Plasmodiophora: Origin and Reproduction | Myxomycetes | Botany

In this article we will discuss about the origin, vegetative body and reproduction in plasmodiophora.

Origin of Plasmodiophora:

Plasmodiophora attacks a number of cultivated cruciferous plants. The best-known species is P. brassicae, which causes the clubrot or finged-and-toe diseases of the cabbage.

The host may be attacked at any time during its active growth, but infection usually takes place at the seedling stage. The amount of moisture present in the soil is an important factor in the occur­rence of the disease. The infection takes place when the soil contains about 60 per cent, moisture or above, but not when the percentage falls to 45 per cent, or below.

Light soils and moorlands are also the favourite welling places of this pathogen than loamy and clayey soils. As the popular name suggests, the infection essentially takes place in the roots. A uniflagellate swarmspore comes in contact with the cell-wall of the root hair, loses its flagellum and then enters into the root hair in an amoeboid fashion. It may also directly penetrate into the older regions of the root cortex.

The affected part begins to grow remarkably, and in the long run becomes abnormally swollen. When the disease is considerably advan­ced s0 as to cause a disturbance in the continuity of the vascular system, the leaves turn yellow, wilt and fall off, and the plant becomes stunted in growth.

Vegetative Body and Reproduction of Plasmodiophora:

The vegetative body is an intracellular multinucleate plasmo­dium.

Reproduction:

Plasmodiophora reproduces both asexually and sexually.

Chupp (1917), and Cook and Schwartz (1930) are of the opinion that the swarmspore, which causes the primary infection of the root, is a haploid body. The nucleus of the amoeboid cell, after it has entered into the root hair, divides repeatedly, and ultimately a multinucleate plasmodium is formed. Later on, the plasmodium breaks up into a number of uninucleate masses, each surrounded by a wall of its own.

During sexual repro­duction each of these uni­nucleate cells, produced asexually, may develop into a gametangium. The gametangial nucleus again undergoes divisions form­ing four or eight nuclei. The cytoplasm is again cleaved, and each uni­nucleate mass is finally metamorphozed into a tiny, uniflagellate zoogamete.

The zoogametes unite in pairs, either within the root hair itself, or after they have migrated into the neighbouring cortical cells. The product of fusion is an amoeboid zygote with one diploid nucleus. This zygote is known as the myxamoeba.

The myxamoeba then increases in size and its nucleus under­goes a few divisions, thus forming a diploid plasmodium. This plasmodium begins to migrate from one cell to migrate from one cell to another within the host tissue and even comes inside the cambial cell. Vertical upward or downward migrating movements of the plasmodia through the cambium have been noted.

Finally, when a plasmodium, after its outward migration, reaches a food-laden cortical cell it lodges within it and increases considerably in size. Thus the swelling in the root is formed. Along with the increase in size of the plasmodium its nuclei divide repeatedly, the last two series of divisions being meiotic. When the nuclear divisions are complete, innumerable vacuoles of various sizes appear within the cytoplasm.

Ultimately, the plasmodium is fragmented into many minute, uninucleate bits, each of which becomes a distinct spore. When the host undergoes degeneration, the spores are liberated as swarmspores, and fresh infections occur in the soil.