PUCCINIA - CLASSIFICATION, VEGETATIVE STRUCTURE, REPRODUCTION, ECONOMIC IMPORTANCE

            A. CLASSIFICATION

Division – Mycota

     Sub-division – Eumycotina

           Class – Basidiomycetes

                 Sub-class – Heterobasidiomycetidae

                      Order – Uredinales

                            Family – Pucciniaceae

                                 Genus – Puccinia

            The genus Puccinia is cosmopolitan in distribution and is a parasitic fungus causing diseases on many crop plants such as wheat, barley, oats, etc., and causes great loses to the cultivated crops. Puccinia is an obligate fungal parasite and are commonly known as ‘rust fungi’ because they produce ‘rust-like’, i.e., reddish-brown coloured spores in the form of rusty spots or streaks on stems, leaf-sheaths, leaves, etc., of the host plants.

            The genus includes about 3000 species. Out of these, 147 species have been reported from India. Wheat rust disease has been extensively studied in India and is found to be caused by three different species of Puccinia. These are – Puccinia graminis, P. striformis (yellow rust of wheat) and P. recondita (Orange or brown rust of wheat). P. graminis tritici which is known as the ‘black stem’ rust of wheat has been thoroughly investigated. It is usually taken as a type species because its life cycle is fully known.

            B. STRUCTURE OF THE VEGETATIVE BODY:- The mycelium is dikaryotic in nature. The mycelium is filamentous, well developed and branched. The hyphae are septate and ramify in the intercellular spaces of the tissues of the stem and leaves of the host.

            It is reported that, the tip of a certain hyphae of the intercellular dikaryotic mycelium come in contact with a cell wall of the host cell and become separated from the rest of the mycelium by a septa. The resultant small, binucleate tip cell swell somewhat and functions as haustorial mother cell. At the point of contact, each haustorial mother cell develops a buldge-like thickening which is closely appressed to the host cell wall. A fine pore appears at the point of contact extending through the cell walls of both the pathogen and the host. A delicate infection peg from the haustorial mother cell passes into the host cell through the pore. After penetration, the infection peg elongates to form the haustorial neck. A wall is formed around it by a fungal pathogen. It then serves as an organ of absorption.

            Each cell of the dikaryotic mycelium contains a pair of nuclei (n+n). The nuclear memnrane is double layered and perforated. The cytoplasm also contains free ribosomes, mitochondria, glycogen particles, lipid bodies and other unidentified particles.

            C. REPRODUCTION:- Puccinia (P. graminis tritici) reproduces by production of spores. Different types of spore formation can be observed in the life cycle. These are uredospores, teleutospores, basidiospores, pycniospores and aeciospores. The first three spores formation take place in primary host plant i.e., wheat plant while the formation of rest of the spores take place in secondary host i.e., barberry plant.

            1. Attack on Primary Host i.e., Wheat Plant:- Dikaryotic mycelium is developed on wheat plants by the germination of aeciospores that comes from the secondary host (barberry plant) or by the germination of uredospores that are disseminated from the primary host (wheat plant) itself and transferred to other host of the same species.

            Three types of spores are formed during the time of infection in a wheat plant. These are – uredospores, teleutospores and basidiospores.

            Uredospores – During late spring, the fungal mycelium develop vertically erect, elongated, reddish-brown pustules or sori just beneath the host epidermis. These are known as uredosori (singular: uredosorus). Each uredosorus produce a single spore at its tip, known as uredospore.

            Each uredospore is unicellular, binucleate (n+n), oval or globose in structure. It consists of a fairly thick wall composed of outer thick spiny exospore and an inner thin smooth endospore. When the spores become fully matured, these are freely exposed due to the rupturing of the host epidermis. The spores then become detached from the sori and carried to other host i.e., new wheat plants by wind, where they germinate within a few hours by producing a germ tube.

            The uredospores are reddish or orange-red coloured which gives characteristic red-coloured rust on the affected part of the host plant.

          Teleutospores – Towards the middle of the summer season, the same mycelium which has been producing uredospores or mycelia originating from germination of uredospores, begin to develop black pustules or sori called teleutosori (singular: teleutosorus). Each teleutosorus produce a single spore at its tip, known as teleutospore.

            Each teleutospore is binucleate, two-celled, spindle shaped and thick walled structure. Each cell of the teleutospore contains two nuclei i.e., dikaryotic (n+n) when young which fuse together at maturity to form a single diploid nucleus (2n).

            The teleutospores are more destructive than the uredospores. They are dark-brown to black in colour which gives characteristic black coloured rust on the affected part of the host plant. The teleutospores are dormant spores and are unable to germinate immediately. They remain dormant on the dead host tissue or on soil till the next growing season.

           Basidiospores – Under favourable conditions of temperature and moisture, each cell of the teleutospore germinates and produces an erect tubular outgrowth from each cell. Such tubular outgrowth is known as epibasidium or promycelium. The diploid nucleus of each cell of the germinating teleutospore moves to the epibasidium and undergoes meiotic division therein. As a result 4 haploid nuclei are formed. This is followed by the formation of transverse septa, separating the 4 nuclei from one another into four un-inucleate basidium proper. A short lateral outgrowth known as sterigmata is developed from each basidium. At the tip of each sterigmata, a single, uni-nucleate, haploid basidiospore is developed. Out of the four basidiospores two are of one mating type and the other two of different mating type.

            Basidiospores are unable to affect wheat plants. But they began to germinate when it comes in contact with the secondary host i.e., the barberry plant.

            2. Attack on Secondary Host i.e., Barberry Plant:- In this stage, the haploid phase of life cycle of P. graminis tritici is observed on the secondary host barberry plant (Berberis vulgaris). In this stage of life cycle the fungus produces two types of spores – pycniospores and aeciospores.

            Pycniospores – The germination of basidiospores produce monokaryotic mycelium which accumulates beneath the upper epidermis of the barberry leaf and develops into a flask shaped pycnium (spermagonium). Numerous vertical hypha known as pycniosporophore develops at the base of the pycnium. At the tip of each pycniosporophore, haploid pycniospores (spermatia) are produced in chain.

            The pycnia open outside the epidermis through a small opening known as ostiole. Some of the sterile hypha called paraphyses are found to be situated in the neck region of the pycnium. Some vertical fertile hyphae, called the flexuous hyphae also come out through the ostiole. They are of the same strain as the mycelium in which they are developed (either + or – strain).

            On maturity of the pycnium, the nectar oozes out through the ostiole and hundreds of pycniospores (spermatia) are released out along with the nectar. The pycniospores are transferred by insects or other means to flexous hyphae of opposite strain. The pycniospore and flexous hypha fuses, leading to the dikaryotization, followed by formation of aecial primordial on the underside of the leaf.

           Aeciospores – As a result of fusion of fusion of pycniospore and flexous hypha, an aecial primordial is formed which develops into a cup-like aecium on the underside of the barberry leaf, rupturing the epidermis. An aecium is composed of a sterile membranous wall known as peridium. The base of the aecium comprises of numerous sterile hyphae from the tip of which arises a chain of spores known as aeciospores.

            At maturity the peridium ruptures and aeciospores are released from the aecium. The liberated aeciospores are disseminated by wind to reach the primary host i.e., the wheat plant. Aeciospores germinate as soon as they are liberated, but their germination cannot affect the barberry plant. They can infect only the wheat plant.

            With the germination of aeciospores a dikaryotic mycelium is developed on the wheat plant, which is responsible for the production of uredia and telia again.

                 D. ECONOMIC IMPORTANCE:- Puccinia is one of the parasitic fungal genus which is of much economic concern, because of its ability to produce rust disease on many cereal crops. Species of Puccinia such as Puccinia graminis, P. recondite, P. striformis, etc., are some of the well known rust fungi which causes different types of rust diseases on wheat plant such as black rust, brown rust and yellow rust respectively. Due to such diseases wheat growing countries including India looses hundreds of tones of wheat production and yield. The fungi also attack other cultivated cereal crops like barley, oats, etc.

Prema Iswary,               

Assistant Professor,     

Department of Botany.

*************