PHYTOPHTHORA - CLASSIFICATION, VEGETATIVE STRUCTURE, REPRODUCTION, ECONOMIC IMPORTANCE

            A. CLASSIFICATION:

Division – Mycota

     Sub-division – Eumycotina

           Class – Oomycetes

                 Order – Peronosporales

                        Family – Phythiaceae

                              Genus – Phytophthora

            Phytophthora, literally means plant destroyer, the term which is taken from a Greek phyton: plant and phthora: destruction. Species of Phytophthora are cosmopolitan and normally live as parasites on flowering plants. They can also be grown on artificial media (hence facultative saprophytes). Some of the species are also found to live as saprophytes on soil, but they may turn to parasitic mode of life if suitable hosts are available (hence facultative parasites).

            The genus Phytophthora contains about 70 species out of which Phytophthora infestans is the most important. This species is a parasite on potato plant and is responsible for the disease in potato known as late blight and early blight diseases.

            B. STRUCTURE OF THE VEGETATIVE BODY:- The vegetative body is typically tubular and coenocytic mycelium. It is profusely branches and consists of aseptate, hyaline, profusely branched, coenocytic, moderately thick hyphae.

            The fungal hyphae ramify in the intercellular spaces between the cells of the host tissue. These are called intercellular hyphae. In addition, there are hyphae that penetrate into the cell and are called intracellular hyphae. From the intercellular hyphae, haustoria develop, which comes in contact with the host cell to absorb nourishment.

            The fungal hyphae is 3-8 µm in diameter.  The hypal wall is 0.1µm thick and consists of glucans and cellulose. The cellulose may or may not be present. The hyphal cytoplasm contains mitochondria, endoplasmic reticulum, dictyosomes, ribosomes, large vacuoles and numerous nuclei.

            C. REPRODUCTION:- Phytophthora reproduces by asexual and sexual methods of reproduction.

            1. Asexual Reproduction:- Asexual reproduction takes place by the production of conidia and chlamydospore formation.

            Conidia formation – In this method, some of the hyphae come out in groups from the internal mycelium through stomata or from broken surface of the leaves of the host plant. These aerial slender hyphae are called conidiophores. Branches arises from the tip of the conidiophores and at the tip of each branch, a single conidia are produced.

              Each conidium is a thick walled, ovoid or lemon shaped structure possessing a terminal beak or papilla-like projections. Its protoplasm is granular and contains numerous nuclei, few vacuoles and oil drops as reserved food. Mature conidia are dispersed by ordinary gush of wind or rain splashes, and are carried through air current to the leaves of another potato plant where it germinates within a few days on favourable condition.

            The germination takes place by two ways – (a) Direct Method – When temperature is high (24⁰C) but moisture content of the atmosphere is less, the conidia germinate directly by producing a multinucleate germ tube at the apex, which penetrate the host tissue through the stomata or by rupturing the epidermis of the leaves, stems, etc. (b) Indirect Method – When temperature is less (12⁰C) but moisture content of the atmosphere is high, the conidia behave as zoosporangia. Inside the zoosporangia, the cleavage of cytoplasmic contents takes place, which results in the formation of uni-nucleate zoospore initials. Each zoospore initial develops into a laterally inserted bi-flagellate reniform zoospore.

            The matured zoospores are liberated from the zoosporangium by the breakdown of the papilla or by the formation of vesicle form papilla and then passing of the zoospores into it. The released zoospores swim in the film of water for sometimes, withdraw their flagella and take a brief rest, get encysted and start producing a fine germ tube when contact with the host epidermal tissues. The germ tube then develops into intracellular or intercellular coenocytic mycelium within the host tissue.

            Chlamydospore formation – Various species of Phytophthora produce chlamydospore. These develop as oblong intercalary or terminal spores. Most have relatively thick walls and dominated by lipid and reserve food vacuoles, which helps them to tolerate under unfavourable condition for growth. On the onset of favourable condition, the chlamydospores germinate to produce a new mycelium.

            2. Sexual Reproduction:- Some species of Phytophthora is homothallic, while others are heterothallic. The former produce sex organs in a single culture but the later require the presence of two compatible strains.

            The sexual reproduction in Phytophthora is of oogamous type, i.e., it produces two types of reproductive structures – antheridia (male sex organs) and oogonia (female sex organs).

            Antheridium – It is a funnel-shaped structure arising as a short lateral hypha from the mycelium with its tip inflated to form a more or less clavate structure. The funnel shaped anthreidium sits on the oogonial stalk forming a collar like structure at the base of oogonium. This type of antheridium surrounding the base of the oogonium is called, amphigynous. When young the antheridium has a thin walled and contains a non-vacuolated with a single nucleus or more often two nuclei, but on maturity the number of nuclei increases to 12. The mature antheridium becomes delimited from the supporting hypha by a septum. Prior to fertilization, all the nuclei except one degenerate. The surviving one functions as the male nucleus.

               Oogonium – It also arise as a short, lateral hypha without any inflation and lies just above the antheridium. The young oogonium contains dense multinucleate cytoplasm. Towards maturity, it increases in size, it protoplasts become vacuolated spherical structure. The nuclei undergo repeated divisions forming the oogonium into two zones – the outer peripheral, hyaline zone with vacuolated multinucleate zone known as periplasm and the central single nucleate zone known as ooplasm. All the nuclei of the periplasm degenerate. Prior to fertilization the single nucleus in the ooplasm divides into two nuclei, out of which one degenerate and the remaining one functions as the egg or oosphere nucleus.    

            Fertilization – During fertilization, the oogonial wall bulge into the antheridium at one point, which is known as receptive spot. The common wall of contact dissolves and produces a pore, through which the antheridium pushes a short fertilization tube. The fertilization tube penetrate the periplasm and reach the ooplasm, its tip opens and releases the male nucleus. Soon the male nucleus comes in contact with the female nucleus and fuse together, to complete the fertilization.

            As a result of fertilization, oospore is formed. A thick wall is formed around the oospore and it undergoes a resting period. Oospore germinates by producing a germ tube. At the tip of the germ tube, multinucleate sporangium develops. The multinucleate sporangium produces numerous uninuclaete biflagellate zoospores. The zoospores get encysted and each germinates into new somatic hyphae.

            D. ECONOMIC IMPORTANCE:- Phytophthora is of great importance in economic point of view. The species Phytophthora infestans, causes the most important disease called the late and early blight diseases of potato. This disease causes a million tones of loss in the production of potato worldwide. It also causes diseases on several important vegetable crop plants belonging to the genus Solanum.

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