Fungi: General Characters

The word fungus (plural ; Fungi) is a Latin word meaning mushroom. The branch of science that deals with study of fungi is called mycology and branch that deals with the study of fungal diseases is called fungal pathology.

Definition

Fungi are group of those plants whose form is thallus and are mostly haploid, build of single cell (unicellular) or cells (multicellular) that posses definite cell wall and nucleus but lack chlorophyll (achlorophyllous) and heterotrophic.

Distribution

They are cosmopolitan in distribution and are ubiquitous i.e. occur in any habitat where life is possible. Fungi are a very large group having about 5,100 genera and more than 50,000 species.

Habitat

Most of them are moisture loving and terrestrial and a few are aquatic (e.g saprolegnia). Many species are parasitic, infecting plants, animals and human-beings and few are epiphytic (e.g. Armillaria on apple tree cause red rot of apple). Grow well at 20-30 degree celsius and at acidic pH 6.0.

Nutrition

They are heterotrophic and according to mode of nutrition they may be

1. Parasites. Those fungi which obtain their nutrition from other living plants or animals are known as parasites and they are further may be

a. Obligate parasites: e.g. Albugo, Ravenelia, Puccinia. It may be further Biotrophs, Hemibiotrophs or Perthotrophs.

b. Facultative saprophytes: e.g. Ustilago, Sphacelotheca, Tolyposporium

2. Saprophytes. It include those fungi which obtain their nutrition from the dead decaying organic matter and they are further may be

a. Obligate saprophytes:. E.g. Mucor mucedo, Agaricus, Morchella

b. Facultative parasites: E.g. Furarium, Pythium

3. Symbionts. Some fungi grow on other living organisms and both are mutually benefited and such association is known as symbiosis. E.g Lichens and mycorhiza.

The reserve food material is in the form of glycogen and oil globules.

Vegetative structure

The thallus consists of long, tubular, branched filaments called hyphae (gk word web) and which together form a net like structure called mycelium. The hyphae are

· Aseptate and coenocytic (Phycomycetes)

· Septate

a. Uninucleate (monokaryotic)

b. Binucleate (dikaryotic)

c. Multinucleate

The septa are of 3 types: complete septum, septum with simple pore and septum with dolipore (Basidiomycetes). In dolipore the edges of septa around pore are swollen and forms septal pore cap or parenthosome. These septal pores form a valve like structure. Septal pores allow quick transport of nutrients from the region of absorption to all parts of the mycelium.

Modification of mycelium

In majority of fungi the hyphae form a net like structure, but in some advanced fungi they may undergo certain modifications is response to functional needs such as

(a) Prosenchyma: When the hyphae lie more or less parallel to one another and unite to form a rather loosely interwoven structure so that their individuality is not lost.

(b) Pseudo-parenchyma: In this the hyphae become closely intertwined and lose their individuality and are not distinguishable from each other. They form a compact mass of parenchymatous like tissue of higher plants in cross section.

(c) Rhizomorph. The hyphae are interwoven and form a root like structure, lose their individuality and the whole mass behaves as an organised unit. It has a higher infection capacity than individual hyphae

(d) Sclerotium (Sclerotia): In this the aggregation and adhesion of hyphae form a compact globose structure. It may survive for long period and represents the resting stage of the fungus. It contain reserve food and thus help in vegetative propagation

(e) Stroma: It is thick mycelia mat in which a large number of fruiting bodies develop.

Flagella:

It is a motile body and is the characteristics feature of some lower fungi and is absent in higher fungi. The flagella are of two types:

(1) Whiplash (acronematic) flagella are smooth with 9+2 organisation,

(2)Tinsel (pantonematic) flagella with numerous minute hair-like mastigonemes on their surface.

Reproduction

They reproduce by asexual and sexual methods. The vegetative reproduction is generally considered under the category of asexual methods. Fungi may show Holocarpic or Eucarpic condition.

Asexual reproduction

It may take place by

a. Fragmentation: The fungal hyphae break up into two or more fragments due to some external force and each fragment develops into a new individual. (Rhizopus, Mucor, Aspergillus, Alternaria),

b. Fission: The vegetative cell elongates and divides into two daughter cells of equal size by simple constriction in the middle with simultaneous nuclear division. (Yeast)

c. Budding; A small outgrowth, the bud emerges out from the parent cell. Nucleus divides into two and one passes to the bud. The bud is finally detached from the mother and grows into new individual. (e.g., Yeast, Ustilago)

d. Oidia: The hyphal tips often divide by transverse wall into large number of small segments, may remain in chain or becomes free from each other, these are known as oidia. Usually thin walled and do not store reserve food. (Mucor mucedo, Collybia, Coprims).

e. Chlamydospores:The chlamydospores are thick walled round to oval in outline, coloured brown or black. They produce either terminally or in intercalary at some intervals throughout the length of hyphae. e.g., Fusarium, Ustilago

f. Sporangiospores: These spores are produced inside the sac like structure called sporangia. They may be

a. Aplanospores (Non-motile spores) are produced inside sporangium (Pilobolus, Rhizopus, Mucor)

b. Zoospores (Motile spores) and are produced inside zoosporangium (Phytophthora, Saprolegnia)

g. Conidia: These are exogenously produced non-motile spores develop by constriction at the end of specialised hyphal branches, called conidiophores. They may produce singly (Phytophthora, Pythium) or in chain (Penicillium, Aspergillus).

Sexual reproduction

It occurs in all fungi except in fungi imperfecti (Deuteromycetes). It involves union of two compatible nuclei of opposite sex which is completed in the following three distinct phases:

(a) Plasmogamy. It involves the union of two protoplasts, brings two haploid nuclei close together in the same cell.

(b) Karyogamy. In the second step, fusion of the two nuclei from the two fusing gametes takes place and a diploid zygotic nucleus is formed. In Phycomycetes karyogamy occurs just after plasmogamy, but in Ascomycetes and Basidiomycetes karyogamy is much delayed and the nuclei of the opposite strains get themselves arranged in pairs (dikaryon) and phenomenon is called dikaryotization.

(c) Meiosis. After karyogamy, reduction division takes place in the diploid nucleus, and haploid re-established. Usually meiosis occurs immediately after fusion of two nuclei.

The compatible nuclei are brought together by the following sexual processes:

1. Planogametic copulation. This involves fusion of two naked motile gametes (planogametes). It is of three types, depending on the nature and structure of the fusing gametes.

a. Isogamy. The fusing gametes are morphologically similar but differ physiologically and are formed on different hyphae (e.g., Synchytrium, Catenaria).

b. Anisogamy. The fusing gametes are both morphologically and physiologically different. The male gamete is smaller and more active than the female gamete (e.g Allomyces).

c. Oogamy. The female gamete (egg) is non-motile and the male game (antherozoid) is motile and are formed in specialised gametangia, known as oogonium and antheridium respectively.

2. Gametangial contact.

In this gametes are never released from gametangia, instead the male and female gametangia come in close contact with the help of a fertilization tube. The gametangia never fuse or lose their identity during the sexual act. The male and female gametangia are known as antheridium and oogonium. e.g. Albugo, Aspergillus, Pythium and Phytophthora

3. Gametangial copulation.

In this the entire contents of two compatible gametangia are fused and lose their identiy. e.g. Mucor, Rhizopus.

4. Spermatization.

In some advanced fungi sex organs are completely absent and the sexual process is accomplished by minute spore-like spermatia (male gametes) and specialized receptive hyphae (female gametes). The spermatia are carried by air, water or insects to the receptive hyphae, and the contents of the spermatium enter the receptive hyphae through a pore. e.g., Puccinia,

5. Somatogamy (Pseudogamy). In this two ordinary two vegetative cell or two vegetative hyphae fuse together and sex organs are not at all formed. e.g Morchella, Peziza and Agaricus.

Heterothallism

The term Heterothallism was first used by A.F. Blakeslee in 1904 when he observed that zygospores could develop in some spp. only when two mycelia of different strains were allowed to come in contact with each other. He made these observations as a result of his studies on zygospore formation in Mucorales. On the basis of his studies, he divided the various species of Mucorales into two groups: Heterothallic and Homothallic.

Heterothallic species are those which require mycelia of two different strains to interact to enable the zygospores to be formed while the homothallic species are those which require mycelia of only one strain to interact for the formation of zygospores. He concluded that the Zygospore formation is a sexual process. In homothallic species, the mycelium is bisexual while the mycelium in heterothallic species is unisexual, (+) and (-) strains represent the two different sexes. Heterothallism may therefore be defined as the condition in which Zygospore formation takes place only when mycelia arising from asexual spores of two genetically different mating types (+) and (-), are allowed to interact.

Cell wall

The composition of cell wall is variable among the different groups of fungi or between the different species of the same group. In the majority of fungi, the wall lacks cellulose but contains a form of chitin known as the fungus cellulose which is strictly not identical with insect chitin.

Generally fungal cell walls consists of chitin, glucans and mannoproteins. In some lower fungi (eg, members of Oomycetes), the cell wall is composed of cellulose and glucan.

Chitin:It is a complex polysaccharide, a polymer of N-acetylglucosamine (nitrogen containing sugar).The suggested formula for fungus chitin is (C22 H54 N21)n. These are laid down in layers and form the basis of the structural rigidity of fungal cell walls. The chitin wall of most fungi is permeable both to water and substances in true solution.

Glucan: Glucans are the most abundant polysaccharides in the cell walls of fungi and their structures are highly variable. In general, glucans are divided into two large classes: alpha bound glucans and beta bound glucans, whose characteristics and functions are rather different. Alpha 1,3 glucans are the most abundant alpha glucans present in the cell walls of fungi, being restricted to dikarya. They exist in the form of structural microfibrils that provide resistance to the cell wall. The structure of beta glucans is more complex. They are linear or branched, and contain mostly β 1,3 and β 1,6 linkages, existing in the form of microfibrils. Microfibrillar glucans with chitin constitute the most important structural components of fungal cell walls and provides rigidity.

Proteins: Proteins represent the third important chemical constituent of fungal cell walls Many cell wall proteins are modified by glycosylation. Since mannose is the main component, such proteins are often called mannoproteins or mannans. Proteins exposed at the cell wall surface can also determine surface properties such as adhesion and recognition.

· The main function of fungal cell wall is to provide cell rigidity and shape, metabolism, ion exchange, and interactions with host defense mechanisms.