Phylum Thallophyta - The Algae - Archimycetes - Plasmodiophorales
The Plasmodiophorales are parasitic Archimycetes in which the plasmodium lives in the cell of a higher plant. By means of fragmentation the
plasmodium may break up, and by passing from one cell to another during cell division of the host plant it may become widely distributed in the host tissues. The group is a small one represented by less than two dozen species. They are mostly wry rare, but two species, Plasmodiophora brassicae, which causes Club Root or Finger-and- Toe disease of Cruciferae, and Spongospom subtenanea, which is responsible for Powdery Scab of Potatoes, are of considerable economic importance.
Plasmodiophora brassicae (Finger-and- Toe Disease or Club Root of Cabbages)
Club Root Disease is the name given to a characteristic swollen malformation of the roots of cruciferous plants. It is particularly common on acid soils and frequently causes great loss to market gardeners. The Fungus gains entry to the young root tissues through the root hairs and stimulates great hypertrophy of the tissues.
In a transverse section through an infected root it will be seen that the normal arrangement of the tissues has been altered by the formation of many additional cells, splitting up the original vascular system and
producing wide bands of thin-walled tissue. This is manifested externally by the swelling or hypertrophy of the root, which is the reason for the popular name given to the disease. In these thin-walled cells lie the myxamoebae, which are at first uninucleate. They increase in size, absorbing the food material in the host cells, and they may divide,. so that if division of the host cell occurs both the daughter cclls,become mfected. When the tissue becomes more permanent the rriyx moe a settles down and grows, without dividing, into a multinucleate plasmodium. When it has exhausted the food supply, meiosis occurs and the whole plasmodium becomes separated into unicucleate protions, around each of whicha wall is laid down. Thus the host cell becomes filled with a mass of spores.
In other genera the spores derived from a plasmodium do not separate, but form a group \yith a more or less regular and characteristic shape, and it is chiefly by this means that the genera are distinguished from one another. In Spongospora subterranea the spore mass forms a ball of irregular shape which has hollows in it and can be compared to a sponge, hence the name Spongospom was derived.
The spores are liberated by the breakdown of the host tissue and are thus set free into the soil. Here they germinate to produce zoospores, which are pyriform in shape and possess each two flagella, one directed fonnrds and another, very much shorter, which may be directed sideways or backwards. These zoospores have the power of penetrating the root hairs, and once inside giye rise to gametangia, which are yery small spherical bodies, the contents of \yhich di\"ide into a small number of motile isogametes. These fuse in pairs
to form fresh myxamoebae. Whether the migration from the root hairs to the root takes place before or after fusion of the gametes is not knO\yn, but the myxamoebae are found mainly in the meristematic cells of the young root, where they stimulate active cell division and in this way are themselves \yidely distributed in the host tissue.
In Spongospora subterranea it is the cortical tissue of the tubers which is most characteristically attacked and the hypertrophy is restricted to the formation of scabs, the surface of \yhich becomes powdery as the sporemasses mature. \iVhen the roots are attacked, however, hypertrophy may sometimes be very marked, and the effect of the disease resembles superficially the Wart Disease, Synchytrium endobioticum
The cytology of Plaslllodiophora and indeed all the members of the order is peculiar and unique. During the development of the plasmodium all the nuclei within it divide simultaneously by a process termed protomitosis, which differs from a normal mitosis in certain features. The metabolic nucleus does not possess a typical reticulum but instead the chromatin is disposed in a peripheral zone, leaying a clear, central, nuclear vacuole in \yhich lies a single spherical karyosome. During the prophase of the diyision the peripheral chromatin becomes denser and the nuclear membrane becomes drawn out into an ellipsoidal shape. At metaphase the chromatin forms a median ring surrounding and enclosing the karyosome, which now becomes drawn out and finally dumb-bell shaped, the nuclear membrane persisting throughout the diyision. The chromatin ring now splits into two rings which move a\ny from one another, keeping pace with the dividing karyosome. Whether this ring is a single unit or whether it is made up of four separate chromosomes has not been finally decided. In the anaphase of the division the two rings become completely separated and the karyosome divides into two, so that in the telophase of the division the chromatin ring of each daughter nucleus again encloses a single karyosome. The nuclear membrane now constricts and separates into two to form the membrane around the daughter nuclei.
Prior to the formation of spores a further change is observable in the behaviour of the nuclei, which is referred to as the akaryote stage. During this stage chromatin is apparently discharged from the nuclei into the cytoplasm, so that both the peripheral chromatin and the karyosome disappear and the nuclei can be recognized only as clear areas in the cytoplasm of the plasmodium. Chromatic discharge at some stage in the life-cycle has been described in other members of the lo\yer Fungi, especially in Synchytrium. The reappearance of chromatin within the nuclei coincides with the prophase of the heterotypic division of meiosis. In these t\yO divisions prior to the formation of spores a typical nuclear spindle appears, and separate chromosomes are apparently formed. No nuclear membrane is present, and it is doubtful if it is reformed between the beginning of the akaryote stage and the production of metabolic monoploid nuclei in the spores themselves.
A nuclear division similar to that in the Plasmodiophorales has been described in certain of the Protozoa, \yhich has led to the suggestion of a common ancestry of the two groups. In the :\Iyxomycetales, on the other hand, the nuclear divisions in the plasmodium are mitotic.
