Plasmodiophoromycetes

Plasmodiophoromycetes-General account with examples

1. Class: Plasmodiophoromycetes Order- Plasmodiophorales Family- Plasmodiophoraceae Genus- Plasmodiophora , Spongospora , Polymyxa Vaishali S.Patil Professor, Department of Botany Shri Shivaji College of Arts, Commerce & Science Akola

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 .