Fungi
Q.1. Give an account of classification of fungi as studied by you giving characteristic features of divisions and subdivisions. (2009)
Ans. Mycota: -
General Characteristic: -
(a) Mycota or fungi are a very large and diverse group of plants.
(b) They are heterotrophic organisms and are distinguished from the algae and most other plants by the absence of chlorophyll and so instead of manufacturing their own food are either saprophytes obtaining their food from dead organic matter or parasites obtaining food from other living plants or animals.
(c) The vegetative phase may be plasmodium or unicellular (in a very few simple fungi, eg, chytrids and yeasts) but in most fungi it ios made up of numerous filaments called hyphae. The hyphae branch freely and form a tangled mass called mycelium.
(d) They differ from the bacteria in that their cels contain definetly organized nuclei with nuclear membrane and nucleoli.
(e) They reproduce both sexually and asexually.
The division Mycota is divided into two sub divisions-The Myxomycotina including the slime moulds and the Eumycotina including the true fungi.
Sub-division Myxomycotina: -
General characteristics: -
(a) The Myxomycotina commonly known as slime moulds or slime fungi combine the characteristic of both plants and animals.
(b) The vegetative body consista of a multinucleate mass of naked protoplasm called a plasmodium.
(c) Most slime moulds are saprophytic and some are parasitic.
(d) The reproduction is plant like. They reproduce eby spores.
Sub-division Eumycotina: -
General Characteristics: -
(a) The Eumycotina are the true fungi. They comprise about 100,000 species.
(b) They are heterotrophic organisms and differ from the slime moulds in lacking plasmodial stages and phagotrophic nutrition.
(c) With the exception of the very few unicellular forms the vegetative thallus of most true fungi is composed of a mass of filaments called hyphae. A mass of hyphae forming the vegetative body is called mycelium.
(d) In the majority of the true fungi the cell wall is chiefly composed of a complex nitrofenous substance called chitin. Some of the true fungi have cell walls composed of cellulose.
(e) With the exception of the few forms eg, Synchtrium the majority of true fungi are eucarpic.
(f) In some true fungithe protoplasm of the hyphae is continuous but in others the hyphae are divided by cross walls called septa into cells. In septate true fungi the septum is perforated by a single central pore or sometimes more than one pores.
(g) The cells contain nuclei and nucleoli with a nuclear membrane.
(h) They reproduce by asexual and sexual methods.
Q.2. Give the economic importance of fungi. (2014)
Ans. Economic Importance of Fungi: -
The fungi are closely related to man, animals and plants. Infact they are intimately linked with us and play an important role in our life. Some fungi are harmful while a few are useful to us.
Some Useful Activities of Fungi: - The fungi are useful to us in many ways:
1. Destruction of Organic Waste: - The saprophytic fungi decompose plant and animal remains. CO2 is released in this process, which is used by green plants.
2. In Industry - 1. Many organic acids and vitamins are prepared by different fungi as Aspergillus niger, A.glances, Aclavatus, Citromyces citricus many acids like gluconic acid, lactic acid etc. are prepared from them. Aspergillus, Fusarium (different species) are the source of riboflovin, and vitamin B.
2. Several fungi have been found to be the basis of entire alcohlic industry in the production of ethyl alcohol by the fermentation of sugar solution by yeast.
C6H1206 ----------> 2C2H5OH + 2CO2
3. Saccharomyces is an important basis of basis of baking industries.
4. Use of Penicillium in preparation of certain cheeses. From yeasts vitamins like thiamine riboflavin, nicotinic acid are obtained.
5. In Medicines: -Many fungi produce certain antibiotic drugs which inhibit the growth of pathogenic microagent is. Some important medicines produced by. fungi are as follows.
(a) Penicillin obtained from Penicillium notatum used kill to bacteria causing pneumonia.
(b) Steptomycin from Streptomyces griesius.
(c) Ergot from Claviceps purpurea.
(d) Chloromycetin from Streptomyces venezuela.
6. In Plastic Manufacturing: - Certain fungi like Odium loctis is widely used in the plastic industry.
7. Nutrition of Plant: - Several members of the Phycomycetes, Ascomycetes Basidiomycetes and fungi imperfects are involved in the formation of mycorrhizae which are believed to be of fundamental importance in .the nutrition of trees like Cycus Zamia, Pinus etc.
Harmful Activities of Fungi: -
The fungi are reponsible for a number of human, animal and plant diseases: -
1. Many fungi cause immense loss to our timber trees by causing rot, e.g., Armillarva mella and species Polypores.
2. Some of mould fungi like Rhizopus Mucor, Aspergillus spoil our foods like jams, pickels, jelly -bread and fruits.
3. Several parasitic fungi are the causative agents of diseases of our crops, fruits and other economic plants. Among them are rust and smuts.
The rust produces yellow, oranges, black pustules on cereal plants and some smuts attack reproductive orange and produces black powderly mass of smut spore. They causes many diseases and a great loss to crops.
Some fungal diseases are as follows : -
(a) White rust of crucifers by Cystopus candidus.
(b) Powdery mildew by Erysiphae species.
(c) Downy mildew of crucifers by Pernospora species.
(d) Late blight of potato by Phytophthora infentense.
(e) Stem galls of coriandrum by Protomyces macrosporus.
4. Some parasitic fungi also cause diseases to animals. Saprolegnia occurs saprophyte on dead fishe. It causes serious disease to crops and gold fishe, thus causing harm.
5. Some fungi cause diseases in human beings. Several species of Aspergillus as A.niger, A. flavus, A. fumigatus are common human pathogens. They cause disease known as aspergilloses similar to aspergillose of lungs ears etc. Many parasitic fungi live in the mucous membranes of throat branchia and lungs. Few fungi cause skin discolouration. A well known skin disease ring worm is also a fungal disease.
Albugo
Q.3. Write short note on Gametangial contact.
Ans. Gametangial contact: -
This type of sexual reproduction is also hetrogamous, but both the gametes are non-motile (aplanogamic)ck. In this mode of sexual reproduction the male (antheridium) and female (oogonium) gametangia come into contact. The two gametangia do not fuse and the male nuclei from the antheridium are directly transferred into the oogonium through a pore formed by the dissolution of the wall between the two at the point of contact, or through a fertilization tube developed from the antheridium that pierces the oogonial wall. The fertilization is accomplished by a number of male nuclei that migrate to the oogonium through the fertilization tube. The male nucleus fuses with the egg resulting in the formation of a thick walled zygote. This type of sexual reproduction occurs in Saprolegnia, Pythium, Phytophthora and Albugo etc.
Q.4. Describe the life history of fungus causing white rust of Mustard. (2005, 07, 11, 15)
Related Questions -
Q. Describe the history of fungus ‘Albugo’. (2013)
Ans. Albugo candida causes white rust of mustard. Its life cycle is as follows -
REPRODUCTION: -
Albugo reproduces by both asexual and sexual methods-
Asexual Reproduction: -
The mycelium after reaching a certain stage of maturity forms a dense mat of hyphae in the mesophyll tissue just beneath the epidermis of the host. Many of these hyphae produce clusters of short, club shaped and unbranched structures called sporangiophores or conidiophores, at right angles to the surface of epidermis of host. Some protoplasm along with nuclei from the mycelium migrates into these so called conidiophores.
The tip of each conidiophore is thin walled and contains dense cytoplasm and 5-12 nuclei. From the tip of each conidiophore is cut off a chain of thin walled, spherical and multinucleate sporangia. The tip of conidiophore which contains 5-11 nuclei together with some cytoplasm enlarges and is then cut off from the rest of the sporangiophore by a transverse septum resulting in the abstriction of a multinucleate sporangium. The portion of the sporangiophore just below the sporangium elongates and enlarges due to the accumulation of more nuclei and cytoplasm and becomes pinched off as a second sporangium. This process is repeated a number of times. In this way from the tip of each conidiphore a chain of sporangia is cut off in basipetal succession i.e. the oldest sporangium lying at the top of chain and the youngest at the base . The conidia of a chain are separated from each other by gelatinous discs formed by the gelatinization of the walls of the sporangia at the point of contact.
Large number of conidia that arise in chains push up the overlying host epidermis which firstly bulges out and then bursts exposing a mass of white conidia on the surface of the host. On this account the fungus is called “white rust”.
When mature, the conidia are separated from each other by the gelatinization of the intercalary discs or pads. Finally the conidia are disseminated by the wind or by water.
The conidia or sporangia are normally globose but they appear cuboid or polyhederal due to the pressure during their formation. Each conidium is thin walled, unicellular and multinucleate structure with dense cytoplasm.
Under the favorable environmental conditions and in the presence of water the conidia germinate within two or three hours. During germination the protoplasmic contents cleave up into 5-8 polyhederal uninucleate pieces. Each such piece becomes transformed into an uninucleate, biflagellate, kidney shaped zoospore. A papilla is developed at one side of the sporangium which later forms a sessile vesicle. The zoospores escape from the conidium and swim about for a while in the film of water on the leaf surface. The zoospores are kidney shaped with two laterally placed flagella. After a very short period of active swimming, each zoospore retracts its flagella, then encysts. The encysted zoospore germinates immediately by a germ tube which penetrates the host tissue and brings about new infection. Sometimes when the temperature is higher the zoospores germinate directly producing a germ tube.
Sexual Reproduction: -
The sexual reproduction is oogamous and takes place by means of antheridia and oogonia, the male and female sex organs respectively. They are formed towards the end of the growing season of the host. The sex organs are formed as globose swellings in the intercellular spaces of the host. The development of the sexual organs within the host tissue is externally indicated by hypertrophy and malformation of the organ affected.
Fig. Sexual reproduction in A. candida A-infected leaf showing white pustules. B-developing sex-organs. C-multinucleate antheridium applied to the side of multinucleate oogonium. D-oogonium showing differentiation of ooplasm and periplasm. E-F-penetration of fertilization tube from an antheridium. Just below the fertilization tube there is egg nucleus, and to the left of it is a body called coenocentrum. G-egg and male nuclei fusing (Plasmogamy). H-karyogamy and formation of oospore. I-oogonium just after the fertilization and showing thick-walled oospore. J-germination of oospore and formation of zoospores within the vesicle. K-zoospore L-encysted zoospore.
The oogonium arises as a globose terminal swelling on the hypha deep within the host tissue. Sufficient amount of protoplasm flows into the developing oogonium. A cross wall soon separates it from the remainder of the hypha.
The antheridium arises as a short lateral, club shaped structure at the end of a hypha lying near the oogonium. It is cut off by a cross wall from the hypha below. It contains 6-12 nuclei.
Both the antheridia and oogonia are first multinucleate. In both the oogonium and antheridium the nuclei undergo one mitotic division. When the antheridium has established contact with the oogonium the cytoplasm within the oogonium becomes differentiated into a central dense portion the ooplasm and a peripheral more vacuolated portion the periplasm. These nuclei undergo second mitotic division with the spindles of dividing nuclei lying in such a way that one daughter nucleus is in the ooplasm and the other in the ooplasm. All the nuclei except the one of the ooplasm disintegrate the remainder nucleus functioning as the uninucleate egg or oosphere. A dark granular body called the coenocentrum appears in the center of the ooplasm.
During fertilization at the pint of contact of the antheridium and oogonium the oogonium develops a papilla like outgrowth which is called the receptive papilla. The receptive papilla is functionless and soon disappears. In the meantime the antheridium develops a fertilization tube which penetrates the receptive spot and enters through the periplasm into the ooplasm. A single male nucleus from the antheridium passes through the fertilization tube into the oogonium and fuses with the egg nucleus resulting in the formation of oospore. Soon after the fertilization the fertilization tube collapses and the coenocentrum is lost. The resulting oospore develops a thick wall which consists of a thin endospore and a thick rough exospore.
The oospores are liberated by the decay and the disintegration of the host tissue. The diploid nucleus of oospore divided repeatedly to form about 32 nuclei. It is believed that meiosis takes place at the time of the first division of the fusion nucleus. At this stage the oospore undergoes a period of rest.
On the approach of favourable conditions nuclear divisions again take place to form about 100 or more nuclei. The protoplat divides to form a large number of uninucleate segments each producing a biflagellate zoospore .The outer thick wall of oospore cracks open and the endospore comes out to form a vesicle into which the zoospores move out. The zoospores are liberated by the rupturing of the vesicle.
The zoospores are uninucleate, reniform and biflagellate structures .After swimming for some time they encyst and then germinate by forming a germ tube.