Phytophthora
Q.5 Describe the life history of the fungus causing the late blight of potato. (2006, 08)
Related Questions -
Q. Describe the life histroy of the fungus Phytophthora causing late blight of patato.(2012, 14)
Q. Write short note on Amphigynous antheridium and paragynous antheridium. (2012)
Ans. Phytopthora infestans causes late blight of potato. It s life cycle is as follows.
Reproduction: - It takes place by following two methods -
(1) Asexual Reproduction: -
It takes place by means of sporangia which are borne on special reproductive hyphae,the sporangiophores.
Some of the hyphae from the internal mycelium of the fungus emerge out singly or in groups of one to five either through the stomata or sometimes through the epidermal cells of the leaves of host plant. These aerial hyphae which are differentiated from vegetative hyphae are called sporangiophores. They may be unbranched or branched sympodially.
Fig. Phytophthora. Asexual reproduction. A-portion of the coenocytic mycelium. B-sporargiophores with sporangia emerging from the stomata of potato leaf. C-sporangium with an apical papilla. D-segmentation of the sporangial contents into uninucleate pieces. E-formation of zoospores within the vesicle. Zoospores have not yet developed cilia. F-escape of zoospores. G-zoospores. H-encysted zoospore. I-germinating zoospore.
Each sporangiophore bears an ovoid or lemon shaped sporangium at its tip. After the formation of a sporangium, the branch continues its growth with the result that the first formed sporangium becomes lateral and a new sporangium is developed at the tip. A succession of sporangia may be produced sympodially at the tips of branches by the continued growth of the sporangiophores.
Each sporangium is hyaline, ovoid or lemon shaped and is provided with a small stalk below and a thin papilla. The sporangia are deciduous. They get detached from the sporangiophores by the rain splashes or air currents and are carried through the air current to the stem and leaves of the other potato plants.
Germination of Sporangium: -
Sporangium of phytopthora is greatly affected by temperature and relative humidity. If the relative humidity falls much below the 100 percent, the sporangia die in a few hours. The germination of sporangia is largely affected by the temperature. When the temperature is high, the sporangia germinate directly by putting forth germ tubes, and if the temperature is low the sporangia germinate indirectly by producing the zoospores.
When the sporangium falls in moist substratum and the temperature is low, it behaves as zoosporangium. The protoplasmic contents of the sporangium undergo cleavage into uninucleate pieces. Each uninucleate piece rounds off and metamorphoses itself as a kidney shaped biflagellate zoospore. Both the flagella are attached on the lateral depression of the zoospore.
The zoospores are liberated by the bursting of the papilla. Each zoospore swims about for some time, and then comes to rest. It drops its flagella, secretes a thick wall and then germinates by a germ tube which penetrates the host leaf through the stomata or the epidermal cells. After penetrating the host leaf, the germ tube develops into a new mycelium.
When the temperature is high, the sporangium behaves as a conidium and germinates directly by putting out a germ tube at the apex which causes new infection.
(2) Sexual Reproduction: -
It is of oogamous type and takes place by means of antheridia and oogonia, the latter developing before the former.
On the basis of the arrangement of antheredium and oogonium, the various species of Phytophthora may be classified into two distinct groups -
(I) The Amphigynous: -
In this the antheredium is first formed at the tip of a hypha then the oogonial incept penetrates, grows entirely through it and swells up to form the spherical oogonium above the antheredium. The mature antheredium thus forms a funnel shaped collar arround the base of the mature oogonium.
Fig. Phytophthora. Sexual reproduction A-somatic hypha. B-development of antheridium and oogonium. C-an antheridial initial with two nuclei. D-swelling of oogonial incept just before to its emergence from antheridium. E-swelling oogonium after emergence. F and G-nuclear conditions at various stages of oogonial development. H-degeneration of peripheral nuclei; a central nucleus remains at the centre. I-central nucleus undergoes a division to form two nuclei. Out of which one nucleus survives and the other degenerates. Papillae, which are formed by antheridium are visible. J-plasmogamy. K-formation of oospore karyogamy). L-germinating oospore.
(II) The Peragynous: -
In this type the antheredium is applied to the side of the oogonium.
Fertilization: -
Due to the budding of antheredial wall a number of receptive papillae are developed. One receptive papilla grows towards the oogonium and becomes closely applied to the side of the oogonium. The common wall is dissolved through which a male nucleus with little cytoplasm is delivered to the female nucleus of oogonium. The two nuclei fuse resulting in the formation of oospore.
The oospore secretes a wall and undergoes a period of rest. In some species eg, Phytopthora infestans the oospores develop parthenogenetically in the absence of antheridia.
The oospore then germinates by forming a germ tube at the tip of which a sporangium is developed. The sporangium germinates by producing biflagellate zoospores by forming germ tubes.
Late Blight of Potato: -
It is the most serious of all diseases of potato. The disease appears on the leaves only after the blossoming period as small black lesions. If the weather is favourable these brownish black lesions enlarges rapidly, involving the whole surface. Infection soon spreads through stem and branches and the whole plant blights. After the aerial parts of the plant have been affected the underground parts especially the tubers are infected. In the early stages the tubers show brownish or purplish discoulouration, but later they get softened, turn completely brown and decay before the harvest.
Q.6. Describe the systematic position of Phytophthora. (2014)
Ans. Systematic Position: -
Division - Mycota
Sub-division - Eumycotina
Class - Oomycetes
Order - Peronosporales
Family - Pythiaceae
Genus - Phytophthora
Mucor
Q.7. Give main characters and systematic position of mucor. (2011,13)
Ans. Mucor: -
Systematic Position: -
Division - Mycota
Sub-division -Eumycotina
Class - Zygomycetes
Order - Mucorales
Family - Mucoraceae
Genus - Rhizopus and Mucor
Characters: -
The vegetative body of Mucor consists of long, slender much-branched coenocytic hyphae. Septa are formed in the older hyphae and at the time of the formation of reproductive structures. The hyphae spread in all directions over the substratum and send branches down into it. The hyphae which penetrate the substratum are called absorptive hyphae, and they absorb nutrient material. The hyphae of Mucor very closely resemble to that of Rhizopus excepting the following:
(a) The absorptive hyphae of Mucor are less specialized than the rhizoids of Rhizopus.
(b) The stolons, characteristic of the Rhizopus are lacking in the Mucor.
Saccharomyces
Q.8. Describe the structure of Saccharomyces and discuss its economic importance. (2010)
Related Questions -
Q. With the help of suitable diagram write an account on structure of Saccharomyces. (2013)
Ans.
The plant body is not composed of hyphae. It is a unicellular fungus but the cells may often be united in chains, thus forming a pseudomycelium. The yeast cells are very polymorphic and are capable of assuming different form and shape depending upon the medium in which they grow.
Single yeast cells are colourless but when grown on solid media, colonies are formed by yeast cells which may be white, cream coloured, or tinged with brownish pigments. They are normally spherical oval or shortly cylindrical. Each yeast cell is small and ranges from 5-10 u in diameter. The cell wall is thin, delicate and is composed of chitin in combination of other compounds such as carbohydrates mannan and glucan. According to Agar and Doughlas the cell wall of S.cerivisiae is made up of two layers an outer dense layer about 0.05u thick and inner less dense layer about0.2u thick, containing microfibrils. Enclosed within cell wall is the granular cytoplasm which in the old cells can be differentiated into an outer ectoplasm and inner endoplasm. Embedded within the cytoplasm are the mitochondria, endoplasmic reticulum and ribosomes. The reserve food materials in the cytoplasm are in the form of glycogen, oil globules and protein particles. The cell contains a large central hyaline area and a small deeply staining body on one side of it. These two structures have been differently interpreted by different scientists. Difference of opinion lies where the vacuole is a part of the nucleus or the vacuole and the nucleus are separate identities. Agar and Doughlas from their electron microscope studies of ultra thin sections of cells of Saccharomyces cerevisiae and schizosaccharomyces octosporus have shown that the nucleus is surrounded by a definite membrane and is distinct from the vacuole.
Economic Importance: -
(1) Yeasts have the ability to ferment a sugar solution poorly supplied with oxygen resulting in the formation of alcohol and carbon dioxide. The oxidation of sugars to carbon dioxide and alcohol by yeast in the absence of oxygen is called alcoholic fermentation. The fermentation is brought about by certain enzymes called zymase and invertase. The other by products of fermentation process are the small quantities of glycerol, fatty acids, ether’s acetic acid and succinic acid. It is this process which is utilized in the baking and brewing industries. In the brewery alcohol is the important product by which beer, wine and other alcoholic beverages are manufactured. The carbon dioxide released during the fermentation is utilized in the raising of bread and thereby increasing its palatability. The bubbles of carbon dioxide cause the dough to raise and the bread to become porous. In baking a special type of yeast called baker’s yeast is used.
(2) Many types of yeast are responsible for synthesizing vitamins. A species of Torulopsis is known to synthesize proteins from molasses if supplied with ammonia.
(3) The yeast cells are pressed into cakes or cubes with an inner matter such as starch. These are sold in the market and are used both in baking industry and at home.
(4) A few species of yeast are used in the treatment of certain skin and intestinal diseases.
(5) Many types of yeast are responsible for the spoilage of cheese, tomato products and other food substances.
(6) A few species of yeasts are parasites of higher plants and cause diseases of tomato, fruits, beans, cotton, and hazelnut.
(7) Several species of yeasts are pathogenic to man causing number of serious disease eg. Blastomycosis and torulopsis etc. They attack the central nervous system and skin of man. Yeast vaginilli causes vaginal thrash. Cryptococcus neoformans causes disease called cryptococeosis which causes mental disorders in man. Candida albicans causes a disease called moniliasis which affects mucous membrane, skin, nails, and lungs etc.
Q.9. With the help of suitable diagram write an account on reproduction of Saccharomyces. (2013)
Ans. Reproduction of Saccharomyces: -
They reproduce by two methods : (i) Asexual reproduction, and (ii) Sexual reproduction.
Asexual reproduction: - Asexually, they reproduce either by budding, in which number of buds are given off by mother cell or by fission, in which equal daughter cells are produced by divisions of mother cells. On this basis, the yeasts are grouped as : Zygosaccharomyces, the budding yeasts and Schizosaccharomyces, the fission yeasts.
Budding: - When the food supply is abundant the yeast cells reproduce rapidly by a peculiar process known as buding. During tis process the nucleus of mother cell divides mitotically but according to some botanists, this process is amitotic. Soon a small protuberance appears on the surface of the vegetative
Fig. A,E-- stage in the process of budding (asexual reproduction) of a cell of Saccharomyces cerevisiae. F-- chain of vegetative cells produced by budding.
cell in the form of a “bud”, and into this passes one of the nuclei. It is then separated from the mother cell by the formation of a wall between the two cells, thus forming a new individual, and the process may be repeated indefinitely. Under conditions of rapid growth, the daughter cell may itself begin to “bud” while still attached to the parent cell. In this way short chains of cells loosely jointed together may be formed. These chains of cells are called pseudomycelium. The pseudomycelium also branches sometimes.
Fission: - In Schizosaccharomyces during reproduction the parental cell elon gates and its nucleus divides to produce two daughter nuclei. Gradually a partition wall is formed just in the middle of cell, dividing it into equal or nearly equal halves. The two daughter nuclei so formed, may remain linked together for sometime and repeat the process or they may separate soon and then divide.
Endospore formation: - Under certain conditions, when there is scarcity of food in surrounding medium, or there is a danger of desiccation, spores are formed from the mother cells. During the process, the protoplasm of vegetative cell divides, usually into four portions, each of which later becomes surrounded by a comparatively thicker wall. These structures are called as endospores. These spores are capable to withstand the adversities of surroundings. On approach of suitable atmosphere, these germinate to produce chains of cells.
Fig. - Stages showing binary fission
Sexual reproduction: - The sexual reproduction takes place when the food supply is scanty. For purpose of sexual reproduction no specialized sex-organs are formed. It occurs by conjugation of two either similar or dissimilar haploid vegetative cells or gametangia the process is called hologamy. This results in the formation of a diploid zygote cell which functions directly as an ascus and produces ascospores.
The number ascospore per ascus varies with the secies. The shape of the ascospores is also variable. They are generally globose or ovoid as in Saccharomyces, Schizosaccharomyces, and Saccharomycodes. In Hahsenula the ascospores are hat-shaped. Nematospora, they are needle like.
In some yeasts, e.g., Zygosaccharomyces chevaueri and Debaromyces, the copulation occurs in between the mother cell and its “bud”. This process is called pedogamy.
According to Lindegren and Lindegren (1943), and Ahad (1953), yeasts may be homothallic or heterothallic.