Describe the life history of Cystopus (Albugo).

Ans. Systematic position :

Mycota – Fungi

Eumycotina – Phycomycetes

Phycomycetes – Oomycetes

Flagellates – Peronosporales

Peronosporales – Albuginaceae

Albuginaceae – Albugo (cystopus)

Albugo (Modern) – (Old)

Occurrence : Albugo or cystopus is the only genus of its family. It is an obligate parasite of many crucifers and causes considerably damage. There are about 26 species of Albugo which are distributed all over the world. A Candida is the most common and important species which attacks on crucifers causing the disease white rust. The fungus attacks the entire plant body except the root.

The disease appears in the form of white pustules on the aerial parts of plants such as leaves, stem, flower etc. The infected parts becomes highly hypertrophied and deformed. The floral parts become flesh and develop violet colour. Stamens look like leaves and petals as sepals and the whole flower becomes several times bigger than the normal size.

The zoospores or spores infect the host plant, enter into the host tissue through stomata by a tubular outgrowth which later on form the intercellular mycelium.

Structure: The mycelium is endophytic, branched, aseptate, coenocytic hyaline, intercellular and with knobs like haustoria for the absorption of food material from host cells. The fungus thus shows the heterotrophic mode of nutrition and absorbs food from the host tissue. The hyphae live and ramify in the intercellular species of the host tissue. The hypal protoplasm is granular. It contains numerous nuclei, oil globules and glycogen. The fungus mycelium grow vigorously. The mycelium which enters in the host tissue through stomata can only be seen by cutting a T.S. of the infected host.

Reproduction: The reproduction may by either asexual or sexual.

1. Asexual Reproduction: The asexual reproduction takes place by zoospore and conidiospore formation, which are formed inside the conidiosporangium. In this case beneath the epidermis the mycelium gives rise to compact clusters of erect conidiophores. These are formed on ends of short sympodially branching which arise from a mass of mycelium gathered in a limited area underneath the epidermis of host. They lie parallel to one another and perpendicular to the leaf or the stem surface. They are sporangiophores or conidiophore. The thin walled apical end of the sporangiophore enlarges and it contains dense cytoplasm with 5 to 6 nuclei. A dependent constriction then appears below the swollen end. It results in the formation of the first conidiosporangium. Just below this another conidiosporangium is cut off in the same manners. This process may be repeated a number of times, forming a chain of conidiosporangia on the above side of each conidiophore.

The conidiosporangia are small smooth spherical multinucleate structures. They are arranged in a basipetalous sequence in the chain. The chains of conidiosporangia grow and press epidermis above. This causes the leaf surface to bulge out and the overlying epidermis eventually bursts over the growing conidiosporangial sorus. This results in the exposure of white conidiosporangial masses causing the characteristic pustules.

The conidiosporangium 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.

The conidiosporangium at the high temperature and unfavourable periods behave like conidiospore and directly infect the host by the formation of germ tube. But in the presence of water and favourable periods the conidiosporangium behaves as zoosporangium. It multinucleate protoplast undergoes division and it divides to form a number of biciliate kidney shaped or heart shaped uninucleate zoospores. The sporangial wall ruptures and the zoospores are liberated. They swim in water for a while and finally they settle down in the host and round off. It germinates and each zoospore puts out a germ tube which enters into host through stomata. The germ tube within the host tissue develops into a mycelium.

2. Sexual Reproduction: The sexual reproduction is of oogamous type and it takes place by definite male and female reproductive organs known as antheridia and oogonia respectively. They arise on separate hyphae called the male and female hyphae. These hyphae lie burried deep in the intercellular spaces of the tissues of the stem or petiole or leaf of the host. Normally the sexual reproduction takes place on the disappearance of conidia and at the end of growing season.

The oogonia arises as a globular enlargement of the tip of the female hyphae. The swelling consists of many nuclei and reserve food material. A cross wall then appears below this inflation. It separates the terminal oogonium from the rest of female hyphae. Afterwards in the oogonium the cytoplasm gets differentiated into two distinct regions, the central rounded dense ooplasm with many nuclei, and outer vacuolate, spongy periplasm. The ooplasm at maturity has a single centrally located nucleus. Regarding the uninucleate condition of mature ooplasm, some workers think that all the nuclei excepting one are extruded from ooplasm and are deposited to ooplasm, while some workers thought that in the later stage of development all the nuclei in ooplasm excepting one degenerate and disappear. The uninucleate ooplasm functions the female gamete or egg.

The antheridium is an elongated club shaped multinucleate structure. The antheridium is developed at the end of a male hypha lying close to oogonium. The end of the male hypha enlarges into a club shaped swelling. The later is then out off by a cross wall from the rest of male hypha. This terminal club shaped cell is called the antheridium. The antheridium comes in direct contact with the oogonium and the size of which is marked by a slight protuberance.

Fertilization: At the time of fertilization a slender fertilization tube pierces the wall of the oogonium and grows until it reaches the oosphere. All the nuclei of the periplasm disorganise. The fertilization tube dips deep into the end introduces a single nucleus along with some cytoplasm. The cytoplasm mingles with the cytoplasm and the male nucleus fuses with the female nucleus and results in the formation of zygote.

Germination of zygote: After fertilization the oosphere (zygote) gets surrounded by a thick wall and oospore is formed. The nucleus of the zygote undergoes a meiotic and several mitotic divisions to form nearly 32 nucle. After this the zygote undergoes a period of rest. On return of favourable conditions the 32 nuclei of oospore undergo repeated divisions to form 100 or more nuclei. A small amount of cytoplasm gathers round each nucleus-as a result of which numerous uninucleate daughter protoplasts are formed. Each of these metamorphosed into biflagellate uninucleate uniform meiospore and is called the zoospore. The zoospores are liberated by the opening of outer thick wall oospore with or without the formation of the vesicle. These zoospores on coming in contact with a suitable host comes to rest and settle down on the host. The cilia are withdrawn. It then rounds off and secretes a wall around it. A germ tube is given out which enters in host tissue through stomata, and this grows vigorously and forms a new mycelium.

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