In this article we will discuss about the classification of claviceps purpurea.
Family Clavicipitaceae:
The family Clavicipitaceae includes fungi parasitic on angiospermic plants, insects, and on the fruit bodies of some fungi. They form conspicuous stroma in which perithecia are embedded. Asci are cylindrical each of which has a cap with perforation through which thread-like ascospores are released.
Genus Claviceps of Clavicipitaceae:
The genus Claviceps is parasitic on rye and other grasses. It is characterized by filiform ascospores which germinate on the flowers of the host. The mycelium ramifies in the ovary and forms an acervulus from peripheral region of which conidia are produced. The conidia are dispersed by insects to the ovaries and thereby the infection is spread.
The mycelium of an infected ovary is ultimately transformed in a hard pink or purplish sclerotium. The sclerotium on germination produces numerous stromata with violet stalks and rose-pink heads in which perithecia are immersed. Each mature perithecium bears several elongated cylindrical asci, each containing eight filiform ascospores.
Some Indian species of Clavicipitaceae:
Claviceps digitarie Hansf.; C. paspali Stev. and Haller; C. purpurea (Fr.) Tul.; C. pusilla Ces.; C. viridis Padwick and Azmatullah Khan.
Claviceps Purpurea (Fr.) Tul:
The species of Claviceps are parasitic on grasses, including rye and barley and occasionally wheat. Claviceps purpurea is a parasitic fungus. It causes ergot disease of rye. The fungus attacks only the inflorescence of its host. Grains of rye are replaced by blunt trilateral horn-shaped black sclerotia known by the name of ergots (Fig. 244A).
These sclerotia are resting stages. In the mature state they are composed of a dirty-white medullary tissue surrounded by a violet-brown rind. The medulla or the central core consists of a prosenchyma formed of looser cylindric prismatic cells of pale-brown colour which are about one to four times as long as breadth (Fig. 141).
The cells are arranged in straight or sinuous longitudinal rows possessing the characters of the fungal hyphae.
The medulla has usually shorter and broader cells towards the periphery of the sclerotium than in its centre. The cells contain large colourless drops of oil. The surrounding rind which is at every point firmly connected with the medulla, is composed of one or two layers of cells with contents showing no oil and has a dark-brown colour.
In between the rind and the medulla there are few layers of pseudoparenchyma of longitudinally arranged or irregular branched rows of isodia- metric cells (Fig. 141) resembling parenchyma of higher plants. The cells are narrow and pale violet-brown in colour.
The sclerotia contain valuable therapeutic properties. They yield about a dozen different alkaloids. These alkaloids have different pharmacological properties. Some of these alkaloids are chiefly used in gynaecology and obstetrics. Most ergot of trade (the sclerotia of C ‘laivceps purpurea) comes from naturally diseased rye, but lately ergot has been produced by artificial infection from the fungus in culture.
Some of the alkaloids contained in the sclerotia are: ergometrine causes rapid contraction of the uterus; ergotoxine, ergotamine have very similar action but much slower with prolonged effect and ergotoline has physiological importance.
Ergotamine tartrate which centres its activity on constricting the dilated blood vessels in the head, is frequently used to relieve headache. For these properties the sclerotia are widely used during childbirth and to control haemorrhage after delivery. The sclerotia are poisonous to live-stock and human beings, often causing disease known as ergotism—ergot poisoning, a disease also known as St. Anthony’s fire.
This is the effect of poisonous alkaloids that are also present in the sclerotia. C. purpurea has been cultured under laboratory conditions but forms neither sclerotia nor alkaloids under these conditions.
The sclerotium on germination usually produces several spherical stalked stromata (sing, stroma), also known as sphaeridia (sing, sphaeridium) (Fig. 244 B & G). The sphaeridium is covered with numerous closely set papillae (Fig. 246A), each papilla surrounding an ostiole leading down to a perithecium which is sunk halfway beneath the surface.
The perithecia form a single layer of numerous flask- shaped cavities which follow closely the contour of the sphaeridium (Fig. 246B). The actual mechanism of formation of the cavities is not clearly known. In all possibility it is affected by the mutual separation of the individual hyphae. The roof of the cavity ultimately develops into the wall of the perithecium, and its floor becomes the hymenium (Fig. 244D).
Details of the formation of perithecia were first elucidated by Killian (1919). According to him, the sex organs are produced from the terminal cell of a hypha. The hypha is usually unbranched, elongated composed of multinucleate cells. It is easily recognizable from the adjacent stromatic hyphae. The terminal cell of the hypha swells (Fig. 245A), the nuclei divide and arrange themselves in pairs.
It becomes broader and thicker and then develops into an ascogonium from whose base branch out one or two antheridia (Fig. 245B to D). The antheridia elongate considerably in which nuclei divide repeatedly.
They -are longer and more slender than the ascogonium. One of these antheridia comes in contact with the ascogonium at a point where the ascogonium forms a small papilla on the side toward the antheridium (Fig. 245E).
The walls at the point of contact dissolve and opening is formed through which the nuclei of the antheridium migrate into the ascogonium (Fig. 245F). The emptied antheridium collapses and disappears completely. Ascogenous hyphae then grow from the ascogonium. The ascogenous hyphae are composed of binucleate cells.
Asci are formed at the tips of ascogenous hyphae by typical crozier formation. The young perithecium soon acquires the form of an elongated cone by growth and forms a canal, the ostiole (Fig. 246C) which is beset all round from below upwards with periphyses.
The perithecia contain long, curved hyaline asci, narrowed at both ends (Fig. 246D) and surrounded by paraphyses which are thickest at the apical end. An ascus contains eight long cylindrical filiform, slightly curved, hyaline ascospores (Fig. 246E). The ascospores under the influence of moisture, are extruded and collect at the ostioles of the perithecia for dispersal.
They are either forcibly ejected into the air by raindrops or dispersed by insects. This constitutes the sphaeridial stage or ascigerous stage or perfect stage. The ascospores during germination under favourable conditions put out germ tubes at several points.
Hyphal branches developed from the germ tubes infect the ovary only at the time of flowering and the infection remains restricted to the pistil only. Gradually the hyphal branches spread through the entire ovary replacing the ovarial tissue by mycelial mat. The ovary is thus changed into a white fungus-body of nearly its own shape.
The mycelium at first occupies the base of the young ovary and then completely replaces the entire ovary.
During the early stage of infection a sweet sticky yellow fluid commonly known as ‘honey-dew’ is exuded from the infected flowers. In this fluid remain embedded minute oval uninucleate conidia cut off from the conidiophores that are developed in the peripheral portion of the soft cottony mycelial mat, an acervulus-like structure/which has partly replaced the ovary tissue (Fig. 246F).
This is known as honey-dew or Sphacelia stage and was originally known as Sphacelia segetum. Before the establishment of connection between this stage and the ascigerous stage, Claviceps purpurea was known by its conidial stage Sphacelia segetum. The full life history of the fungus was established by Kuhn in 1863.
The sweet exudate which accumulates on the surface of the inflorescence like dew-drop is also known as honey-dew. It is mixed with conidia. Insects on being attracted by the honey-dews visit diseased inflorescences and invariably eat the honey-dews.
At the same time the conidia are carried by the insects from infected to healthy plants and thereby the infection is spread. On the completion of the conidial stage the mycelial mat develops considerably even beyond the ovary and is ultimately metamorphosed into a compact tissue which becomes hard purple or bluish-black sclerotium, the sclerotial stage (Figs. 246G and 244A).
A sclerotium is several times larger than the normal grain of the host plant. After a period of rest, usually lasting till the following season the sclerotium germinates. Claviceps purpurea is differentiated into a number of physiologic races. Life cycle of Claviceps purpurea is presented in Figure 247.
