Kingdom Fungi

The science dealing with the study of fungi is called as mycology. Clausius (1601) may be regarded as one of the earliest writers to describe fungi. Bauhin (1623) also included the account of known fungal forms in his book Pinax Theatric Botanica. The fast systematic account of fungi came from Pier Antonio Micheli (1729) who wrote &aposNova Plantarum Genera&apos. He is described by some workers as founder of mycology. Kingdom fungi has been called mycota by Von Arx (1981) and mycetae by Alexopoulos.

Thallus organization 

The plant body of true fungi (Eumycota) is thallus. It may be non-mycelial or mycelial. The non-mycelial forms are unicellular, however, they may form a pseudomycelium by budding. In mycelial forms, the plant body is made up of thread like structures called hyphae (sing. hypha). The mycelium may be aseptate (non-septate) or septate. When non-septate and multinucleate, the mycelium is described as coenocytic. In lower fungi the mycelium is non-septate e.g., Phycomycetes. In higher forms it is septate e.g., Ascomycetes, Basidiomycetes and Deuteromycetes. In some forms the plant body is unicelled at one stage and mycelial at the other. Their organization is sometimes described as dimorphic.

When the entire mycelium is converted into reproductive structure, the thallus is described as holocarpic. However, if only a part of it becomes reproductive, the thallus is called as eucarpic.

Specialised formation

In higher forms the mycelium gets organised into loosely or compactly woven structure which looks like a tissue called plectenchyma. It is of two types :

Prosenchyma : It comprises loosely woven hyphae lying almost parallel to each other.

Pseudoparenchyma : If the hyphae are closely interwoven, looking like parenchyma in a cross-section, it is called as pseudoparenchyma.

In addition to above, the fungal mycelium may form some specialized structures as under :

Rhizomorphs : It is a &aposroot or string-like&apos elongated structure of closely packed and interwoven hyphae. The rhizomorphs may have a compact growing point.

Sclerotia : Here the hyphae gets interwoven forming pseudoparenchyma with external hyphae becoming thickened to save the inner ones from desiccation. They persist for several years.

Stroma : It is thick mattress of compact hyphae associated with the fruiting bodies.

Cell organization

The cell wall of fungi is mainly made up of chitin and cellulose. While chitin is a polymer of N-acetyl glucosamine, the celulose is polymer of d-glucose. Precisely, the cell wall may be made up of cellulose-glucan (oomycetes), chitin-chitosan (Zygomycetes), mannan-glucan (Ascomycetes), chitin-mannan (Basidiomycetes) and chitin-glucan (some Ascomycetes, Basidiomycetes and Deuteromycetes). Besides, the cell wall may be made up of cellulose-glycogen, cellulose-chitin or polygalactosamine-galactan.

The cell wall is closely associated with the inner layer, the plasma membrane. In fungi, plasma membrane bears coiled membranes outgrowth called lomasomes (Moore and McAlear, 1961).

Fungi cells are eukaryotic. They possess all eukaryotic organelles such as mitochondria, E.R., ribosomes, microbodies, lysosomes, vacuoles and reserve food particles (glycogen, lipid etc.). Golgi body or dictyosome are also not typical. In many cases they are unicisternal. The cells lack chloroplast. However, a reddish pigment, neocercosporin has been isolated from the fungus Cercospora kikuchii. The vacuoles are bound by tonoplast. The genetic material is DNA.

Near the hyphal tip the cytoplasm contains small vesicular structure called chitosomes (Barker et.al, 1974). They contain cell wall material.

Fungi possess true nucleus having definite nuclear envelope. The nuclear envelope persists during nuclear division. It has called karyochorisis by Moore (1965). The nuclear spindle is formed within the nuclear envelope in both mitosis and meiosis. The spindle poles either contain centrioles are spindle pole bodies (SPB) but lack microtubular organization.

Nutrition

The fungi are achlorophyllous organisms and hence they can not prepare their food. They live as heterotrophs i.e., as parasites and saprophytes. Some forms live symbiotically with other green forms.

Parasites : They obtain their food from a living host. A parasite may be obligate or facultative. The obligate parasites thrive on a living host throughout their life. The facultative parasites are infact saprophytes which have secondarily become parasitic.

Some forms produce rhizoids for absorbing food. The parasitic fungi produce appressoria for adhering to the host. For absorbing food, the obligate parasites produce haustoria. The haustoria may be finger-like, knob-like or branched. Each haustorium is distinguishable into a base, stem and body.

Saprophytes : They derive their food from dead and decaying organic matter. The saprophytes may be obligate or facultative. An obligate saprophyte remains saprophytic throughout it&aposs life. On the other hand, a facultative saprophyte is infect a parasite which has secondarily become saprophytic.

Symbionts : Some fungal forms grow in symbiotic association with the green or blue-green algae and constitute the lichen. A few fungal forms grow in association with the roots of higher plants. This association is called as mycorrhiza. They are two types – Ectotrophic mycorrhiza and Endotrophic mycorrhiza e.g., (VAM).

Reproduction

(1) Vegetative reproduction

Fragmentation : Some forms belonging to Ascomycotina and Basidiomycotina multiply by breakage of the mycelium.

Budding : Some unicelled forms multiply by budding. A bud arises as a papilla on the parent cell and then after its enlargement separates into a completely independent entity.

Fission : A few unicelled forms like yeasts and slime molds multiply by this process.

(2) Asexual reproduction

Oidia : In some mycelial forms the thallus breaks into its component cells. Each cell then rounds up into a structure called oidium (pl. oidia). They may germinate immediately to form the new mycelium, e.g., Rhizopus.

Chlamydospores : Some fungi produce chlamydospores which are thick walled cells. They are intercalary in position. They are capable of forming a new plant on approach of favourable conditions, e.g., Rhizopus, Saprolengnia Ustilago.

Sporangiospores : These are thin-walled, non-motile spores formed in a sporangium. They may be uni-or multinucleate. On account of their structure, they are also called as aplanospores. e.g., Rhizopus, Mucor.

Zoospores : They are thin-walled flagellate spores which occur in aquatic fungi. These spores formed in a zoosporangium. The zoospores are of several types :

$\bullet$ Uniflagellate with whiplash type flagellum e.g., Allomyces.

$\bullet$ Uniflagellate with tinsel type flagellum e.g., Rhizidiomyces.

$\bullet$ Biflagellate with a tinsel type and a whiplash type flagella e.g., Saprolegnia.

$\bullet$ Biflagellate with two whiplash type flagella e.g., Plasmodiophora.

Conidia : In some fungi the spores are not formed inside a sporangium. They are born freely on the tips of special branches called conidiophores. The spores thus formed are called as conidia. e.g., Penicillium.

Bi-celled conidia are formed in Trichothecium. In Fusarium it is possible to differentiate smaller microconidia from larger macroconidia.

(3) Sexual reproduction : With the exception of Deuteromycetes (fungi imperfecti), the sexual reproduction is found in all groups of fungi. During sexual reproduction the compatible nuclei show a specific behaviour which is responsible for the onset of three distinct mycelial phases. The three phases of nuclear behaviour are as under :

Plasmogamy : Fusion of two protoplasts.

Karyogamy : Fusion of two nuclei.

Meiosis : The reduction division.

These three events are responsible for the arrival of the following three mycelial phases :

Haplophase : As a result of meiosis the haploid (n) or haplophase mycelium is formed.

Dikaryotic phase : The plasmogamy results in the formation of dikaryotic mycelium (n + n).

Diplophase : As a result of karyogamy the diplophase mycelium (2n) is formed.

In some fungi plasmogamy, karyogamy and meiosis do occur in a regular sequence but not at specified time or points in life cycle. Such a cycle is described as parasexual cycle and phenomena celled parasexuality recorded by Pontecorvo and Roper.

The fungi reproduce sexually by the following methods

$\bullet$ Planogametic copulation : In this type of copulation fusion occur between flagellate gamete. It is of three types :

(i) Isoplanogametic copulation : Both the fusion gametes are similar and flagellate. e.g., Synchytrium.

(ii) Anisoplanogametic copulation : The fusing gametes are flagellate, similar in structure but different in size. In which one is small and other one is large. e.g., Allomyces.

(iii) Ooplanogametic copulation : It is union between a small flagellate male gamete and a large nonflagellate food laden female gamete or ovum. . e.g., Monoblepharella.

$\bullet$ Gametangial contact : It involves fusion of two gametangia. In lower forms the female gametangium is called as oogonium. The male gametangium is termed as antheridium. A contact develops in between the two gametangia and then the male nucleus is transferred into the female directly or through a tube. e.g., Phytophthora, Albugo.

$\bullet$ Gametangial copulation : In this case the fusion occurs in between the two gametangia. When it occurs in some holocarpic forms where the entire thallus acts as gametangium, the phenomenon is called as hologamy. In others, dissolution of cell wall in between the two gametagial brings about gametangial copulation. e.g., Rhizopus, Mucor.

$\bullet$ Spermatization : Here the uninucleate male gametes called spermatia are formed in special structures called spermogonia or pycnidia. The female gametangium is called as ascogonium which has a long neck called trichogyne. The spermatium attaches itself with the trichogyne and transfers the male nucleus, thus bringing about dikaryotisation. e.g., Puccinia graminis.

Somatogamy : In higher fungi there is reduction of sexuality to the maximum level. Here two hyphae of opposite strains are involved in fusion thus bringing about dikaryotization. e.g., Penicillium brefeldianum, Agaricus.

Clamp connection

In Basidiomycetes, the dikaryotic cells divide by clamp connections. They were first observed by Hoffman (1856) who named it as &aposSchnallenzellen&apos (buckle-joints). A lateral pouch like outgrowth arises which projects downward like a hook. This pouch or clamp becomes almost parallel to the parent cell. The two nuclei now undergo conjugate division in such a way that one spindle lies parallel to the long axis of the cell and the other somewhat obliquely. As a result, one daughter nucleus enters into the clamp. Now, septae appear separating the clamp and the lower hyphal cell. The upper cell has both the nuclei. The clamp with a nucleus now fuses with the lower cell. The septum between the pouch and the lower cell is dissolved and thus the lower cell now contains both the nuclei of opposite strains. The entire process takes some 23-45 minutes.

Heterothallism

Blakeslee, (1904) while working with Mucor sp. observed that in some species sexual union was possible between two hyphae of the same mycelium, in others it occured between two hyphae derived from &aposdifferent&apos spores. He called the former phenomenon as homothallism and the latter as heterothallism. Thus, the homothallic species are self-fertile whereas the heterothallic are self sterile. In heterothallic species the two &aposthalli&apos are sexually incompatible. They are said to belong to opposite strains. Blakeslee designated them as + and – i.e., belonging to opposite strains or mating types. Bipolar heterothallism found in Mucor and Rhizopous.

Classification

The classification of fungi based on the characteristics of the life cycle involved like.

Nature of somatic phase, kinds of asexual spores, kinds of sporangia, nature of the life cycle and presence or absence of perfect or sexual stage

Important features of classes

(1) Phycomycetes (Oomycetes/Algal fungi) : It is also called lower fungi, mycelium is coenocytic. Hyphal wall may contain chitin or cellulose (e.g., Phytophthora). Asexual reproduction occurs with the help of conidio-sporangia. Under wet conditions they produce zoospores. Under dry conditions, the sporangia directly function as conidia. Zoospores have heterokont flagellation (one smooth, other tinsel). Sexual reproduction is oogamous. It occurs by gametangial contact where male nucleus enters the oogonium through a conjugation tube. The fertilized oogonium forms oospore. e.g., Saprolegnia, Albugo (Cystopus), Phytophthora, Phythium, Sclerospora, Peronospora, Plasmopara.

(2) Zygomycetes (Conjugation fungi) : Mycelium is coenocytic. Hyphal wall contains chitin or fungal cellulose. Motile stage is absent. Spores (Sporangiospores/aplanospores) are born inside sporangia. Sexual reproduction involve fusion of coenogametes through conjugation (Gametangial copulation). It produces a resting diploid Zygospore. On germination, each zygospore forms a germ sporangium at the tip of a hypha called promycelium e.g., Mucor, Rhizopus, Pilobolus, Abscidia, Saksenea.

(3) Ascomycetes (Sac fungi) : These are unicellular as well as multicellular fungi. In the latter, mycelium is septate. The asexual spores formed in chains are called conidia. The spores are formed exogenously, i.e., outside sporangium. They detach from the parent and form new mycelia. Sexual reproduction is through ascospores, which are formed endogenously (within the mycelium) in a sac like structure called ascus (pl. asci). The gametes involved in sexual reproduction are nonmotile compatible and are generally represented as + and –. The fusion of gametes is followed by reductional division that produces haploid ascospores. The fruiting body called ascocarp.

The ascocarp are of four types :

(i) Cleistothecium : It is an ovoid or spherical fruiting body which remains completely closed e.g., Aspergillus.

(ii) Perithecium : It is a flask shaped fruiting body which opens by a single pore called ostiole. e.g., Neurospora.

(iii) Apothecium : It is a saucer-shaped fruiting body. The asci constitute the fertile zone called hymenium e.g., Peziza.

(iv) Ascostroma : It is not a distinct fruiting body. It lacks its own well defined wall. The asci arise directly with a cavity (locule) of stroma. It is also called as pseudothecium e.g., Mycosphaerella.

(4) Basidiomycetes (Club fungi) : They are the most advanced fungi and best decomposers of wood. These are called club fungi because of a club shaped end of mycelium known as basidium. They have septate multinucleated mycelium. Septa possess central dolipores and Lateral clamp connections. The sexual spores called basidiospores are generally four in number. They are produced outside the body (exogenuous) unlike ascomycetes where they are endogenous. Two compatible nuclei fuse to form zygote, which undergoes meiosis and forms four basidiospores. The fruiting body containing basidia is a multicelular structure called basidiocarp. The common members are edible mushrooms (Agaricus), smut (Ustilago) and rust (Puccinia).

(5) Deuteromycetes (Fungi imperfecti) : The group include all those fungi in which sexual or perfect stage is not known. Mycelium is made of septate hyphae. Asexual reproduction commonly occur by means of conidia. e.g., Alternaria solani, Fusarium, Gibberella, Colletotrichum falcatum, Helminthosporium oryzae, Cercospora personata.

Practical Aspects

(1) Harmful aspects

Crop diseases : Several important crop plants are destroyed by fungal diseases. Some important ones are listed here under :

Diseases in human beings : Several diseases in human beings are found to be caused by fungi, infecting different parts of the body. Some of them are given here under as :

Spoilage of food : Some forms like Rhizopus, Mucor, Aspergillus, Cladosporium grow on food articles and spoil them. Cladosporium grows even at a temperature of – 6°C.

Aflatoxins : They are produced mainly by Aspergillus flavus and A. parasiticus. They are well know for their carcinogenic effect. e.g., Aflatoxin B₁, B₂, M₁, M₂, G₁, G₂.

Poisonous fungi : Some fungi are extremely poisonous e.g., Amanita phalloides (&aposdeath cup&apos), A. verna and Boletus satanus. Forms like Coprinus, Psilocybe are less poisonous. The fungus Amanita phalloides produces toxins like a-amanitin, phalloidin etc. which are very poisonous.

Ergotism : The fungus causing &aposergot&apos disease of rye (Secale) is Cleviceps purpurea. It contains many poisonous alkaloids in their sclerotia. It causes poisoning in human beings. It&aposs acute condition is called as &aposSt. Anthony&aposs fire&apos.

Hallucinogenic drugs : The hallucinogenic drug LSD (Lysergic acid diethylamide) is extracted from Cleviceps purpurea as also from Inocybe. Besides, the mushroom Amanita muscaria is also hallucinogenic.

Rotting of wood : Rotting of wood is caused due to degradation of lignin and cellulose. It is brought about fungi like Polyporus sp., Fomes sp. and Ganoderma sp., Forms like Fusarium, Penicillium leave stains on the wood.

Allergies : Spores of Mucor, Aspergillus, Penicillium, Puccinia etc., present in the atmosphere cause allergies.

Deterioration of articles : Forms like Aspergillus, Cladosporium, Rhizopus, Chaetomium, Alternaria deteriorate cork, rubber, leather, textile and even plastics.

(2) Useful aspects

Food : Forms like Agaricus bisporus, Morchella esculenta, Lentinus edodes, Clavatia gigantia, Volvariella volvacea are edible. The yeast Saccharomyces cerevisiae is used for making &aposyeast cake&apos. When mixed with cereal flour, the yeasts produce a preparation called incaparina. The Single Cell Protein (SCP) obtained from yeasts, Penicillium, Fusarium etc. are used as substitute of protein food. Rhizopus oligosporus when processed with soyabeans yield a food preparation called &apostempeh&apos. It has high protein contents.

Flavoring of food : Penicillium roquefortii and P. camemberti are employed for flavouring cheese.

Brewing and baking : Yeasts are generally used in bakeries and breweries. e.g., Saccharomyces cerevisiae.

Organic acids : Several organic acids are commercially produced by fungi, some of which are given hereunder :

Antibiotics : The antibiotics are chemicals produced by living organisms that kill other living organisms. The first known antibiotic is penicillin that was extracted from Penicillium notatum by A. Flemming, (1944). Raper (1952) also extracted the same antibiotic from P. chrysogenum. Besides, several other antibiotics have been extracted since then.

Other chemicals : Various chemicals have been obtained from different kinds of fungi. Yeast are good source of glycerol and enzymes like zymase, invertase and lipase. Cellulases are obtained from Aspergillus. Some alkaloids are also obtained from fungi e.g., Ergotinine, Ergotetrine and Ergobasine from Cleviceps purpurea. Gibberellins (plant hormones) are obtained from Gibberella fujikuroi. Another hormone, trisporic acid is obtained from Mucor mucedo.

Biological assays : The fungi can detect the presence of certain chemicals present in the medium even in traces e.g., Aspergillus niger for Mn, Pb, Zn, Cu, Mo etc.

Vitamins : Various vitamins have been obtained from different kind of fungi.

Description of some important fungus

Rhizopus/Mucor

Habitat : They are cosmopolitan and saprophytic fungus, living on dead organic matter. Rhizopus stolonifer occur very frequently on moist bread, hence commonly called black bread mold Mucor is called dung mold. Both are called black mold or pin mold because of black coloured pin head like sporangia. Besides, it appears in the form of white cottony growth on moist fresh organic matter, jams, jellies, cheese, pickles, etc.

Structure : The vegetative body or thallus consists of well branched, aseptate and multinucleate (coenocytic) mycelium on the surface of substratum. The mature mycelium is distinguishable into three types of hyphae :

(1) Stoloniferous hyphae (Stolons) : These hyphae grow horizontly on the surface of substratum. They are relatively stout and less branched than other hyphae. Certain portions of the stolons called nodes, give out rhizoids and sporangiophores.

(2) Rhizoidal hyphae : They arise in clusters from the lower side of each node and are repeatedly branched. The rhizoids penetrate the substratum and serve as anchors for the superficial mycelium. These hyphae secrete enzymes like amylase and maltase into the substratum and absorb the digested food.

(3) Sporangiophores : They are erect, aerial, unbranched reproductive hyphae that arise in clusters from the upper side of each node. Each sporangiophore develops single terminal sporangium which is filled with spores. In Mucor there is no such distinction. In Mucor, the hyphae develop singly. There is no holdfast or apparant node. The hyphal wall is made up of chitin or fungus cellulose, a polymer of glucosamine (C₈H₁₃O₅N)n. Inner to the chitin wall is a thin layer of plasma membrane. The granular protoplasm has many nuclei, glycogen and oil droplets, mitochondria, endoplasmic reticulum and ribosomes.

Reproduction : They reproduces by vegetative, asexual and sexual methods.

(1) Vegetative reproduction : It takes place by fragmentation. If stolon breaks accidentally into small segments, each part grows into a new mycelium.

(2) Asexual reproduction : It occurs by three types of non-motile mitospores, sporangiospores, chlamydospores and oidia.

Sporangiospores : The sporangiospores are also called aplanospores. They are thin walled, non-motile, multinucleate spores formed in a sporangium. A vertically growing mycelium acts as sporangiophore. Its tip now shows accumulation of food and nuclei. A septum now appears separating the outer sporangium from the inner columella. The sporangium dehisces irregularly due to collapse of columella and the spores are dispersed.

Chlamydospores : These are the perennating spores formed when the fungus starts facing dry conditions. The protoplasm of hyphae collects at certain places, rounds off accumulates a lot of food materials and develops thick wall to become chlamydospores.

Arthrospore or Oidia : In liquid, sugary and acidic pH medium the hyphae of Rhizopus and Mucor undergo septation and form small rounded reproductive cells called oidia. They multiply by budding like yeast. The budded condition is called torula stage. It takes part in alcoholic fermentation. On transfer to a suitable solid medium, each oidium forms a new mycelium.

(3) Sexual reproduction : Sexual reproduction takes place by conjugation between two multinucleate but single celled gametangia. The gametes are isogamous and non-motile.

The species of Rhizopus may be heterothallic (R.stolonifer) or homothallic (R. sexualis). But mostly heterothallic in both Mucor and Rhizopus. In homothallic species sexual union in brought about between two hyphae of the same mycelium whereas in heterothallic species it occurs between two hyphae derives from different compatible strains i.e., positive (+) and negative (–).

The two mycelial branches growing towards each other are called progametangia. Their tips become rich in food and nuclei. They enlarge and come in contact with each other. A septum is laid down separating the terminal gametangium from the proximal suspensor. The gametangium has dense cytoplasm and many nuclei whereas the suspensor has vacuolated cytoplasm with fewer nuclei. Each gametangium behaves as an coenogamete. The two gametangia fuse with each other. Plasmogamy is followed by pairing of nuclei of opposite strains. The unpaired nuclei degenerate. This is followed by karyogamy. The zygospore so formed develops a dark coloured thick wall and undergoes rest. It is also believed that karyogamy is delayed till the germination of zygospore.

On the arrival of favourable conditions the zygospore germinate. The outer wall ruptures and the inner protrudes out in the form of promycelium. The promycelium grows vertically upward and forms a terminal germsporangium. It is generally believed that meiosis occurs in the germ sporangium. Each diploid nucleus forms four haploid nuclei, of which three degenerate.

Occasionally, failure of gametangial copulation results in parthenogenous development of zygospores which are called azygospore (parthenospores).

Practical Aspects

 (1) Spoilage of food : Exposed bread and other food particles are spoiled by Rhizopus.

(2) Soft rot : Rhizopus species attack sweet potato, apple and strawberry producing soft rot or leak disease. Germinating maize grains are also attacked.

(3) Mucormycosis : Mucor pusillus and M. ramosissimus may attack internal human organs, including lungs alimentary canal and nervous system

(4) Fermented foods : Temph (a solid food from soyabean) and sufu (chinese cheese) are prepaired with the help of Rhizopus and Mucor respectively.

(5) Chemicals : Citric acid prepared by Mucor from molasses, fumaric acid and cortisone by Rhizopus stolonifer, Lactic acid by R. stolonifer and R.nodosus and alcohol by R. oryzae and M. javanicus.

(6) Antibiotic : Ramysin is produced by Mucor ramannianus.

(7) Waste water treatment : Growth of Mucor arrhizus removes heavy metal contamination of water.

Yeast

Habitat : Yeast is a saprophytic fungus of ascomycetes, found on substratum which is rich in sugars e.g., sugarcane, juice, fruits (palms, grapes), milk, etc. Some species are found on animal excreta.

Structure : Yeast was first described by Antony Von Leeuwenhoek in 1680. Yeast are nonmycelial or unicellular, which is very small and either spherical or oval in shape. However, under favourable conditions they grow rapidly and form false mycelium or pseudomycelium. Individual cells are colourless but the colonies may appear white, red, brown, creamy or yellow. The single cell is about 10 mm in diameter. It is enclosed in a delicate membrane which is not made up of fungal cellulose but is a mixture of two polysaccharides known as mannan and glycogen.

The cytoplasm is granular in appearance due to the presence of droplets or granules of fat, glycogen and volutin. The volutin is nothing but nucleic acid. The glycogen is the chief reserve food material and its bulk increases during alcoholic fermentation and at times it may form as much as 30% of the weight of the yeast cell.

Yeasts are facultative aerobes i.e., they are anaerobes but can also survive under aerobic conditions and respire aerobically as well. The yeast cells secrete extracellular enzyme zymase which converts complex sugars into simple soluble sugars that can easily be assimilated.

Reproduction : Yeast reproduces by vegetative or asexual and sexual methods.

(1) Vegetative reproduction : Yeast reproduce vegetatively either by budding or by fission.

By budding : It is the common method of reproduction in budding yeasts (e.g., Saccharomyces) under favourable conditions (i.e., when growing in sugar solution). During this process a small bud like out growth appears at one end of the parent cell which gradually enlarges in size (unequal division of cytoplasm takes place) The nucleus enlarges and divides amitotically into two daughter nuclei.

Sometimes a bud may produce another bud over it which is still attached to parent cell forming a false mycelium or pseudomycelium.

By fission : It is a common method of reproduction in fission yeasts (e.g., Schizosaccharomyces). During fission the parent cell elongates and its nucleus divides into two daughter nuclei. The two nuclei separate apart. It is followed by a transverse cytokinesis by formation of a transverse septum which develops centripetally. The two cells separate apart and behave as uninucleate vegetative thalli.

(2) Sexual reproduction : Sexual reproduction in yeasts takes place during unfavourable conditions, particularly when there is less amount of food.

The sex organs are not formed in yeasts and the sexual fusion occurs between the two haploid vegetative cells or two ascospores which behave as gametes. The two fusing gametes are haploid and may be isogamous or anisogamous. Such kind of sexual reproduction is called gametic copulation. It is the best example of hologamy i.e., the entire vegetative thallus is transformed into reproductive body. The sexual fusion leads to the formation of diploid zygote. The zygote behaves as an ascus and forms 4 – 8 haploid ascospores. These liberate and function as vegetative cells.

Guilliermond (1940) has recognised three types of life cycle in yeasts.

(1) Haplobiontic life cycle : This type of life cycle is common in Schizosaccharomyces octosporous, a homothallic species.

(2) Diplobiontic life cycle : This type of life cycle is found in Saccharomyces ludwigii.

(3) Haplodiplobiontic life cycle : This type of cycle is found in Saccharomyces cerevisiae in which haploid and diploid both types of generations are found.

In addition to above, in Schizosaccharomyces pombe, two adjoining sister cells fuse and this phenomenon is called adelphogamy. In some yeasts e.g., Debaryomyces, the mother and daughter cells fuse to form the zygote and this phenomenon is called pedogamy.

Practical Aspects

Useful activities

(1) Baking industry : Yeast are used in manufacture of bread. Kneaded flour is mixed with yeast and allowed to ferment. Yeast convert starch into sugars and sugar into CO₂ and alcohol with help of enzyme zymase, CO₂ is released when effervescence takes place due to which bread become spongy and gets swollend and is of light weight.

(2) Brewing industry : Brewer&aposs yeast or Beer yeast is Saccharomyces cerevisiae and wine yeast is Saccharomyces ellipsoidens. They perform alcoholic fermentation.

$\underset{Glu\cos e}{C_6{H}_{12}{O}_6}\underset{Zymase}{\stackrel{Yeast}{\to }}\underset{Ethylalcohol}{C_2{H}_{5}OH+2C{O}_2}$

(3) Food yeast : Yeast from brewing industry is harvested and used as food yeast. It is rich in protein and vitamins-B (Riboflavin). Special food yeasts are Torulopsis (protein), Endomyces (fat) and Cryptococus (both).

Harmful activities

(1) Fermentation of fruits and fruit juices by yeast cells makes their taste unpleasent.

(2) Parasitic species of yeast like Nematospora causes diseases in tomato, cotton and bean.

(3) Parasitic yeast cause diseases in human beings (e.g., cryptococcois, blastomycosis and torulosis).

Albugo

Habitat : Albugo is a member of phycomycetes. It is an obligate parasite and grows in the intercellular spaces of host tissues. It is parasitic mainly on the members of families Cruciferae, Compositae, Amaranthaceae and Convolvulaceae, The disease caused by this fungus is known as white rust or white blisters.

The most common and well known species is Albugo candida which attacks the members of the mustard family (Cruciferae). It is commonly found on Capsella bursa pastoris (Shepherd&aposs purse) and occasionally on radish, mustard, cabbage, cauliflower, etc. The reserve food is oil and glycogen.

Structure : The plant body of the fungus is mycelial and eucarpic. The mycelium is intercellular, branched, aseptate and multinucleate (coenocytic). The mycelium produces finger like or globular haustoria which enter into the host cells to absorb the food material. The mycelial wall is made up of cellulose-glucan.

Reproduction : The fungus Albugo reproduces asexually as well as sexually.

(1) Asexual reproduction : Asexual reproduction takes place by the formation of sporangia or conidia. The fungal mycelium collects below the host epidermis and forms sporangiophores (conidiophores). These are club-shaped and multinucleate. The sporangiophores cut sporangia in basipetal manner, that is, the oldest and first formed are at the top and the youngest and last formed are at the base.

They appear as white powdery mass on the host tissue. These smooth, rounded structures are disseminated by wind. Germination of sporangia occurs when they reach a suitable host.

If water is available sporangia forms about 4 – 12 (usually eight) zoospores within two minutes of their formation. The zoospores are kidney shaped and laterally biflagellate (unequal).

If water is not available, the sporangia germinate directly by forming germ tubes.

(2) Sexual reproduction : The sexual reproduction is oogamous type and takes place with the help of antheridia and oogonia. The antheridia and oogonia are multinucleate in the beginning but become uninucleate by disorganization of nuclei. The antheridia are club-shaped and the oogonia are globular. Both sex organs develop terminally on the hyphae. There is a single egg (oosphere) surrounded by periplasm. At the time of fertilization, a receptive papilla develops on one side of the oogonium through which the fertilization tube enters into the oogonium. Inside the oogonium the male nucleus fuses with the egg nucleus. The diploid zygote develops a warty wall and becomes the oospore. The diploid nucleus undergoes meiosis, followed by several mitotic divisions. After a period of rest, the oospore germinates and produces reniform, biflagellate zoospores. The zoopores are first released into a vesicle and then to the outside. They swim for some time, encyst and then germinate to form germ tubes. Most part of the life cycle of Albugo is gametophytic. The sporophytic phase is limited only to the oospore stage.

Lichens

The term lichen was first given by Theophrastus for superficial growth on bark of Olea europea (olive) tree. A lichen is structurally organised entity consisting of the permanent association of a fungus and an alga. The fungal component of a lichen is called mycobiont (mostly ascomycetes) and the algal component is called phycobiont (mostly blue-green alga). The fungus provides the structural covering that protects alga from unfavourable conditions, i.e., drought, heat etc. It also traps moisture from the atmosphere and anchors the lichens to a substrate. The alga prepares organic food (e.g., mannitol) by the process of photosynthesis from carbon dioxide. If the algal component is a cyanobacterium (blue-green alga), it fixes atmospheric nitrogen in addition to preparation of food. The relationship between the two is that of consortium, symbiosis or mutualism. Micheli (1729) describe 300 species of lichens. Dual nature of lichen was studied by Schwendener (1897). Ahmadjian (1963) considers fungus to be a controlled parasite. The phenomenon of controlled parasitism is called helotism. The study of lichen is called lichenology. 

Habitat : Lichens are cosmopolitan in distribution. Their growth is very slow. Some lichens growing in arctic regions are believed to be 4500 year old. The lichens which grow on stones are called saxicolous (e.g., Dermatocarpon) and those growing on barks of tree are called corticolous (e.g., Usnea, Parmelia, Graphis). Some lichen developing on wood are called lignocolous (e.g., Cyphelium) and those growing on soil are called terricolous (e.g., Cladonia, Lecidea). A few liches are aquatic (e.g., Peltigera, Verrucaria margacea). The lichens generally do not grow near smoky industrial areas where atmosphere is polluted. Cladonia rangiferina, commonly known as reindeer-moss grows luxuriantly in tundra region and form the food of animals like the reindeer and caribou (musk ox). Some of the common Indian genera are: Cladonia, Parmelia, Usnea, Physcia, Anaptychia, Lecidia, etc. Lichens are highly pigmented. They may be bluish, green, grey, yellow, orange, red and brown in colour. Some are white (Gyrophora).

Classification : Hole (1967) divided lichens into 3 classes :

(1) Ascolichens : When fungal partner belongs to ascomycetes. Most lichens are ascolichens. Ascolichens are further divided into :

Gymnocarpeae : Fruiting body is apothecium.

Pyrenocarpeae : Fruiting body is perithecium.            

(2) Basidiolichen : When the fungal partner belongs to basidiomycetes.

(3) Lichen Imperfecti : When the fungal component belongs to fungi imperfecti.

Structure

(1) External structure : The lichens vary in their size and shape. However, three main types are recognised on the basis of their habit, growth, form and mode of occurrence.

(i) Crustose or Crustaceous lichens : These lichens occur as crust over rocks, soil or tree barks, e.g., Graphis, Haematomma.

(ii) Foliose or Foliaceous lichens (Leafy lichens) : They are leaf like lobed structure which are attached to substratum by rhizoid like organs, e.g., Parmelia, Paltigera.

(iii) Fructicose or Filamentous lichens : They are branched shrubby lichens but small base e.g., Cladonia, Usnea.

(2) Internal structure : The bulk portion of lichen thallus is formed by fungal partner. The alga constitutes about 5% of the lichen body. Internally the lichens are of two types :

(i) Homoiomerous thalli : Algal cells and fungal hyphae are uniformly dispersed throughout the thallus, e.g., Collema.

(ii) Heteromerous thalli : The algal cells are restricted to algal zone only. In these forms fungal component is dominant. Usually the heteromerous thalli show 4 distinct zones.

(a) Upper cortex : Formed by compactly interwoven hyphae either without interspaces between them or interspaces filled with gelatinous substances. A cuticle like layer is present on the surface. In some species e.g., Parmelia breathing pores are present.

(b) Algal layer : Present just below the upper cortex forming photosynthetic zone of the thallus. This layer is also called gonidial layer.

(c) Medulla : Occurs nearly in the middle of the thallus beneath the algal layer, the hyphae are loosely interwoven in this layer, Parmelia, Graphis.

(d) Lower cortex : Comprising of closely packed dark coloured hyphae rhizoids arise from this layer.

Special structures and Propagules : The following specialized structures and propagules are associated with lichen thalli :

(1) Breathing pores : The upper surface of some lichens, particularly the foliose lichens is provided with pores. Here the fungal hyphae are loosely arranged. They help in aeration.

(2) Cyphellae : These are small, almost circular depressions present on the lower side of the thallus. The medullary hypae are not exposed through these depressions due to the presence of corticating hyphae. They are meant for exchange of gases e.g., Sticta. Similar structures without any cortical border are called pseudocyphellae.

(3) Cephalodia : These are gall like outgrowths present on the upper surface of the thallus. They are distinguishable into cortex and medulla with similar fungal but different algal components from that of the main thallus. Lichens having two algal and one fungal partner are called diphycophilous. The cephaloida are meant for retaining moisture e.g, Peltigera.

(4) Isidia : These are coral like, simple or branched outgrowths present on the upper surface of the thallus. They have the same algae and fungal component as that of the main thallus. They help in photosynthesis as also in vegetative propogation e.g., Parmelia, Peltigera.

(5) Soredia : It is a powdery mass comprising both algal and fungal components formed in a postule like structure called soralium. The soralia arise from the algal zone lying just below the upper cortex e.g., Physia, Parmelia.

Reproduction : Lichens reproduce both by asexual and sexual methods.

(1) Asexual reproduction

Fragmentation : The main thallus breaks into small pieces which grow as independent thalli.

Rejuvination : Plants like Cladonia show this unique phenomenon. It becomes young again. The older parts of the thallus die whereas the young branches continue to grow.

Conidia : In serveral lichens the fungal component forms conidia on conidiophores. They form new fungal mycelium which with suitable algal component form the lichen.

Pycniospores : The conidia formed in a flask shaped structures lying embedded in the thallus (pycnidia) are called as pycniospores. The pycniospores form new fungal mycelium which consitute the lichen on coming in contact with suitable algal component.

(2) Sexual reproduction : Sexual reproduction in lichen is performed only or mainly by its fungal component. So, the structure of the reproductive organs is dependent upon the type of their fungal partner.

Practical Aspects

 (1) Pioneer of vegetation : Lichens are considered as pioneers in initiating a plant succession on rocks. These are the first plant group which play an important role in the formation of the soil. So lichens are called as formers of nature or soil builders.

(2) Food and Fodder : Reindeer moss (Cladonia rangiferina) of the arctic region is used as food for reindeer and cattle. Iceland moss (Cetraria icelandica) is ground up and mixed with wheat and made into cakes in Iceland. Rock tripe (Umbillicaria) has been eaten by travellers when they face starvation in arctic regions. Evernia is used by Egyptians for making bread and Umbilicaria esculenta is regarded as a delicacy in Japan. Species of Parmelia are used as curry powder in India.

(3) Medicinal uses : Dog lichen (Peltigera canina) was used as medicine for hydrophobia in ancient days and Lungwort (Lobaria pulmonaria) was used for the diseases of lungs. Usnic acid obtained from Usnea (old man&aposs beard) and Cladonia sp. is used as broad spectrum antibiotic. It is effective against gram positive bacteria. Lobaria pulmonaria, Cetraria icelandica are used in respirotory diseases particularly T.B., Roccella montagnei in angina, Parmelia sexatilis in epilepsy and Usnea barbata in urinary troubles.

(4) In perfumery : Ramalina and Evernia, having sweet scented thalli, are used in the preparation of Dhup, Havan Samagri and soap. Perfumes are extracted from Evernia prunastri and Lobaria pulmonaria.

(5) In tanning and dying : Lichens like Cetraria icelandica and Labaria pulmonaria are used in tanning. A red dye is obtained from Ochrolechia sp. whereas Parmelia sp. yield a brown dye. Litmus used as acid-base indicator is obtained from Roccella montagnei and Lasallia pustulata. An orchill dye is obtained from Roccella and Lecanara which is purified as orcein and used as a biological stain.

(6) In brewing and distilling : The lichens contain carbohydrates in the form of lichenin. Cetraria islandica and Cladonia rangiferina (yield upto 66% of the polysaccharides) are used to obtain alcohol in Sweden and Russia.

(7) Indicators of air pollution : Lichens are very sensitive to $S{O}_2$ and grow only in $S{O}_2$ free atmosphere. So lichens like Usnea are used as indicators of air $S{O}_2$ pollution.

(8) As poison : Some lichens are poisonous also such as Letharia vulpina due to vulpinic acid, Cetraria juniperina due to pinastrinic acid, Parmelia molliuscula due to selenium, Xanthoria parietina due to beryllium and Everina furfuracea due to chlorine.

(9) Other uses : Some lichen yield important chemicals e.g., salazinic acid (Ramalina siliquosa), Lecanoric acid (Parmelia subrudecta) and squamatic acid (Cladonia crispata) etc. In hot season, Usnea gets dry and becomes highly inflamable. It easily catches fire and causes forest fires.

Mycorrhizae

The term &aposmycorrhizae&apos was coined by Frank (1885). It is an association between a fungus and the root of a higher plant e.g. Pine, Birch, Eucalyptus, Ficus etc. The actively growing roots of higher plants get infected by fungi. As a result, the roots are modified (i.e., become tuberous, nodulated, coralloid, etc.) to accommodate fungi. The root cells and fungi directly transmit nutrient substances to each other. Mycorrhiza is an example of symbiosis or mutualism. 

Types of mycorrhizae : Mycorrhizae are classified into three categories :

(1) Ectotrophic mycorrhiza : It occurs only in about 3% of plant species, majority of which are forest trees, viz. pines, spurs, firs, oaks, beeches, birches, eucalyptus etc. The fungus partner is commonly a basidomycetes. In this type of mycorrhizae, the fungus completely encloses the rootlet in a sheath or mantle of tissue formed of compact hyphal cells and penetrates only between the cells of root cortex. The ectomycorrhizal fungus cannot exist saprotrophically in nature without a plant host association. Such roots are devoid of root hair, root cap and may become unforked, bifurcate, nodular or coralloid.

(2) Endotrophic mycorrihiza : In this kind of mycorrhizae the fungus does not form an external mantle but lives within the root. The endomycorrhizae are further divided into three groups :

(i) Ericaceous mycorrhizae : The fungus forms dense intracellular coils in the outer cortical cells.

(ii) Orchidaceous mycorrhizae : These are associated with orchid roots. The fungus forms association from the time when the orchid seeds germinate.

(iii) Vesicular-arbuscular mycorrhizae (VAM) : The fungi of this group mostly belong to zygomycetes. This type is significant in agriculture because it occurs in a large number of crop plants. The fungal hyphae develop some special organs, called vesicles and arbuscules, within the root cortical cells.

(3) Ectoendotrophic mycorrhiza : This type of mycorrhiza sharing characteristics of both ecto and endotrophic mycorrhizas. The fungus forms a hyphal mantle and Hartig net as do the ectotrophic mycorrhiza and also establish haustoria and hyphae coils in the epidermal and cortical cells, like the ectorophic mycorrhizas. The external hyphae deliver organic compounds absorbed from the humus to the root cells. One of the best studied examples of ectoendotrophic mycorrhizas is the mycorrhiza of Monotrapa indica, the Indian pine.

Advantages of mycorrhizal association

(1) The fungal hyphae increase a plant&aposs uptake of certain nutrients from the soil, particularly phosphorus, copper, zinc, nitrogen and potassium.

(2) The mycorrhizal hyphae permeate the soil and help the absorption of water by host more efficiently.

(3) The mycorrhizal plants need less fertilizer and can even grow better on the infertile soils. They withstand high doses of heavy metals and acid rain pollution.

(4) The fungi produce various growth promoting substances which help the plants to grow better.

(5) Due to mycorrhizal association, the higher plants develop resistance to soil borne diseases (due to phytolaxins released by fungi), drought resistance and tolerate salinity, pH and temperature extremes.