In this article we will discuss about the life cycle pattern of uredinales, explained with the help of suitable diagrams.
In the usual course of the life cycle, as a result of infection of the host by sporidia, monokaryotic mycelia are produced which give rise to spermogonia (sing, spermogonium). The spermogonia are also known as pycnia (sing, pycnium). The spermogonia are subepidermal or subcuticular structures which may be acervular and pycnidial in form (Fig. 262).
The pycnidial type is rather common. The whole structure in gross appearance is much like the pycnidium being globose to flask- shaped with ah ostiole and is composed of pseudoparenchymatous tissue. The spermogonia are reddish or orange in colour.
The inner wall of the spermogonium is lined by elongated uninucleate hyphal structures called spermatiophores from the apices of which are abstricted in series single-celled uninucleate spermatia also known as pycniospores (Fig. 262A & D). Slender thread-like periphyses protrude from the ostiole of the spermogonium.
Just below or among the periphyses are developed slender and branched hyphae known as receptive hyphae (Fig. 269A) or flexuous hyphae.
The receptive hyphae, along with the periphyses grow out through the ostiole and extend beyond the host surface. The spermatia developed in the spermogonia ooze out through the ostiole along with a sticky sugary exudate which sometimes gives off a fragrant odour.
Insects attracted by the sticky exudate mixed with spermatia visit the spermogonia and serve as agencies through which the spermatia are transferred from one spermogonium to the other. The function of spermatia was questionable for quite some time.
Graigie (1927) observed that spermatia are carried from one spermogonium to the other possibly by insects which suck the ‘nector drops’ (exuded by spermogonia), in which the spermatia are held.
Again along with the development of a spermogonium in its vicinity, an aecial primordium or a protoaecium (Fig. 269A) which has hyphal connection with, it, develops. But further growth of the aecial primordium into aecium bearing aeciospores never takes place unless dikaryotic condition is established by spermatization between a sermatium and a receptive hypha.
Craigie also demonstrated that the four sporidia that are developed from a promycelium during the germination of a teleutospore belong to two groups: two of them possess (+) nuclei and the other two (—) nuclei, one in each.
The sporidium bearing (+) nucleus produces a (+) spermogonium and the one with (—) nucleus gives rise to- a (—) spermogonium and dikaryotic condition is established only between a (+) spermatium and a (—) receptive hypha and vice versa.
It has been demonstrated by Brown (1935) that the dikaryotic condition may be established between hyphae of (+) and (—) spermogonia. Immediately after the establishment of dikaryotic condition, it is perpetuated into the aecial primordium through the hyphal connection that is already present between the spermogonium and the aecial primordium.
The aecial primordium then develops into an aecium (pi. aecia) (Fig. 269).
The aecia are usually deeply sunken in the host tissue. They are cup-shaped structures and are commonly known as cluster cups. The whole structure is usually red or yellow. The outer layer of the cup is composed of sterile tissue and is termed the peridium (Fig. 270A).
At the base of the cup a hyphal plexus is developed from which vertically elongated cells arise. From these cells aeciospores (Fig. 270A) are produced in chains. In long chain the spores are separated from each other by intercalary sterile cells known as disjunctor cells (Fig. 270B). The whole interior of the young aecium is filled up by numerous parallel chains of spores (Fig. 270A).
The peridium at first completely encloses the aecium. But as the aeciospores within the aecium increase in number, the cup is thrown open by bursting an area of the peridium.
Thus a cup with a toothed margin with its edges torn-apart is developed. Peridium may also be lacking in an aecium. The aecia of the Melampsoraceae lack peridia so that they are diffuse instead of cup-shaped. Such aecia are termmed caeomata (sing, caeoma). Rusts which produce uredinoid aecia are brachycyclic, e.g., Puccinia puncliformis is a brachy-Puccinia.
The aeciospores are unicellular usually globular, or angular by compression. They are light orange, yellow, smooth-walled with thin places, and are dikaryotic (Fig. 269B).
The aeciospores become separated by breaking down of the disjunctor cells between them. ‘They are capable of germination at once and on germination give rise to germ tubes which emerge through the thin places in the spore wall, and may infect susceptible hosts, leading to the development of mycelium.
The aeciospores may infect the same kind of host on which the aeciospores were produced (autoecious), or entirely unrelated hosts (heteroecious).
The dikaryotic mycelium produced on germination of aeciospore develops within the host producing localized infection. Ultimately a subepidermal hyphal plexus is formed known as uredium (pl. uredia) or uredosorus (p1. uredosori) (Figs. 263B & 266), from which arises a bed of spores, the uredospores (Fig. 263A).
The uredospores are yellow to orange-red, stalked, unicellular dikaryotic oval or globose with echinulate or verrucose wall bearing 2 to 15 variously placed germ pores (Fig. 263A). They may be produced in chains with intercalary cells which separate them from each other (Fig. 263B). The uredospores with maturity are exposed through ruptured epidermis of the host surface.
These spores falling on susceptible hosts may germinate at once by producing a germ tube (Fig. 263G) which develops into mycelium. The infection is usually stomatal and production of uredia may be continued for some time. The uredospores are usually short-lived and function to spread summer infection. They are also sometimes known as summer spores or repeating spores.
In a few species there are also resting forms of thick-walled uredospores which are also known as amphispores. These spores are more persistent than those of the usual uredospores.
Toward the latter part of the growing season another kind of spore appears, often in the same sorus with the uredospore and from the same mycelium producing what is called mixed sorus. This kind of spore is known as teleutospore or reliospore.
Ultimately the dikaryotic mycelium produces only the teleutosori (sing, teleutosorus) or telia (sing, telium) (Fig. 267) bearing teleutospores. The telia are generally darker than uredia, and when mature may be exposed like the uredia or may still be covered by the host epidermis. They may also form crusts or cushions or columnar masses.
The teleutospores are of various forms in different genera, one or more-celled, and are extremely variable in shape, thickness of wall, surface marking and colour (Fig. 264).
They may be sessile (Fig. 264 J) or stalked (Fig. 264 A to I), may be completely free from one another (Fig. 264 A to I), embedded in a gelatinous matrix, united laterally forming small groups, layers (Fig. 264J), or columns (Fig. 265B). The teleutospores are almost colourless to a dark reddish-brown.
They vary in size and in external characteristics of their walls which may be smooth, spiny, or variously sculptured (Fig. 264). The teleutospores when combined together to form a telial column are not readily detachable (Fig. 265B). But they are dikaryotic and are uniform in the character of the germination which is very different from any other rust spores.
The teleutospores are resting spores and most rust fungi perennate in the telial stage.
All rust fungi except those included under the Uredinales Imperfecti produce teleutospores. The teleutospores are usually considered as the perfect stage of the rust fungi, since karyogamy and meiosis take place during their germination. The morphology and behaviour of the teleutospores furnish the principal characteristics for the differentiation of families of the order Uredinales.
In teleutospore germination of most rust fungi, each cell of the teleutospore produces a hypha-like structure of limited growth known as promycelium (Fig 265A). The promycelium may be borne externally, external promycelium or internally, internal promycelium from a teleutospore.
In certain rust fungi the teleutospore on germination becomes septate without the formation of an external promycelium. Karyogamy takes place during teleutospore germination and the diploid nucleus migrates in the promycelium.
Where four haploid nuclei of two groups (+) and (—) are produced as a result of meiosis. Septa are laid down in the promycehum separating the four haploid nuclei into four cells. Each cell then sends out a sterigma on which one-celled thin-walled oval to rounded spore is developed. The four spores of two groups (+) and (—) so formed are known as sporidia (sing, sporidium) or basidiospores.
The sporidia at maturity are dispersed by wind. Morphologically the promycelium is a basidium bearing its four sterigmata and basidiospores. The sporidia germinate immediately by germ tubes which on suitable hosts give rise to spermogonia of two groups (+) and (—) completing the life cycle.