In this article we will discuss about:- 1. Introduction to Plasmodiophora Brassicae 2. Symptoms of Disease Caused by Plasmodiophora Brassicae 3. Life Cycle.
Introduction to Plasmodiophora Brassicae:
Plasmodiophora is the best-known genus under the family Plasmodiophoraceae. The well- known species P. brassicae is an obligate endoparasite which causes club root disease of crucifers like cabbage, cauliflower, turnip, radish etc.
The disease is also known by different common names in different countries like ‘Finger and Toe’ and ‘Hamburg’ in England; ‘Club root’, ‘Club foot’ and ‘Clump foot’ in U.S.A.: ‘Kapoustnava Kila’ in Russia: ‘Kelch’ and ‘Kropf des Kohles’ in Germany, etc.
The disease was first reported in Scotland in 1 780. After many years, detailed scientific work about the pathogen and disease on cabbage was done by Woronin (1878).
Symptoms of Disease Caused by Plasmodiophora Brassicae:
The symptoms are visible in the root of affected plants (Fig. 4.12). The roots become abnormally increased and swell up at different regions due to hypertrophy and hyperplasia.
In Cabbage, the root bears malformation or clubs of different shapes, hence the disease is called club root disease. The development of lateral roots becomes suppressed.
The drooping of the leaves during sunny days, with gradual progress of.disease, leads to permanent wilting. The disease shows some variations in symptoms in different hosts.
In section, the infected tissue shows some hypertrophied cells containing plasmodia or spore masses (Fig. 4.13).
Life Cycle of Plasmodiophora Brassicae:
The life cycle of Plasmodiophora brassicae is given in Fig. 4.14. The life cycle consists of two phases – A. Primary phase occurs in the root hair and B. Secondary phase occurs in the cortical cell of root.
A. Primary Phase:
a. Germination of spore. The spores of P. brassicae are very small (up to 4 pm in diameter), spherical, uninucleate structure (Fig. 4.14A). The spores may germinate immediately or remain dormant for several years. During favourable condition the spore swells up several times and the protoplast comes out of the spore wall through a rift (Fig. 4.14B) as a single zoospore or swarm cell.
The zoospores are uninucleate and spherical or pear-shaped in structure with two unequal flagella of whiplash type, placed at the anterior end (Fig. 4.14C).
b. Infection to root hair. The zoospores swim in a thin film of water. On contact with suitable host, the zoospore looses its flagella and becomes amoeboid, called myxamoeba (Fig. 4.14D). The myxamoeba enters into the root hair by dissolution of cell wall (Fig. 4.14E).
c. Gametophytic thallus or Gametothallus. Within the root hair cell, the uninucleate myxamoeba enlarges to form a thallus. The protoplast enlarges in volume and its nucleus undergoes repeated mitotic divisions.
Thus a small, multinucleate, naked mass of protoplasm is formed, called plasmodium. The plasmodium is haploid and thus it is called as gametothallus (Fig. 4.14F). Many gametothalli may be present in a single root hair due to multiple infection. The gametothalli do not fuse together.
d. Sexual reproduction. During sexual reproduction, each nucleus of gametothallus accumulates some cytoplasm around itself and behaves as an individual unit, called gametangium (Fig. 4.14G). Thus the gametangium is initially very small, uninucleate spherical structure (Fig. 4.14H). The nucleus of each gametangium undergoes mitotic divisions and forms 4-8 daughter nuclei.
Each daughter nucleus accumulates some cytoplasm and behaves as an individual unit (Fig. 4.141). Each unit develops into a spindle-shaped, biflagellate gamete. Gametes are alike and are called isogametes (Fig. 4.14J). The gametes are smaller in size than the zoospores.
The gametes are liberated out by bursting the gametangial wall and root hair cells. The gametic union (only plasmogamy) takes place in the soil, results the formation of binucleate and quadri- flagellate structure (Fig. 4.14K, L) [whether the fusing gametes come from same or different gametangia has not been established].
B. Secondary Phase:
The binucleate and quadritlagellate structure then enters again into the host tissue, initially through epiblema towards the meristematic zone. It retracts the flagella in contact with the host tissue and after entering inside host cell it becomes amoeboid, called myxamoeba or plasmodium (Fig. 4.14M).
The myxamoeba can spread from cell to cell through the cell wall. Initially, it remains in cortical cell and later invades to the other tissue. Within the host cell, the myxamoeba enlarges and forms a naked multinucleate plasmodium by repeated division of its haploid nucleus (Fig. 4.14N). At maturity, it almost occupies the entire lumen of the host cell and surrounds the host nucleus.
Later on, the infected cell undergoes hypertrophy (i.e., enlargement of the cell) following hyperplasia (i.e., repeated division of cells). Thus several cells are formed and the internal plasmodium also distributed to each cell by fragmentation, a vegetative process. The group of hypertrophied cells is called Krankheitsherd.
Consequently, the neighbouring uninfected cells get stimulated and undergo hypertrophy and hyperplasia, thereby tuberous swelling or malformation takes place. During single point infection in cabbage, these malformations look like spindle-shaped clubs.
Due to rapid growth of club, cork layer formation in root becomes hampered, thereby it gets infected by soft root bacteria or fungi. Due to secondary infection, the decay of the club leads to the formation of toxic material, which causes wilting of the plant.
Spore formation. After maturation of the haploid plasmodium inside the host cell, the haploid nuclei get associated in pairs and karyogamy takes place at this stage. Thus many zygotic (2n) nuclei are formed. The diploid nuclei then undergo meiosis and form haploid nuclei. (There are different suggestions regarding the position of fusion and meiosis in the life cycle of P. brassicae).
The cytoplasm undergoes cleavage and accumulates around each nucleus. The entire protoplast is then transformed into uninucleate small bodies, surrounded by smooth wall, called resting spores (Fig. 4.140, P). The mature spores are set free, after decay and decomposition of club root. The spores can survive for a long time in soil. During favourable condition, the spores germinate and produce zoospores.
