Life Cycle of Puccinia Graminis Fungi

Dikaryophase in the Life Cycle of Puccinia:

This phase in the life cycle of Puccinia is confined to the primary host which is wheat. It consists of dikaryotic mycelium and two spore stages, uredineal and telial.

Dikaryotic Mycelium

It is internal and thus invisible until it is ready to reproduce.The binucleate secondary or dikaryotic mucelium is filamentous, well developed and branched.

The hyphal branches, which are septate, ramify in the intercellular spaces of the tissues of the stem and leaves of the host plant (wheat Fig. 14.14 B). The septal pore is simple. Each cell contains a pair of nuclei (n+n) constituting a dikaryon.

The nuclear membrane is double layered and perforated. Besides the two nuclei, the cytoplasm contains free ribosomes mitochondria, glycogen particles, lipid bodies and other unidentified particles.

To obtain nutrition the intercellular hyphae develop intracellular food absorbing organs called haustoria.

List of important diseases caused by species of Puccinia:

Haustorial Apparatus:

Allen (1923) reported that the tips of certain hyphae of the intercellular dikaryotic mycelium come in contact with the cell walls of the host cells and become separated from the remainder of the mycelium by septa.

The resultant small, binucleate tip cells swell somewhat and function as haustorial mother cells. At the point of contact, each haustorial mother cell develops a buldge-like thickening which is closely appressed to the host cell wall.

A fine pore appears at the point of contact extending through the cell walls of both the pathogen and the host. A delicate infection peg from the haustorial mother cell passes into the host cell through the pore.

After penetration, the infection peg elongates to form the haustorial neck. A wall is formed around it by the fungal pathogen. The plasma membrane of the host invaginates as the developing neck elongates.

The latter eventually ceases to elongate. Its apical portion expands to form the body region of the haustorium. Dickinson (1949, 1955) stated that the formation of haustoria by the infection hyphae is induced by contact stimulus.

The intercellular mycelium probably by enzymatic degradation enters the host cells. The haustoria may be knob-like or finger-like, rarely convoluted. Rykenberg and Truter (1973) reported that the intracellular saccate or extensively lobed body of the haustorium is connected by a narrow penetration tube or the stalk to the extracellular haustorial mother cell.

The hyphal wall and the plasma membrane of the haustorial mother cell are continuous along the entire length of the penetration tube and haustorial body constituting the haustorial wall and haustorial membrane respectively.

A collar-like structure attached to the host cell wall and made of the same material is produced by host cell in response to the presence of the penetration tube. It surrounds the penetration tube at its proximal end.

The haustorial body is binucleate. On the outer face of the hyphal wall (or haustorial wall) in the body region only is formed an electron dense sheath matrix consisting of a granular amorphous material.

The sheath matrix is also designated as the extrahaustorial sheath. Since the fungus obtains nutrition from the host the infected wheat plants exhibit stunted growth.

Despite the serious damage that the rust causes the host plant never succumbs completely. The yield of wheat crop is, however, reduced. Each cell of the fungus mycelium has a pair of nuclei (dikaryon).

The transverse septa between the cells have each a central pore without a dolipore parenthesome complex. The clamp connections, however, have not been reported on the dikaryotic mycelium.

The mycelium is generally localised. It grows and ramifies near the point of infection extending only short distance into the host tissue.