In this article we will discuss about the life cycle of ustilago with the help of suitable diagrams.
Mycelium of Ustilago:
According to the nuclear behaviour, the mycelium of Ustilago passes through two distinct stages of development. These are the primary and secondary mycelia.
The primary mycelium consists of hyaline, slender, septate hyphae with a single haploid(n) nucleus in each cell. This kind of mycelium is also called monokaryotic mycelium or haplomycelium.
It is formed by the germination of a basidiospore (fig. 14.2 A). It might be of a plus or a minus strain according as it is developed from a plus or a minus strain basidiospore.
It seldom develops very extensively. In most species the primary mycelium soon becomes converted into a secondary mycelium. The primary myeclium is thus of very short duration.
The secondary mycelium consists of hyphae with two haploid (n+n) nuclei in each cell. Such hyphae are called dikaryotic hyphae. These dikaryotic hyphae are septate and extensively branched.
The septa between the cells have each a central pore. The dolipore septal complex is, however, absent in the smuts. Through these pores the adjacent cells communicate with each other.
The mycelium of most species of Ustilago found within the host is generally dikaryotic or secondary
mycelium. It develops extensively within the tissues and spreads to the various parts of the host.
In fact the secondary mycelium constitutes the most conspicuous and important part of the somatic or vegetative phase of the majority of species of Ustilago. In many species, septa develop clamp connections.
The hyphae ramify in the spaces between the host cells. They are thus intercellular. The intercellular hyphae may develop distinct haustoria which penetrate the walls of the host cells and absorb nutrition.
The host cells, however, are not destroyed. In some species the haustoria are absent. In Ustilago maydis, the hyphae are intracellular. They penetrate the cells and obtain nutrition directly from the protoplasm of the host cells.
The growth of the parasitic mycelium within the host tissues causes little or no disturbance to the vegetative development of the host plant. In some species, the mycelium is scattered throughout the various parts of the host.
It is said to be systemic. In others it spreads near the point of infection and is called localised.
Dicaryotisation or Diploidisation in Ustilago:
The process whereby the primary mycelium produced by the germination of basidiospores changes into a secondary mycelium is called discaryotisation or diploidisation.
The process is ‘initiated by the pairing together of two haploid cells of opposite strains of a species. They copulate and one of them becomes binucleate.
The two nuclei in the fusion cell constitute a dicaryon. They do not fuse in the vegetative phase. The resultant dicaryotic or binucleate cell develops into a dicaryotic hypha which by further growth forms the dicaryotic or secondary mycelium. The formation of a dicaryotic cell is a prerequisite to normal infection in Ustilago.
In U. maydis (com smut) copulation to form the dicaryotic cell occurs inside the host tissue (com plant) but in all other species, in gereal, it occurs outside the host.
The various methods of diploidisation in Ustilago are detailed below:
1. By hyphal fusions (somatogamy) between primary mycelia (A). In U. maydis, the basidiospores or sporidia fall on the surface of the host (com plant) and germinate to produce haploid mycelia.
The latter penetrate the host epidermis and grow horizontally beneath. Dicaryotisation takes place within the host by means of hyphal fusions [somatogamy between hyphae of suitable mating (opposite) strains].
It may take place immediately after penetration within the host or after there has been some growth of primary mycelia. Subsequent migration of nuclei into the fusion cells initiates the dicaryotic phase.
Binucleate cells thus are formed by elongation and repeated cell division by clamp connections from the secondary mycelium.
2. By Fusion between the Germ Tubes of two germinating basidiospores (B-C). As the basidiospores germinate the germ tubes of the basidiospores of opposite strains meet and fuse.
The intervening walls at the point of corftact dissolve. The nucleus of one germ tube migrates into the other. The latter becomes binucleate. It grows into a secondary mycelium. Example of this type is U. hordei.
3. By Conjugation between the basidiospores. In some species the basidiospores multiply by budding to produce secondary spores (sporidia). The secondary spores or sprout cells of opposite strains (copulate).
The common wall between them dissolves at the point of contact or they send copulation tubes towards each other. The nucleus of one migrates into the other through the connecting link (I).
The binucleate sporidium, or sprout cell, on germination, produces the secondary mycelium. U. receptacularum and U. violocea are common examples.
4. By union of the basidiospores of one strain with the germ tube of the basidiospores of another strain.
5. By the union of infection threads. U. tritici is an example (H). The promycelium or basidium does not bear the basidiospores. Its haploid cells grow into small, slender hyphae one each. These are called the infection threads.
Two neighbouring infection threads of the opposite strains fuse. The nucleus of the one passes into the other. As a result one of the infection threads becomes binucleate. It grows to form the secondary mycelium.
Similarly in U. nuda fusion between the compatible cells of the epibasidium takes place by conjugation tubes (D). The conjugated binucleate cell forms a binucleated hypha which infects the host.
6. By fusion between the two haploid cells of the same epibasidium (E1, E2.) In this case fusion takes place between two haploid cells of opposite strains of the same basidium. U. hordei and U. carbo are the examples.
7. By fusion between two basidia formed by the germination of smut spores of opposite strains (G). U. nuda is an example.
8. In U. violacea the binucleate cell may arise by the union of a basidiospore with one of the basidial cells of the opposite strain (F).
Reproduction in Ustilago:
Sex organs are absent in Ustilago. Plasmogamy, karyogamy and meiosis, the three fundamental events of the sexual process do occur. Plasmogamy takes place by the fusion of two haploid compatible cells.
The binucleate cell thus formed by repeated divisions produces the secondary or dikaryotic mycelium. The intercellular hyphae of the latter feed on the host plant, accumulate reserve food materials and reaching a certain stage of development enter the sporulation stage.
1. Sporulation (Fig. 14.3):
In wheat, oat and barley, the invading secondary mycelium becomes active at the flowering time of the host. It grows vigorously and reaches the inflorescence region where it branches profusely and infects embryonic spikelets.
The parenchymatous tissue in the embryonic spikelets is destroyed and occupied by the hyphal masses. By the time, the head or ear emerges from the host leaf, it is generally completely destroyed (B).
Sporulation starts in the centre of the hyphal mass and progresses outwards as hyphal proliferation continues. The hyphae divide by additional septa into shorter binucleate segments called the spore fundaments.
These hyphae are called the sporogenous hyphae. They are closely interwined. The binculeate protoplast of each segment functions as the spore initial. The spore formation in Ustilago is thus endogenous and the sproes are formed singly inside the hyphal segments.
Sporulation is preceded by the thickening of the hyphal walls and their subsequent gelatinisation. The sporogenous hyphae thus lose their identity. The spore initials (binucleate cell protoplasts) lie in a hyaline, gelatinous matrix.
They are, at first, variously shaped but become globular as they enlarge. Each secretes a new wall around it to become a teliospore or brand spore. By the time the spores are morphologically mature, the gelatinous material disappears.
The spores are closely appressed into a hard, compact mass called a smut ball or sorus. The spores in the sorus are readily separable by slight pressure. The sorus or smut ball is covered by a peridium of host cells in U. hordei.
Virtually all the hyphae in hyphal mass are converted into spores after necrosis of the host tissues. No peridia or columellae of the fungal origin are formed. In U. hordei the group of spikelets at each node of the rachis forms a single irregularly shaped spore mass or sorus.
The differentiation and development of spores in other species of Ustilago such as U. avenae, U. tritici and U. nuda may closely follow the pattern of events described above in U. hordei.
The only difference is that in U. avenae and U. tritici, the fungal hyphae grow profusely between the anticlinal walls of the host epidermis and destroy the latter. The mature sori in these two species are thus naked, U. nuda with a growth pattern similar to U. hordei has the smut balls or sori enclosed in a fragile peridium derived from the host tissue.
The smuts in which the sori are covered by the membranous covering or peridium are called covered smuts. In loose smuts, the sori are naked. Each smut ball or sorus contains numerous, thick-walled, rounded spores. The thick spore wall is differentiated into two layers (D). The outer exine or exosporium is thick. It may be smooth, reticulate, or spiny. The inner intine or endosporium is always thin.
The binucleate smut spores are generally the resting spores. They remain dormant under adverse conditions. Some mycologists call the smut spores as teleutospore. The older mycologists termed them chlamydospores. The use of the term chlamydospores for the smut spores of Ustilago appears to be inappropriate.
The smut spores are binucleate structures produced only by the binucleate cells of the secondary mycelium which originates as a result of plasmogamy (sexual fusion). They are thus reproductive in nature and homologous to the teleutospores of rusts rather than to the chlamydospores.
The smut spores are dispersed by wind, insects, or washed by water. When all the spores are blown off small rachis are left behind on the infected ear. In U. tritici, the smut spores do not function as resting spores. They serve as means of propagating the disease during the growing season.
They fall on the stigmas of the flower and soon germinate to infect the ovules in the ovaries of the healthy plants. The spores of the covered smuts are liberated by the rupture of the walls of the grains at the threshing time.
2. Germination of smut spore to form the Basidium (Fig. 14.4).
This smut spores carried by the wind may fall on the soil, on the grain, and other favourable places. Under suitable conditions such as warmth and moisture they germinate.
Duran and Safeeulla (1968) reported that in most smuts optium temperature for smut spore germination ranges from 20 to 30°C. Light also stimulates smut spore germination. (ii) Karyogamy. Prior to germination the two nuclei (one of plus and the other of minus strain) in the smut spore fuse to form a synkaryon. It is diploid.
The thick-walled smut spore with a synkaryon represents the encysted probasidium or hypobasidium stage (A). It absorbs moisture and swells up. The exosporium or the epispore layer ruptures. The endospore or the endosporium protrudes in the form of a short, cylindrical hypha, the promycelium.
The promycelium is also called the epibasidium or metabasidium. (iii) Meiosis. The diploid nucleus migrates into the epibasidium and divides twice. These two divisions constitute meiosis (B) and (C). The resultant four nuclei in the epibasidium are thus haploid. Since segregation of the sexual strains takes place during meiosis two of these nuclei are of plus strain and two of minus strain.
They are arranged in a row (C). Septa are laid between the nuclei (D). The epibasidium at this stage is composed of four haploid cells.
The basidiospores of some species such as U. maydis are capable of multiplying by budding like the yeast cell (F). The new spores formed by budding are called secondary spores or conidia.
In U. tritici which parasitizes wheat the basidiospores are lacking. The haploid cells of the epibasidium or the promycelium, instead produce slender, short hyphae (Fig. 14.5 E). These are called the infection threads.
Germination of basidiospores and infection of the Host:
The basidiospores or the secondary sporidia produced from the them by budding germinate either on the soil or on the young host plant (U. maydis) itself. Each basidiospore produces a fine germ tube, also called the infection tube.
The germ tube is haploid (monokaryotic). In most speices it cannot infect the host tissues. Exception is U. maydis. Infection is generally brought about by the dikaryotic germ tube.
Dikaryotisation or diploidisation of the germ tubes if brought about differently in different species of Ustilago.
The following examples will illustrate the point:
1. Ustilago hordei:
It causes Covered smut of Barley. The basidiospores germinate in the soil or on barley grains as the latter are sown. The germ tubes produced by them are unable to bring about infection.
Diploidisation is brought about by fusion between the haploid germ tubes of the two basidiospores of opposite strains (Fig. 14.2 B-C). As a result one of the tubes becomes dikaryotic (binucleate).
The dikaryotic germ tube is capable of infecting the young barley seedling at a very early stage as it emerges from the grain. It gains entry into the host seedling through the hypocotyl and reaches the coleoptile.
U. hordei thus provides an example of infection at the seedling stage. Loose smut of Oats caused by U. avenae is also an example of infection at the seedling stage.
2. Ustilago tritici (Fig. 14.5):
The smut spores germinate on the feathery stigmas of the flower. Each produces a four celled promycelium or epibasidium (D). The cells of the epibasidium do not bear basidiospores.
Instead each basidium cell produces a slender tubular outgrowth, the infection thread (E). It is haploid. The infection threads of the same basidium with nuclei of opposite strains fuse to form a binucleate (dikaryotic) hypha (F).
The latter grows through the style until it reaches the ovary which it penetrates. In the ovary it ramifies in the intercellular spaces of the ovary tissue. By the tenth day of its origin it gains entry into the ovule.
It is an example of infection through the flower. The mycelium lies dormant in the grain (Fig, 14.6 A), and is again activated when the grain germinates (Fig. 14.6 B).
It spreads and grows along with the seedling (Fig. 14.6 C) till the latter matures and produces flower. The mycelium finally invades the ovaries (Fig. 14.6 D) and ovules.
Inside the ovaries it produces millions of smut spores which are exposed by the decay of host tissues. When the wind blows the spores are carried away leaving the naked rachis (Fig. 14.3 C).
3. Ustilago maydis (Com smut Fig. 14.4):
It is an example of general primary infection through many embryonic tissues of the host. The smut spores (teliospores) which are produced in summer are, at first, binucleate when young.
The two nuclei eventually fuse. The mature spores are thus uninucleate and diploid. They lie dormant in winter on com debris or other favourable places in the soil. They germinate in the following planting season of maize.
The thick spore wall ruptures. Through the split emerges a short cylindrical germ tube known as the promycelium or epibasidium. (A). Immediately the diploid nucleus migrates into the latter and undergoes meiosis (B-C).
The resultant four haploid nuclei are distributed uniformly and are arranged in a row. Septa are laid between the nuclei in the epibasidium (D). Each cell of the latter bears a haploid basidiospores.
The basidiospores are capable of budding. These basidiospores or the secondary basidiospores are carried by the wind. They happen to fall on a young com plant.
There each basidiospore germinates to produce a uninucleated germ tube (Fig. 14.2 A). It enters the host through a stoma or pierces the wall of the epidermal cell and brings about primary infection.
Infection can take place any time during the growing season and through any young and meristematic part of the host (stem, leaves, ears, tassels, etc.).
The haploid germ tubes from two basidiospores of plus and minus strain fuse in the tissue of the host and produce a binucleate cell (dikaryotic cell). This is diploidisation by somatogamy or somatogamous copulation (Fig. 14.2 A).
The resultant binucleate or dikaryotic cell grows by elongation and cell division by clamp connections to form a full-fledged secondary mycelium. The cells of the secondary mycelium (dikaryotic mycelium) are binucleate.
The secondary mycelium plays a dominant role and carries on the life cycle of the fungal parasite. It ramifies intercellularly and even intracellularly throughout the tissues of host.
It is reported that some of the hyphae of the secondary mycelium that reach the surface of the host, produce several crops of binucleate conidia during the growing season.
The mature binucleate conidia are dispersed by wind. Falling on the host the conidia initiate new or secondary infections. The disease spreads in this way. Eventually the secondary mycelium develops extensively at certain points.
At these points the extensive development of the mycelium causes swellings called galls or tumours (Fig. 14.8). These tumours can appear on any portion of the host e.g. stem, leaves, ears, tassels. Each swelling contains an indefinite number of smut spores.
Sexuality in Ustilago:
No sex organs are developed in Ustiiago. The sexual process is represented by three fundamental phenomena characteristic of it, namely, plasmogamy, karyogamy and meiosis.
(a) Plasmogamy (Fig. 14.2):
Heterothallism is common in the genus Ustiiago. The mycelia though morphologically alike are different physiologically. Physiologically they are unisexual. There is, however, no apparent distinction into male and female mycelia.
They are different only in their sexual behaviour. The difference of sex is thus very rudimentary. It is denoted by the signs plus and minus. Such mycelia are said to be heterothallic.
Plasmogamy in heterothallic species is brought about by different methods of diploidisation. It may be accomplished by conjugation between basidiospores of opposite strains (B-C).
Union may as well take place between a basidiospore of one strain and a cell of the basidium of opposite strain (F). There may be fusion between basidia of different smut spores (G).
Diploidisation is also brought about by somatogamous copulation between vegetative cells of the two hyphae of opposite strains. In either case a binucleate condition is established in one of the conjugating cells.
The binucleate cell is also called the dikaryotic cell. The dikaryotic condition once established is maintained for a considerable period in the life cycle. Plasmogamy therefore initiates dikaryophase in the life cycle.
The binucleate cell by elongation and division generally by clamp formation develops into a secondary mycelium.
(b) Karyogamy:
With karyogamy the dikaryophase ends. The two nuclei in the smut spore fuse. This fusion between the two nuclei may be regraded as a culmination of the sexual process begun at the time of diploidisation. It is equivalent to the fertilisation process.
The diploid nucleus formed in this way is called a synkaryon. The smut spore with a synkaryon is the probasidium or hypobasidium. It represents the transitory diplophase in the life cycle of smuts.
(c) Meiosis:
The diploid smut spore germinates to form the promycelium or epibasidium. The synkaryon in the epibasidium undergoes meiosis to form four halpoid daughter nuclei.
The walls are laid between the nuclei. The epibasidium thus becomes a fourcelled structure. Each cell of the epibasidium bears a haploid basidiospore. With meiosis the transitory diplophase comes to an end in the life cylce of Ustilago.
Alternation of Generations in Ustilago:
The life cycle of Ustilago illustrates the important biological phenomenon of alternation of generations. There are two distinct phases in the life cycle.
The sexual phase or the gametophyte phase is represented by the haploid four-celled epibasidium, basidiospores, germ tubes of basidiospores and the haplo or primary mycelium in some species (U. maydis).
It ends with plasmogamy which initiates the dikaryophase in the life cycle. The dikaryophase in smuts consists of the dikaryotic or secondary mycelium and the binucleate smut spores.
With karyogamy which consists in the fusion of the two nuclei in the smut spore ends the dikaryotic phase. The smut spore with the synkaryon (probasidium) represents the transitory diplophase.
The dikaryotic phase together with the transitory diplophase consitutes the sporophyte phase. The sporophyte phase ends with meiosis. With meiosis starts the future gametophyte.
These two phases alternate with one another in the life cycle of Ustilago. One regularly follows the other. Hence Ustilago is said to exhibit alternation of generations in its life cycle.