Ustilago
Q.15. What is loose smut? Describe the life history of the fungus causing loose smut of wheat. (2006)
Ans. Ustilago tritici causes loose smut of wheat. Its life cycle is as follows-
Life Cycle of Ustilago Tritici: -
The smut spores which are formed in enormous numbers are spherical to oval, 5-9 u in diameter and have fine walls. The teleutospores from the diseased plants are blown about and fall on the feathery stigma of he althy inflorescence. On germination over the stigma they produce promyceliafrom which arise infection threads. No sporidia or basidiospores are produced. Conjugation occurs between the compatible haploid infection threads and the dikaryotic infection hypha is produced.
The secondary mycelium which is composed of binucleate cells spreads through the body of the host plant. Though the infection occurs at the seedling stage of the host, it has little or no effect on vegetative development of the host. At the time of sporulation the hyphae form dense tangled masses in certain portions on host such as inflorescence leaves and stems. The cells of this mycelium become transformed into a mass of black, thick walled spores, the brand spores or the chlamydospores or the teleutospores or the smut spores. They are spherical and binucleate structures.
Germination of Teleutospores: -
The germination of teleutospores may occur immediately or following a dormancy period. The smut spores that are carried by wind may fall on the soil, on the grain and other favourable places leading a saprophytic existence.
Under suitable conditions such as warmth and moisture they germinate. The exosporium becomes ruptured. The endosporium protrudes in the form of a cylindrical hypha the promycelium or epibasidium, into which the diploid nucleus migrates. Meiosis occurs and the four haploid nuclei are formed in the epibasidium. U.maydis is heterothallic. The segregation into plus and minus strains takes place during the meiosis before the formation of the basisdiospores. Septa are laid down separating the epibasidium into four uninucleate cells. The haploid nucleus in each cell of the epibasidium divides mitotically into two daughter nuclei. One nucleus migrates into a bud which arises at the side of each epibasidial cell, and the other nucleus remains in the parent cell.The haploid uninucleate buds produced are the basidiospores or sporidia. The basidiospores are not differentiated morphologically but they are of two kinds. Two plus and two minus basidiospores are formed.
Fig. Life cycle of U. maydis.-A infected cob of maize showing tumours or galls; B-binucleate teleutospore; C-mature teleutospores containing diploid nucleus (after fusion), the composition of genes in teleutospore is also shown (AaBb); D and E-germination of teleutospore (chlamydospore) following meiosis; F-four nucleate stage of germinating chlamydospore to produce promycelium; G and H-development of sporidia or basidiospores; the possible segregation of the compatible genes AaBb is also shown; I-germination of basidiospore; J-the germ tube enters the host cells through stoma and grows. Two types of basidiospores showing genetic composition (AB and ab) grow simultaneously undergo process of dikaryotisation, and K-the dikaryotic cells grow by elongation and cell division by clamp connections to form full fledged secondary mycelium resulting to a condition as shown in A.
In U.tritici the basidiospores are not formed. The haploid cells of the epibasidium give rise instead to short hyphae called the infection threads.
The basidiospores or sporidia or secondary basidiospores are dispersed by wind and thus they may happen to fall on the soil or on the host plant. They bring about new infection of the host plants which depends upon the types of infection. Each basidiospore give rise to a five germ tube which is monokaryotic. In most species it can not infect the host tissue except U. maydis. The infection is established only when the diploidization has already taken place between the cells of the germ tubes. In U.tritici the fusion of two infection threads takes place.
Once penetration has occurred the mycelium makes its way to the growing apex of the plan and maintains its growth so as to keep pace with that of its host causing little or no disturbance to the host plant.
Modes of Infection: -
Smuts differ in manner of infecting the hosts. They may penetrate the host through seedlings, blossom or locally. In U.tritici blossom infection occurs.
Blossom Infection: -
In this type the infection takes place in the flowers at blossoming time. The teleutospores from the diseased plants at the time of healthy plants are in blossom are blown about and fall on the feathery stigma of healthy inflorescence. On germination over the stigma the teleutospores produce promycelia from which arise infection threads which enter the style and eventually develop in the embryo. The infection has no injurious effect on the developing grain which matures normally. The mycelium remains in the grain as dormant mycelium thus carrying the fungus over seasons unfavourable to growth. When such seeds are sown, the mycelium within becomes active, grows rapidly thus keeping pace with the development of the host plant. As the ear formation starts, the mycelium grows vigorously and teleutospores are produced there. The entire head of grain becomes a black, loose mass of teliospores.
Symptoms of Loose Smut: -
The disease manifests itself only when the plants are in ear. Diseased ears emerge out of the boot leaf earlier than the healthy ones. The flower heads become masses of blackish powder, the smut spores. When these spores are blown off, only the central axis of the spikelet is left behind. Spore masses remain covered by a delicate silvery membrane which usually bursts before the emergence of the ear to expose the black powdery mass of spores.
Puccinia
Q.16. Describe the life history of the fungus causing black rust of wheat. (2007)
Related Questions -
Q. What is heteroecism? With the help of diagrams describe only that part of the life history of Puccinia graminis found on barberry or wheat plant. (2009, 11, 12)
Ans. Heteroecism: -
The development of different stages of a parasitic species on different host plants is called heteroecism.
Life Cycle of Puccinia Graminis: -
According to the nature of the spores, the life cycle of the P.graminis is divided into the following five stages.
(1) Uredo Stage: -
The wheat plant becomes infected by means of aecidospores that are produced by the fungus on the alternate host leaf. Theses aecidiospores are disseminated by wind and can infect only wheat plants. Since the aecidiospore is binucleate, the mycelium formed from it is also dikaryotic. The hyphae produced by aecidiospores 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 or pustules on stems and leaf sheaths. These pustules are known as uredosori or uredia. 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 specialized 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, forming elongated, rusty reddish brown, streak-like pustules. The rusty red colour of uredosori gives the characteristic name ‘rust’ to the fungus and the disease it causes. It is through these streaks that the rusty coloured masses of uredospores are seen externally. The uredospores are intermixed with sterile hyphae, the paraphyses.
Development of the Uredospore: -
During the formation of uredospore, the terminal cell of each sporophore divides transversely to form two cells the lower foot cell and an upper cell which divides again forming a lower stalk cell and an upper spore initial. The stalk cell forms the stalk and the upper spore initial develops into the uredospore.
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 germinate to produce a new crop of uredospores.
Germination of Uredospore: -
The uredospore germinates within four or five days by sending out a germ tube, if sufficient moisture is present. The germ tube does not enter the host tissue directly. The germ tube grows over the surface of the epidermis and on reaching a stoma the tip swells up to form an elongated appressorium. The protoplast of the germ tube migrates into the appressorium and the empty germ tube is cut off from the appressorium by cross wall. 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 ramify 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 produce successive crops of uredospores in a season, causing a widespread infection from plant to plant throughout the field.
(2) Teleuto Stage: -
Later in the summer, after the host plant matures, the same mycelium that gave rise to the uredospores now begins to give 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 the 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 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.
Development of the Teleutospores: -
The development of the teleutospore is similar to development of the uredospore except that the upper cell cut off from the basal cell, divides further to form two binucleate cells which form a two-celled teleutospore.
The teleutospores are stalked dark brown to black in colour, thick-walled and two-celled, each cell containing a pair of nuclei.the apex of each teleutospore is rounded or sometimes pointed. Each cell of the spore has a germ pore. The germ pore is situated apically in the upper cell, while in the lower cell it lies just below septum. These are more firmly fixed to their stalks as compared to the uredospores. When young each cell has tow 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.
(3) Basidial Stage: -
With the return of warm weather during the early spring, the teleutospores germinate on the ground. When moisture and temperature conditions are favourable each cell of the teleutospore forms an elongated germ tube like structure called a promycelium. The diploid or fusion nucleus of the teleutospore migrates to the promycelium where it undergoes meiosis to form four haploid nuclei (n).
The promycelium beomes 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 basidiopore is formed. The nucleus and most of the cytoplasm moves to the basidiopore through the sterigmata.
At maturity, the basidiospores are liberated forcibly by the usual water drop method and are carried away by the wind. These basidiopores can not infect the wheat or any other member of what family but are capable to infect only the young leaves, fruits or twigs of the alternate host i.e. the barberry.
(4) Pycnidial or Spermogonial Stage: -
The basidiospore germinates on the surface of the barberry leaf in the presence of moisture, forming a slnder 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 is obtained from the germinating basidiospore. Many basidispores land and infect the same barberry leaf so that both (+) and (-) 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 primordial of the spemogonia(pycnia). After these primordia of the spermogonia have developed small yellow or red patches appear on the upper surface of the infected leaves. In addition to forming primordia of spemogonia the monokaryotic hyphae grow towards the lower surface of the barberry leaf and form the structure calles “aecia primordial”, opposite the primordial of spermogonia. Aecia primordial which are initially monokaryotic do not develop further until they become dikaryotic.
A few days after infection, the primordial of the spermogonia give rise to spermogonia or pycnidia. The spermogonia which are produced are either of (+) 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 pseudoparenchhymatous mass of uninucleate cells from which arise a large number of elongated uninucleate cells called spermatiophores. From the tip of each of these spermatiophores are constricted off successively uninucleate, conidium-like bodies, called pycnidiospores or spermatia. 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 like the strain of mycelium, which produced previously the spermogonium. All spermatia from a single spermogonium carry the same strain. Each spermatium is small, oval to spherical in shape, smooth – walled and has a single nucleus with very little cytoplasm. The hyphae adjacent to the ostiole develop into straight pointed 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 is essentially the female gametangia. Previously pycnidiospores were generally regarded as functionless male cells but in 1927 the researches of Craigie, Buller and their associates demonstrated that they actually function as male cells and bring about diploidization 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 uninucleate hyphae, the basal cells, and a lower larger parenchyma like cells. The aecia primordial, which are initially monokaryotic, do not develop further until they become dikaryotic.
From the opening of the spermogonia a number of receptive hyphae project. The insects particularly the flies are attracted to the leaf by 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 pycnia. If a pycniospore of one strain comes into contact with a receptive hypha projecting from a pycnium of opposite strain or vice versa, they fuse and the nucleus of pycnidiospore passes into the receptive hypha and moves downward, establishing the dikaryon condition. It probably divides repeatedly and the resulting nuclei are to pass through pores in the hyphae cross-walls. It has been demonstarated that aecial primordial fail to develop if spermatization does not occur. On the other hand in some cases the binucleate condition has been demonstrated to be the result of the fusion of two primary hyphae of opposite strains.
(5) Aecidial Stage: -
Fusion of the spermatium with the receptive hypha is followed by migration of spermatial nuclei into the receptive hyphae and presumably through the septal perforations of the mycelium to the basal cells of aecial primordia which thus become dikaryotized. In the resulting binucleate cells, one nucleus belongs to the (+) strain, the other to the(-)atrain. If due to any reason dikaryotization fails, the protoaecidium aborts and aecidial cup is not formed.
The dikaryotized (binucleate) 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 neighbouring marginal cells at the base of aecidium divide and form peridium. The peridium thus surrounds the cells in the aecidium. The aecidial cup remains embedded in the leaf tissue and partly projects above it. The edge of peridium bends upward and a bell shaped structure is 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, binucleate; thin walled and polyhederal in shape. The aecidiospores are unable to reinfect barberry but they can infect only the wheat plants thus repeating the life cycle.
Therefore in the life cycle of P.graminis five different types of spores are produced. These spore forms in roman numerals are referred as follows -
- Stage 0- spermogonium
- StageI-Aecium bearing aecidiospores
- Stage II-Uredium bearing uredodpores
- Stage III-Telium bearing teleutospores
- Stage IV-Basidium bearing basidiospores.