Systematic position
Kingdom-Fungi
Phylum-Basidiomycota
Class-Teliomycetes
Order-Uredinales
Family-Pucciniaceae
Genus-Puccinia
Vegetative structure
The mycelium is well developed and consists of septate and intercellular hyphae, obtaining the nourishment by sending small, round or branched haustoria into the cells. During the lifecycle, two types of mycelia are produced, the monokaryotic mycelium and dikaryotic mycelium. The dikaryotic type of mycelium occurs in primary host and the monokaryotic type is found in alternate host.
Reproduction or Lifecycle
The lifecycle of the Puccinia graminis is divided into the fallowing five stages on the basis of nature of spores.
Uredo stage
The wheat plant becomes infected by means of aecidiospores, that are produced by the fungus on the alternate host(barberry) leaf. Since the acediospore is binucleate, the mycelium formed from it is also dikaryotic. The hyphae produced by acediospores ramify through the host tissues, forming haustoria that draw nourishment from surrounding cells of the wheat plant but do not kill them. In the early summer, the binucleate mycelium leads to the production of elongated, rusty, and reddish brown streaks on stems and leaf sheaths. These pustules are knowns as uredosori. The uredopustules develop from a mass of dikaryotic hyphae which collect just below the epidermis of the host. From the base of the uredosorus arise specialised hyphae, the sporophores which bear at their apices oval, thick walled and brown uredospores. The epidermis of the host plant becomes ruptured by the pressure of the underlying uredospores. The rusty red color of uredosori gives the characteristic name rust to the fungus and the disease it causes. The uredospores are intermixed with sterile hyphae, the paraphyses.
Each uredospore is stalked, broadly ovoid, brown and is finely echinulate. It has four germ-pores round the equatorial plane. The uredospores, thus produced are disseminated by wind currents. They are carried to other wheat plants where they where they germinate to produce a new crop of uredospores.
Germination of Uredospore
The uredospore germinates within 4 or 5 days by sending out a germ tube, if sufficient moisture is present. The germ tube grows over the surface of the epidermis and forms an elongated appresorium on reaching a stoma. The appressorium gives rise to a fine branch that passes through the stomata and swells up to form a substomatal vesicle. From the vesicle, arise one or more hyphae which remify through the intercellular spaces of the host tissue, producing a much branched mycelium made of binucleate cells.
The uredospores are also called repeating spores, as they are produced in successive crops in season, causing a widespread infection from plant to plant throughout the field.
Teleuto stage
Later in the summer, after the host plant matures, the same mycelium that gave rise to the uredospores, now begins to give rise to another kind of spores, the teliospores or teleutospores. They are produced either in the same pustule that has been producing uredospores, or in new sori or pustules, called teleutosori or teleutopustules. These telia form elongated, dark brown to black pustules on the stems and leaves of the host plant. Since the teleutospores are dark brown spores, producing dark brown to black to black pustules on the surface of stems and leaves of the wheat, it has resulted in the common name 'black stem rust of wheat' for this fungus.
The teleutospores are stalked, dark brown to black in color, thick-walled and two called, each cell containing a pair of nuclei. Each cell of the spore has a germ pore. When young, each cell has two nuclei which fuse when the spore is mature, resulting in a single diploid nucleus in each cell.
Unlike uredospores, the teleutospores are not capable of immediate germination. These two celled spores function for the survival of the fungus during winter, and do not usually germinate until the next spring.
Basidial stage
With the return of warm weather during the early spring, the teleutospores germinate on the ground. When moisture and temperature conditions are favorable, each cell of the teleutospore forms an elongated germ tube like structure called a promycelium. The diploid or fusion nucleus of teleutospore migrates to the promycelium where it undergoes meiosis to form four haploid nuclei.
The promycelium becomes divided into a row of four uninucleate cells. From the side of each of the four cells of the promycelium, arises a short, narrow tube, the sterigma on which a single basidiospore is formed. The nucleus and most of the cytoplasm moves to the basidiospore through the sterigmata.
At maturity, the basidiospore are liberated forcibly by the usual water drop methods and are carried away by the wind. These basidiospores cannot infect the wheat or any other member of wheat family but are capable to infect only the young leaves, fruits or twigs of the alternate host, i,e., the barberry.
Pycnidial or Spermogonial stage
The basidiospore germinates on the surface of the barberry leaf in the presence of moisture, forming a slender germ tube. The germ tube penetrates into the epidermis directly and there it branches freely, forming a haploid, monokaryotic, intercellular mycelium, i.e. composed of uninucleate cells. The mycelium carries either(+) or (-) strain which it obtained from the germinating basidiospore. Many basidiospores land and infect the same barberry leaf so that both (+) or (-) mycelia are developed side by side in the barberry tissues. About 4 days after infection of the host, the monokaryotic mycelium branch and grow vigorously, producing dense mats of uninucleate hyphae here and there between the upper epidermis and palisade tissue of the host leaf. These are the primordia of the spermogonia. After these primordia of the spermogonia have developed, small yellow or red patches appear on the upper surface of the infected leaves. In addition of forming primordia of spermogonia, the monokaryotic hyphae grow towards the lower surface of the barberry leaf and form structure, called aecia primordia, opposite the primordia of spermogonia.
A few days after infection, the primordia of the spermogonia give rise to spermogonia or pycnidia. The spermogonia which produces either (+) or (-) strain, depending on the nature of basidiospores. A mature spermogonium, is oval to flask shaped and projects from the surface of the leaf by a pore like opening, the ostiole. At the base of the spermogonium, is a pseudoparenchymatous mass of uninucleate cells from which arise a large number of elongated uninucleate cells, called spermatiophores. From the tip of each spermatiosphores are consticted off successively uninucleate, conidium like bodies, called pycnidiospores. The spermatia which are formed in chains, are produced in large numbers within the spermogonial cavity. They ooze out on the leaf surface in small droplets of nectar through the ostioles. The spermatia are of (+) or (-) strain. Each spermatium is small, oval, smooth-walled and has a single nucleus with very little cytoplasm. The hyphae adjacent to the ostiole develop into straight ponted paraphyses, that protrude through and beyond the ostiole. In addition, several long hyphae, called receptive hyphae, extend upward through the pore of the spermogonium which are essentially the female gametabgia. The pycnidiospores function as male cells and bring about diploidisation of the monokaryotic mycelium.
Each aecium primordium, which has been previously formed on the lower surface of the host leaf by the same mycelium that gave rise to the spermogonium on the upper surface of the leaf, consists only of an upper, closely packed weft of uninucleate hyphae, the basal cells, and a lower group of large, parenchyma like cells.
The insects are attracted to the leafy sugary nectar. Some spermatia stick to the legs and proboscis of the visiting insects and thus are carried from one pycnium to the receptive hyphae of other pycnium. If a pycniospore of one strain comes in contact with a receptive hypha projecting from a pycnium of opposite strain or vice versa, the nucleus of the spermatium passes into the receptive hypha, but it does not fuse with the nucleus already present in the latter. Thus, the dikaryotic condition forms.
Aecidial stage
This dikaryotic mycelium then grows intercellularly towards the lower side of the leaf, where it makes the basal cells of the aecial primordia dikaryotised. If due to any reason dikaryotisation fails, the protoaecidium aborts and aecidial cup is not formed.
The binucleated basal cells are popularly called aecidiospore mother cells or sporophores, and from them are cut off terminally the chains of binucleate spores, the aecidiospores, alternating with sterile cells. As a result, closely packed chains of cells are formed at the tip of aecidiospore mother cells. In each case, the oldest aecidiospore is present at the top and the youngest at the base or near to the tip of aecidiospore mother cell.
Simultaneously, the neighboring marginal cells at the base of acedium divide and forms peridium. The peridium thus surrounds the cells in the aecidium.
The aecidial cup remains partly embedded in the leaf tissue and partly projects above it. The edge of peridium bends upward and bell shaped structure formed. These aecidial cups appear on the lower surface of the infected leaf as white cups, filled with golden yellow powder of spores. The developing aecidium exerts pressure on the leaf tissue and ruptures the peridium as well as the lower epidermis and as a result, the exposed binucleate aecidiospores fall from the aecidium.
The aecidiospores are unicellular, thin walled and polyhedral in shape. The aecidiospores are unable to reinfect barberry, but they can infect only the wheat plants, thus repeating the lifecycle.
 |
| Life cycle of Puccinia graminis |
