Puccinia

                                                                Puccinia   

Diagrams are same as drawn in the Record

         

                                                                                                    Division : Eumycota

                                                                                                            Sub Division : Basidiomycotina

                                                                                                            Class : Teliomycetes

                                                                                                            Order : Uredinales

                                                                                                            Family : Pucciniaceae

                                                                                                            Genus : Puccinia

The species of Puccinia are obligate parasites on higher plants. They are popularly known as Rust fungi, because of characteristic reddish, brown colour of their spores. The genus includes about 700 species and of them about 147 species are recorded in India.

They are of great economic importance as they cause destructive rust diseases of major corps such as wheat, barley, sorghum, maize, bajra, groundnut, sunflower etc.

The rusts have a pleomorphic life cycle, having more than one independent form or spore – stage in the life cycle. Many rust have a complex life – cycle producing five different types of spores, produced at 5 reproductive stages. The spores are –

            Stage 0       : Spermogonia  ( Pycnidia) bearing spermatia ( Pycniospores) and receptive hypae

            Stage I        : Aecia bearing aeciospores

            Stage II      : Uredinia bearing Uredospores

            Stage III    : Telia bearing teleutospores

            Stage IV    : Promycelia bearing basidiospores

The rust fungi which produce all the five types of spores in their life cylcle are called as macrocyclic rusts. If one or two spore stages are missing in the life cycle, it is described as microcyclic rusts.

The species which complete their life cycle producing all the spores types on a single host  are called as autoecious speices, ex – P. Asparagi, P. helianthi. The species which complete their life cycle producing all the spores types on two different unrelated host plants, are called heteroecious, Ex – P. graminis

In the case of heteroecious rust, the host which bears uredial and telial stages (sexual or perfect stage) is called the primary host. The host which bears pycnidial and aecial stage is called the secondary host or alternate host.  Some autoecious rust produce the same type of spores on several unrelated host plants. These unrelated host plants are called collateral hosts.

Life Cycle of Puccinia graminis tritici

P. graminis tritici is the casual organism of the black rust disease of wheat. The fungus is polymorphic in nature, as it produces five types of spores during its life cycle. It is therefore also called macrocyclic rust. The life cycle is completed on two different host plants, hence called heteroecious rust.

The wheat plant on which the parasite passes its dikaryotic phase is called the primary host and the Barberry (Berberis vulgaris) is the secondary or alternate host. The fungus can survive in the absence of the alternate host, but it can produce new races by hybridization only on barberry.

Out of the five spore types in the life cycle, urediniospores and teliospores are produced on the wheat plant, whereas pycniospores and aeciospores are produced on the alternate host.

Life Cycle on Wheat

When aeciospores released from the infected barberry plants falls on the wheat leaves, they germinate under favourable conditions and germtubes enter the leaves through stomata. the mycelium is intercellular, septate and branched. The septal  pore is simple.

The mycelium is dikaryotic, each cell contains a pair of nuclei (n + n) constituting a dikaryon. To obtain the nutrition the intercellular hyphae develop intracellular food absorbing haustoria.

Reproduction

Within about 7 -21 days of infection, the dikaryotic mycelium reproduces by sporulation. The spores produced are of two kinds – the uredospores and teleutospores.

Urdineal Stage:

The dikaryotic mycelium begin to aggregate below the epidermis of the infected organ such as leaf, leaf sheath, stem. From the mycelium, many vertical hyphae called sporophores arise towards the epidermis. They are arranged in a close palisade –like layer. The tip of each hyphal stalk swells to form a single binucleate uredopsore or uredeniospore. The uredospores are thus formed in groups. Each group is called a uredosorus or uredinium.

Structure of Uredospore:

Each uredospore is a unicellular, dikaryotic, oval and brown structure. It has a thick, echinulate or spiny wall. The wall consists of two layers – outer thick spiny exospores having four germ pores, and an inner delicate, hyaline endospore. The uredospores contain cytoplasm, oil globules and brown pigment. In masses the uredospores appear rusty red colour.

The huge number of uredospores exert pressure on the epidermis which is, at first, lifted but finally ruptured in form of slits or blisters. They appear in form of reddish – brown patches on the stems and leaves and called as pustules. This stage is called the ‘Red Rust of Wheat’. As a result of this, the entire field crop appears to be burnt by fire (Gr. Urere = to burn).

The uredospores get detached from their stalks and are disseminated by wind.

Germination of Uredospores

On falling on another wheat plant and in the presence of moisture and optimum temperature ( 20 -25⁰ C)  the uredospore germinates within a few hours. The endosporium comes out in form of a slender tube through germ pore. More than one germ tubes may be produced by the same uredospore.

The germ tube by elongation grows over the surface of the host leaf till it reaches a stomata where its tip swells to form an appresorium. The two nuclei and the protoplasm of the germ tube migrate to the appresorium.

The peg – like outgrowth, the infection peg arises from the appresorium. It enters the stomatal aperature. Reaching the substomatal cavity the tip of the infection peg swells into a vesicle. The contents of appresorium pass through the infection peg into the vesicle. One or more hyphae develop from the vesicles, grow  into dikaryotic mycelim and ramify in the intercellular spaces.

This mycelium is again capable of producing urediniospores. These spores are thus ‘’ repeating spores” and they multiply and propagate the disease in the field as long as the weather is favourable. This repeated cycle recurs several times in a single growing season.

Telial Stage

Late in the growing season another kind of spore develops form the same dikaryotic mycelium. This spores are termed as teliospores or teleutospores. The pustules are called telia or teleutosours.

The teleutospores are developed among the uredospores in the same uredosorus. Gradually , as the season progresses more and more teleutospores are produced , whereas the number of uredospores is reduced. Finally, the sori conatin only the teleutospores. These sori are called the teleutosori. 

The cells of the mycelium producing the teleutospores are called the telia. The teleutospores are dark brown or black colour. The teleutospores exert pressure on the epidermis, which is ruptured and the telia appear as black raised streaks along the leaf sheaths and stems. Hence, this stage is called the black stem rust.

Structure of teliospore

The teleutospores are black or dark brown, stalked, two-celled, spindle shaped structures. The spore wall is thick and smooth. There is a single germ pore in the wall of each cell. It is at the apex in the upper cell and below the septum in the lower cell.

This stage in the life cycle, in which teliospores ( Gr. Telos = end) are produced is called the telial stage, because these are formed towards the end of the growing season.

The telial stage is considered the perfect stage of the Uredinales because it is in the teleutospores that karyogamy and meiosis takes place.

As the spores mature, the two nuclei in each cell of the teleutospore fuse to form a diploid nucleus. After the harvesting period mature teleutospores remain dormant on straw, stubble and survive even the severest winters.

Basidial stage

After the resting period the teleutospores germinates on return of favourable conditions – high humidity, presence of moisture and freezing condition prior to germination.

The teliospores germinate to give rise to the basidial stage in the life cycle of Puccinia.

Each cell of the teleutospore containing a diploid nucleus represents the probasidium or hypobasidium. A short, slender hypha of limited growth grows out through the germ pore from each cell, this is called the promycelium or epibasidium.

The diploid nucleus of the teleutospore migrates into the promycelium. There the diploid nucleus undergoes meiosis form four haploid nuclei. Septa appear between the nuclei dividing the epibasidium into four uninucleate haploid cells.

From each  of the three lower epibasidial cells develops a short narrow tube, the sterigma. From the terminal cell of the epibasidium the sterigma arises from the apex. The free tip of each sterigma swells to form a basidiospore.

Two out of the four basidiospores on each epibasidium are of plus strain and the other two of minus strain.

Each basidiospore is a small, unicellular, uninucleate haploid structure.

The basidiospores are forcibly discharged into air. They are carried by wind to the leaves of alternate host barberry which they infect. The basidiospores remain viable only for a few days. They cannot infect the wheat plant and thus perish soon if the alternate host is not available.

Life Cycle of Barberry Plant

The basidiospore germinates on the leaf of the alternate host – barberry provided the moisture and temperature conditions are suitable. Each basidiospore gives out a germtube, which enters the host tissue directly through the epidermis.

Once within the host tissue it grows vigorously and branches freely to form the primary mycelium, also called monokaryotic or haplomycelium. The primary mycelium is septate, uninucleate. The mycelial hyphae ramify in the intercellular spaces between the mesophyll cells of the leaf.

They produce haustoria which penetrate the cells of the host tissue and obtain nutrition for the growth of the mycelium. The basidiospores are either of plus or minus strain. Several basidiospores of different strain infect the same barberry leaf. Naturally mycelia will be to two different strains, and develop side by side.

Spermagonial or Pycnidial Stage

After 3-4 days of infection, the hyphae begin to collect beneath the upper epidermis. They form a dense mat. Fromt he mycelial mat arise groups of hyphae which develop into small, flask-shaped structures called the spermagonia or pycnidia. The pycnidia like the mycelia form which they arise, are of plus or minus strain.

When pycnidia mature, the infected areas become swollen and are seen as small, orange yellow bumps on the upper surface of infected leaf.

Each spermogonium consists of a wall surrounding a cavity. It opens to the upper surface of the host leaf through small pore called the ostiole. From the wall of the spermagonium arise three kinds of hyphae:

i.                 Spermatiophores or pycnidiophores:

Numerous, elongated, uninucleate, hyphae arise from the cells of the wall. They project into the cavity of the spermagonium and are called the spermaiophore are pycniosphores. They are closely packed and arranged in a palisade – like layer.

Each spermatiophore by successive divisions forms  number of small cells one after the other. These are spermatia or pycnidia. Each spermatium has a single nucleus and very little cytoplasm. The mature spermatia fall into the spermagonial cavity. They are non-motile and cannot infect either host.

ii.               Periphyses:

They are long, delicate sterile hyphae which develop near the ostiole form the spermagonial wall. They are unbranched, tapering, orange-coloured, and called the  periphyses. They project through and beyond the ostiole end.

iii.             Receptive hyphae

Adjacent to the ostiole and among the periphyses, develop another kind of hyphae. They are slender, delicate, cylindrical with blunt free tips. Being flexous these are named the receptive or flexuous hyphae.

Spermatisation:

The mature spermatia exude from the ostiole of spermagonium in a drop of sticky, thich liquid called nectar. The nectar with its scent and sugary content attracts the insects to the leaf. The spermatia stick to the leags of the visiting insects are thus are dispersed from leaf to leaf or from one spermagonium to another.

When a compatible spermatium or pycniospores is deposited on the receptive hyphae, the intervening wlls at the point of contact dissolve. The spermatium nucleus passes through the opening into the receptive hypha.

The spermatial nucleus reaches the basal cell of the receptive hyphae, which thus comes to possess two nuclei which lie side by side in a pair. This coming together of opposite strain ( - and + ) is called the spermatization.

One of these nuclei is of minus strain and the other of plus strain. This pair of nuclei of opposite strains is called a dikaryon.

The spermatial nuclei from the basal cells of the receptive hyphae by repeated mitotic divisions forms several nuclei. The daughter nuclei so produced are passed on to the rest of the cells of the mycelium through the septical pores of the hyphae. The result is that theentire mycelium becomes dikaryotic.

Aecidial or Aecial Stage

Along with the formation of spermagonium, the haploid primary mycelium also produces a globose mass of hyphae, within the lower epidermis. This mass is called aecial primordium.

Each aecial primordium consists of a closely packed theft of hyphae, called basal cells; and a group of larger parenchyma cells. As a result of spermatization, the basal cells of the aecial primordium become dikaryotic. Dikaryotization leads to formation of aecia and aeciospores.

The dikaryotized basal cells are called the aecidiospore mother cells. The aecidiospore mother cell increase in length. The two nuclei in it divide conjugately. A small daughter cell is then cut off at its terminal end and it is called the aeciospores initial. Two of the nuclei remain in the aecidiospore mother cell and other two pass into the aecidiospore initial. The initial cell increases in size and divides into two, upper bigger binucleate aecidiospore cell and lower smaller binucleate, sterile disjunctor or intercalary cell.

Each aecidiospore mother cell undergoes a series of such divisions and closely packed chains of cells are formed at the tips of the aecidiospore mother cells. The mass of aeciospores is surrounded by a membrane called peridium. All these structures form a cup – like aecium or aecial cup.

The yound aecium is closed and buried deep in the leaf tissue. When the aecium matures, the spore chain pushes through the roof of the peridium and the peridium hangs down the lower epidermis. The aecial cups are visible externally as circular spots of reddish purple colour on the ventral surface of the leaf.

Each aeciospores is a polyhedral, binucleate, unicellular, thin-walled, orange coloured structures. The spores absorb water, round off suddenly and thus are jerked out of the aecidium. They are disseminated by the wind. The aeciospores are unable to germinate on the host barberry plant on which they are produced. They infect wheat plant. Under suitable conditions, the aeciospores germinates and produce dikaryotic mycelium in wheat to repeat the life cycle.