somatic structure

Systematic position Division — EumycotaSubdivision — Mastigomycotina Class — OomycetesOrder — PernosporalesFamily — AlbuginaceaeGenus — Albugo

ANC,Patna/B.Sc.(Bot-Hons) Part-I-Sushil

Somatic structure

The somatic structure is represented by mycelium.

The mycelium is well developed, branched, intercellular, aseptate and coenocytic.

The hyphal wall consists of cellulose.

The protoplasm of the hyphae is granular and contains many nuclei, oil globules and glycogen.

Many spherical or knobs like haustoria develop from the intercellular mycelium, which penetrate into the host cell and absorb the food.


Asexual reproduction Asexual reproduction is accomplished by

sporangiospores.

The intercellular mycelium forms a palisade of hyphae below the host epidermis.

The tips of these hyphae develop into short, erect, thick walled and club shaped structure called sporangiophores.

Sporangiophores:

- lie perpendicular to the host surface

- closely packed into a palisade like layer beneath the host epidermis.

- The apical end of the sporangiophore is multinucleate, thin walled and enlarged with dense cytoplasm.


Contd. - A deep constriction appears below the swollen

end, which results in the formation of a multinucleate and spherical or oval sporangium.

• Another sporangium is formed by the tip of thesporangiophore in the same manner.

• The process is repeated several times and a chainof sporangia is formed at the tip of eachsporangiophore.


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•The arrangement of sporangia is basipetal.•A mucilaginous pad or disc, called disjunctordevelops between the successive sporangia.The epidermis of the host is raised due to thepressure of the developing sporangia and it isfinally ruptured. The sporangia are now visibleon the host surface as a white powdery mass.•The sporangia are smooth, colourless andmultinucleate.


Contd. The sporangia get separated by the shrinking and

drying of the mucilaginous disc and are then blown away by wind or washed away by the rain water.

The sporangia germinates on a suitable host within a few hours under appropriate conditions.

The sporangium behaves as a zoosporangium under moist condition and at low temperature. It absorbs water and swells. Its multinucleate protoplast divides to form a number of daughter protoplasts, each of which develops into a zoospore.

A papilla develops from the sporangium and the young 4-12 zoospores are released in a sessile vesicle.


Contd. Each zoospore is a biflagellate structure. Of the two

flagella, one is short and of tinsel type, whereas the other is large and of whiplash type.

The released zoospores settle on some suitable host, become deflagellated, and each of them secretes a wall. The encysted zoospore forms a germ tube, which penetrates the host epidermis and develops into a fresh mycelium.

Under dry condition and at high temperature the sporangium germinates directly into a germ tube, without forming any zoospore. This is uncommon in Albugo.


Sexual Reproduction

The sexual reproduction is oogamous.

The sex organs are antheridia and oogonia which develop quite deep into the tissues of the stem or petiole of the host. Both the sex organs develop near each other.

Oogonia: They develop by the inflation or swelling of the female hyphae in the intercelluler spaces of host. Each inflated hyphal tip is multinucleate and gets separated by a cross wall just below the inflation. The separated swollen portion represents the oogonia.


Contd. The cytoplasm of young oogonium is uniformly

vacuolate in which the nuclei are evenly distributed .In the later stages its central portion becomes dense,round, and represents ooplasm. The nuclei keep ondividing in the ooplasm and migrate towards itsperiphery.

One of the nuclei remains in the denser ooplasm,whereas the other moves towards the outer region.

Outer peripheral region of the oogonium becomesmore vacuolated and represents periplasm. Theooplasm represents the egg. Many nuclei are presentin the egg when it is separated.


Contd. But soon all egg nuclei, except one, disintegrate in A.

candida. However, according to some other workers all the egg nuclei migrate to the periplasm.

Antheridia: The antheridia develop very close to an oogonium as an elongated, club shaped and multinucleate bodies, at the end of a male hyphae. The swollen antheridial tip is soon cut off by a septum. Out of its many nuclei only one remains functional.

Meiosis occurs in gametangia and not in the zygotic nucleus.


Fertilization A slender fertilization tube develops in the

antheridium at the point of its contact with that of theoogonium and it grows through the oogonium wall.

It penetrates the egg after passing through theperiplasm.

The functional male nucleus enters the egg, fuses withthe functional female nucleus and forms diploidzygotic nucleus. The fertilized egg secretes a thick wallaround itself and changes into an oospore.

Oospore: The oospore is surrounded by a thickornamented wall. The single oospore nucleus isdiploid.


Germination of oospore The oospore undergoes a long resting period of several

months before germination. During this period the oospores are released because the host tissue first shows hypertrophy and then might decay.

The zygotic diploid nucleus shows repeated mitotic divisions to form many nuclei.

Germination of oospore occurs in the following spring. At the time of germination the outer oospore wall bursts and the inner endospore comes out and forms a thin spherical vesicle.

Inside the vesicle large number of zoospores are released. The vesicle is soon dissolved and the zoospores swim freely.

Zoospore is uninucleate, reniform and biflagellate. On reaching a suitable host, their flagella are withdrawn. They get encysted and germinate by forming germ tube. The mycelium is diploid.


somatic structure

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