Botany - B.Ye. Yakubenko 2017

Part Two. System of the Organic World
Chapter V. System of the Organic World
Laboratory Class Topic 10.24. Division Chytridiomycota. Class Chytridiomycetes

Objects. Order Chytridiales, representative Olpidium brassicae (Wor.) Dang.

Tasks:

1. Examine and study the general appearance of cabbage seedlings infected with black leg.

2. Examine and study The Structure of Olpidium brassicae.

3. Draw the external appearance of the infected plants and the fungus at various Stages of the parasite's developmental cycle.

Equipment and Materials: MBR-1 or Biolam microscopes, magnifiers, herbarium and fixed material infected with Olpidium, charts, and other supplies.

Macroscopic examination of cabbage seedlings infected with Olpidium. Using a magnifying Glass or low power under a Microscope, carefully examine the herbarium specimens and fixed material of the cabbage seedlings. Pay special attention to the General condition of the plants. Morphological changes are scarcely noticeable on the young leaves, buds, hypocotyl, and upper stem. The leaves are lighter in color, while fresh material exhibits wilting and a loss of turgor. Careful inspection of the lower stem, ROOT collar, and root system reveals significant changes: the stem is wrinkled, black, narrowed, and breaks or bends easily. The Root System is poorly developed: the upper part of the taproot is wrinkled and brownish-black, lateral roots are fewer in number, and second- and higher-order branching is observed only on individual roots.

Methods FOR PREPARING microscope slides of Olpidium brassicae. Take a glass slide and coverslip and clean them. Place a drop of Water on the slide. Take a piece of elderberry pith and make a longitudinal cut in it to a depth of 1–1.5 cm. Cut a piece of the main root with the root collar or a stem section with the root collar from fresh or fixed cabbage seedlings. Using a sharp razor or blade, first level the cutting surface, and then make a series of cross-sections through the root collar. Using a magnifying glass or microscope, select the best sections. Place them in the drop of water, cover with a coverslip, and secure the slide on the microscope stage.

Microscopic examination of the slide. Using low and high magnification of the microscope, study the STRUCTURE OF THE root collar infected with Olpidium. Beneath the epidermis in certain Cells of the cortical parenchyma, plasmodia of the parasite can be seen in places as clumps of Cytoplasm. In other, larger cells, single or groups (3–6) of zoosporangia can be observed protruding beyond the stem with their necks. Numerous zoospores are visible inside the zoosporangia (Fig. 73). Draw them and label their constituent parts.

Having studied the MACROSCOPIC AND MICROSCOPIC structure, let us examine the individual stages of The life cycle of Olpidium brassicae. The Development of the parasite begins with the infection of young plants by zoospores, which encyst upon landing on the root surface. The spore dissolves the wall of the root Cell and pours its protoplast into it. Inside the host cell, the parasite's Nucleus divides repeatedly, resulting in a multinucleate plasmodium. It develops a wall, transforming into a zoosporangium. The latter forms a long neck that pierces The Cell walls of the host plant's primary cortex and protrudes outward. Uniflagellate zoospores emerge from the zoosporangium through this neck to initiate a new infection.

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Figure 73. Olpidium brassicae:

a — “black leg” disease on cabbage seedlings; b — naked protoplasts (plasmodia); c — zoosporangium in the cells of the root collar; d — zoospores

Under conditions of delayed germination, zoospores may fuse in pairs like Gametes (isogamy). The resulting biflagellate zygote infects plants in the same manner as a spore. However, the parasite's protoplast, growing within the host cell, develops a thick stellate wall and transforms into a resting spore (cyst) that overwinters. In spring, the cyst undergoes Meiosis and germinates into numerous zoosporangia, which release zoospores that infect plants. Infected roots turn black, thin out, rot, and the plant dies.

Conclusion. The vegetative body of chytridiomycetes is represented by a plasmodium. Primary infection occurs via zoospores that develop from the zygote (cyst). Secondary infection and the spread of the disease (in parasitic fungal forms) are carried out by zoospores formed by the fungal plasmodium.

Self-Control Questions

1. What represents the vegetative body of chytridiomycetes?

2. Why is the disease caused by Olpidium called the black leg of cabbage seedlings?

3. Name the methods of reproduction of Olpidium brassicae.

4. What type of mycelium is characteristic of chytridiomycetes, and can it be considered typical for these organisms?

5. How does the infection of plants by chytrid Fungi occur, and in what state?