(Mastigomycotina)
1. Albugo ( Cystopus )
Q.1. With the help of neat and labelled diagram describe the life cycle of cystopus indetail. (2005)
Related Questions -
Q. What is White Rust ? Describe the sexual reproduction of fungus causing it. (2012)
Q. Describe the sexual reproduction in the fungus which causes ‘white Rust’. Also mention the symptoms of this disease. (2010)
Q. Describe the discase symptoms of white rust disease of crucifera (2002)
Q. Describe the systematic position and important characters of cystopus. (2002,08,09)
Q. Describe the formation of oospore of cystopus. (2007)
Q. Describe the sexual reproduction of cystopus with the help of diagrams only. (2007)
Q. Write short note on white rust of mustard. (2013)
Ans.1. Systematic Position: -
Kingdom - Mycota
Division - Eumycotina
Sub-division - Mastigomycotina
Class - Oomycetes
Order - Peronosporales
Family - Albuginaceae
Genus - Albugo
Characteristics:-
The plant body of the fungus is mycelial and eucarpic. The mycelium is intercellular, branched, septate and coenocytic. The mycelial wall is made up of cellulose-glucan. The cells show characteristic fungal eukaryotic organization. The haustorium of the fungus is distinguishable into a head and a narrow short stalk. The stalk or head lacks nucleus. The base of the haustorium is surrounded by a collar - like sheath formed by the extension of host cell wall. Cystopus is the imperfect stage of Albugo, while Albugo is sexual or perfect stage. It is obligate parasite which is found on the plant of family cruciferae but some times it is also found in the plants of compositae family as a endoparasites.
Albugo or Cystopus is the only genus of in the family. It infects the members of family cruciferae and compositae. Albugo candida is the most common and important species which attack on cruciferaes causing the disease white rust.
Symptoms:-
(1) The disease shows in the form of white, irregular pustules on the lower surface of leaves as well as on the stem.
(2) The stem becomes malformed and highly hypertraphied .
(3) stamens converted into leaves and pistils into thick fleshy structures. The flowers becomes sterile than normal size.
(4) The growth of the entire plant is stunted.
Structure of Mycelium: -
Mycelium is endophytic, branched aseptale, coenocytic hyaline, intercellular and with knob like and haustoria for absorption of food material from host cells. The hyphal protoplasm is granular. It contains numerous nuclei, oil globules and glycogen.
Reproduction: -
The reproduction may be either asexual or sexual.
1. Asexual Reproduction: -
Asexual reproduction takes place by zoospores and conidiospores formed in conidiosporangium. Beneath the epidermis, the mycelium gives rise to compact clustures of erect conidiophores. They are formed at the end of short sympodially branching hyphae which arise from a mass of mycelium. These structures are called sporangiophores or conidiophores.
The conidiosporangia are small smooth spherical multinucleate structures. They are arranged in a basipetal sequence in the chain. The, conidia are now liberated in the presence of moisture by the disintegration of gelatinous pads. They are then blown away in the air by wind or washed away by rain water and can directly infect the host cells by the formation of germ tube. But in presence of water conidiosporangium behaves as zoosporangium. It protoplast undergoes division and form biciliate kidney shaped uninucleate zoospores. Zoospores are liberated and swim in water and finally they settle down and germinates and such zoospores puts out a germ tube which enters host through stomata and infect the host cells.
Fig. Asexual reproduction in Cystopus.
Sexual Reproduction :-
The sexual reproduction is of oogamoüs type and it takes place by male and female reproductive organs known as antheridia and oogonia respectively. They arise on separate hyphae called male and female ‘hyphae.
Oogonia :-
It arises as a globular enlargement at the tip of the female hyphae. It consists of many nucleic and reserve food material.The cytoplasm of oogonium gets differentiated into central rounded ooplasm with many nuclei and outer vacuolated spongy periplasm.
Fig. - Sexual reproduction in cystopus
Antheridium :-
It is an elongated club shaped multinucleate structure. The antheridium is developed on the, male hypha close to oogonium.
Fertilization :-
When the antheridium and oogonium comes in contact to each other a thin fertilisation tube is developed from the internal wall of the antheridium. This tube passes through the thin spot in the oogonial wall and enters the multinucleate periplasm. It then dips deep into the ooplasm. Prior to this a spherical and granular cytoplasmic body appears in the centre of the oospore. It is known as the coenocentrum. The fertilisation tube reaches the coenocentrum and ruptures the tip to introduce a single male nucleus which fuses with the female nucleus.
The young oospore is delimited from the vacuolate periplasm by an electron dense cell wall. Further development of oospore it marked by the deposition of U-layers, two on the outer and two on the inner side of the first layer of the young oospore.
The mature oospore has thick highly differentiated 5- layered wall. The thick highly diferentiated oospore wall together with the surrounding additional layers contributes to the longevity of oospore. After the formation of thick wall the oospore undergoes meiosis. Its diploid nucleus divides repeatedly to form 32 nuclei. In this 32 nucleate stage, the oospore enter the resting stage and tides over the period unfavourable for growth.
Germination of Oospore:-
After fertilization the oospore (zygote) gets surrounded by a thick note wall and oospore in formed. The nucleus of zygote undergoes meiotic and several mitotic divisions to form neary 32 nuclei After this zygotes undergoes a period of rest and germinates in favourable conditions.
The outer wall of zygote bursts and a vesicle comes out in which kidney shaped zoospores are present. After sometime cilia of zoospores become degenerates and spherical in shape and after sometime it germinates through germtube and attacks new healthy plant.