2. •They are obligate endoparasites of the seed plants (Cabbage,
Nasturtium), ferns (Azolla), algae (Vaucheria) and Fungi (Achlya,
Saprolegnia).
•The somatic phase is a moving, naked, multinucleate mass of
protoplasm feeding in an amoeboid fashion. It is known as the
Plasmodium.
•It grows within the host cells and is holocarpic. Two types of plasmodia
namely primary or gametangial plasmodia and secondary or
cystogenous Plasmodia occur in the life cycle. The former are haploid
and the latter diploid.
•The entire protoplasm of the primary Plasmodium divides to give rise to
thin-walled gametangia, each of which contains a single motile
biflagellate, uninucleate structure which functions as a gamete. It is
called a zoospore and the Plasmodium bearing it as zoosporogenous
Plasmodium.
3. •The cystogenous Plasmodium divides by meiosis to form thick- walled
resting meiospores. Some scientists call them resting sporangia or cyst.
•On germination each cyst gives rise to a zoospore or cyst zoospore.
Both the gametes and the cyst zoospore are uninucleate, biflagellate
and similar in structure.
•Both the flagella are of whiplash type but of unequal length
(heterokont) and inserted anteriorly, the shorter near the longer one. The
longer flagellum terminates in an end piece. The shorter flagellum lacks
the end piece. It has blunt tip. When the swarm cell is in motion, the
longer flagellum trails behind and the short blunt flagellum is directed
forward.
•The nuclear division occurring during the somatic phase of both
gametangial and cystogenous phases is cruciform. Sexual reproduction
when present is isogamous and takes place by planogametic copulation.
The class includes a single order Plasmodiophorales.
4. Plasmodiophora (Club root of cabbage)
caused by Plasmodiophora brassicae
The resting spores or sporangia (cysts) occurs in the root
hairs. It is a tiny hyaline, spherical, uninucleate, haploid
structure .The sporangial wall is differentiated into 3 layers
but in many cases it showed only 2 layers. The outer layer is
electron-dense and granular. It included the surface spinules.
The middle layer is less electron-dense. A circular germ pore
is present in the sporangial wall with the middle layer (inner in
the 2 layered sporangial wall) thickened to form a circular
electro-lucent plug-like structure in the region of the pore
orifice.
5. It is surrounded by a ring of electron-dense material
contiguous with the inner wall layer. Within the sporangial
wall is the sporangial membrane which completely invests the
cytoplasm replete with ribosomes. The other organelles
present in the cytoplasm are mitochondria, endoplasmic
reticulum and dictyosomes. It has a single haploid nucleus.
Numerous lipid globules of various sizes is another
conspicuous feature of the spore (sporangial) cytoplasm. The
mature spores are set free, when the clubs undergo decay. In
the soil they germinate to initiate the life cycle.
The spores may germinate immediately or remain dormant in
the soil for several years.
6. Spore(5.3-A) absorbs water and swells considerably (B),
particularly on one side along which a slit appears in the spore
or sporangial wall. Through the slit, the uninucleate
protoplasm emerges (C) as a single swarm cell or zoospore
which is spherical or pear-shaped (D) in form.
On germination, the plug dissolves to enable the zoospore
to emerge through the germ pore. The refractile (lipid)
globules in the sporangial cytoplasm gradually disappear as
the germination begins. A small papilla emerges through the
germ pore in the spore wall. It enlarges to form a thin vesicle
as the spore contents flow into it. The flagella develop on the
spherical or pearshaped zoospore as it completely emerges. It
may also be termed as the primary or cyst zoospore.
7. Infection takes place at the seedling stage in the host. The
biflagellate, heterokont (with unequal flagella) primary
zoospore (cyst zoospore) swims in a film of water in the soil
or creeps over soil particles like an amoeba.
On contact with the root hair of suitable host seedling
(Cabbage or other crucifer seedling), the zoospore becomes
attached and amoeboid, in this state it is called myxamoeba.
The myxamoeba retracts its flagella and enters the root hair
directly by a partial dissolution of the root hair wall.
The cyclosis of host cytoplasm sweeps the myxamoebae away
from the penetration site soon after injection. The young
myxamoebae in the root hairs are separated from the host
cytoplasm by two kinds of simultaneously occurring
intrafacial membrane arrangements.
8. Within the root hair the uninucleate myxamoeba
enlarges to form a thallus. The protoplasm increases in size
and the haploid nucleus divides repeatedly. The resultant small
naked, multinucleate acellular mass of protoplasm constituting
the thallus is called the primary Plasmodium. It is a haploid
structure and thus termed gametothallus.
Sexual Reproduction-
After a certain period of growth, the cytoplasm of the
gametothallus, while still lodged in the root hair, cleaves into
as many uninucleate daughter protoplasts as there are nuclei
(usually 4-10) in the Plasmodium.
Each daughter protoplast secretes a membrane around it
and functions as a gametangium. The gametangia are small
spherical, thin-walled uninucleate structures. The haploid
nucleus of the gametangium undergoes mitosis to form four to
eight daughter nuclei .
9. Each daughter nucleus gathers a small bit of cytoplasm
around it and develops into a spindle-shaped motile,
biflagellate, isogamete which is smaller in size than the
zoospores. The two flagella are of unequal size and of
whiplash type inserted at the anterior end.
Thus in one gametangium up to eight isogametes are
formed.They have no cell walls. The whole process takes 2—8
days. The thin gametangium wall bursts and liberates the
gametes in the root hair. All the stages occur in the root hairs
of the host. These constitute the haplophase.
After liberation, the isogametes copulate in pairs. the
isogametes are released in the soil outside the host through a
pore formed at the point of contact of gametangium and the
root hair cell wall. Pairing of gametes takes place outside the
host in the soil. Pairing of gametes is soon followed by the
fusion of their membranes and cytoplasms. This is termed
10. plasmogamy. The latter thus results in the formation of
binucleate fusion cell. Karyogamy takes place immediately
before cyst formation.
Diplophase in the life cycle begins with the formation of
quadriflagellate zygotes. The single nucleus in zygote is
diploid.
The quadriflagellate zygotes are generally found in the
cortical cells of the root. The flagellated zygotes perforate the
epiblema cell walls and reach the meristematic zone of the
root. When the quadriflagellate zygote is formed in the soil, it
reinfects the root. Once in the root cortex, the zygotes retract
their flagella and become amoeboid. The amoeboid zygotes
are called myxamoebae .The myxamoebae spread from cell to
cell by direct penetration of cell walls. At first they are found
in the cortical cells and later in the other tissues of host root
and stem.
11. Secondary Plasmodium-
Within the invaded cortical cell, the myxamoeba by
subsequent growth accompanied by repeated karyokinesis
(mitosis) of its diploid nucleus grows to form a relatively
large, naked, multinucleate diploid Plasmodium.
It is called the secondary Plasmodium or sporothallus or
cystogenous Plasmodium. In the mature state, it may occupy
the entire lumen of the infected host cell and surround the host
cell nucleus. It lies immersed in the cytoplasm. The host cell
may contain more than one plasmodia. It consist of two
closely appressed osmophilic membranes, ribosomes,
endoplasmic reticulum, mitochondria, dictyosomes, lipid
droplets & nucleus.
12. Formation of Clubs: The presence of the secondary
plasmodia in the cortical host cells elicits many metabolic
responses in the infected cells such as hypertrophy &
stimulates active and repeated division. This excessive
multiplication of infected or diseased host cells is termed
hyperplasia. They stimulates the neighbouring uninfected
parenchyma cells of the host to divide and redivide. There is
excessive proliferation of infected host cells in the form of
tuberous swellings or malformations on the diseased roots.
These malformations are called clubs or galls.
The club-tissue is thus likely to be invaded by soft rot
bacteria or other rot producing organisms which bring about
decay of clubs resulting in the formation of materials toxic to
the plant and cause wilting of the tops. Subsequently it may
result in permanent wilting or retarded growth.
13. Vegetative Multiplication of the Parasite-At the time of
division of infected or diseased host cells (hyperplasia), the
secondary plasmodia may also divide. The formation of new
plasmodia by a vegetative method of fragmentation results in
the spread and propagation of the disease. The secondary
Plasmodium of each enlarged cell of eventually becomes
converted into a mass of resting sporangia or cysts. The
resting spores or cysts (sporangia) thus perennate in the soil.
They germinate to produce zoospores on the offset of suitable
conditions .
