formed within the sac-like structure called sporangium, developed at the tip of special erect hypha called sporangiophore. Example – Mucor, Rhizopus, etc.

e) Chlamydospores – They are unicellular, non-motile, thick walled, dark coloured, non-deciduous resting spores, formed due to the enlargement of terminal or intercalary hyphal cell. They accumulate much food reserves and form a thick wall. During favourable conditions the chlamydospore resume growth and develop into a new individual. Example – Fusarium, Phytophthora, Mucor, etc.

f) Ascospores – These are unicellular or multicellular and non-motile spores formed endogenously in a definite number (4 to 8) after meiosis within a special type of structure called ascus. It is the characteristic spores of Ascomycotina. Example – Saccharomyces, Peziza, Ascobolus, etc.

g) Basidiospores – These are non-motile, unicellular spores formed exogenously on a special type of fruiting body called basisium. It is the characteristic spores of Basidiomycotina. Example – Agaricus, Coprinus, etc.

h) Zygospores – These are sexually produced diploid spores,  which are formed as a result of union of two morphologically undifferentiated gametes. It is the characteristic spores of Zygomycotina. Example – Mucor, Rhizopus, etc.

i) Oospores – These spores are formed as a result of fusion of two morphologically differentiated gametes i.e. male and female gametes. It is the characteristic spores of Oomycotina.

3. Sexual Reproduction:- It involves the production of new individual by the method of fusion of two morphologically different gametes i.e., male and female gametes. The various methods of sexual reproduction in fungi are as follows –

a) Gametic copulation – It involves the fusion of two gametes, may be in between two isogametes (two gametes indistinguishable in size and structure) and anisogametes (two gametes distinguishable in size and structure). When fusion occurs between two isogametes, it is called isogamy and when fusion occurs between two anisogametes, it is called anisogamy. Example – Monoblepharis, Monoblepharella, etc.

b) Gametangial contact – In this process, the two gametangia of opposite sex i.e., antheridia and oogonia, come in contact to each other, next the male gamete migrate from antherediun to the oogonium through a pore developed at the point of wall contact via fertilization tube. Soon after the migration of the gametes the gametangium, ie., both the antheredium and the oogonium disintegrate. In some cases only the 

antheredium may disintegrate and the oogoniun continues development in various ways. Example – Phytium aphinidermatum, P. debaryanum, etc.

c) Gametangial copulation – In this method, fusion of entire contents of the two copulating gametangia takes place. This process takes place in any one of the two ways – i) transfer of the whole contents of the antheredium to the oogonium, and ii) by fusion of the two gametangia into one cell.

d) Spermatization – In this process, small, uninucleate, unicellular spore-like bodies called spermatia are carried by various agencies like insects, water, wind, etc., to the female gametangia or a typical receptive hyphae to which they remain allied.  The contents of the spermatia passes to those receptive hyphae through a pore formed due to the dissolution of the walls of contact. Example – Padospora anserine, Puccinia, etc.

e) Somatogamy – In most of the higher fungi where sex organs are absent, cells takes over the sexual reproduction. In this process the nucleus of the cell passes into the corresponding somatic cell by dissolving the walls between them and thus a dikaryotic condition is established.