PENICILLIUM - CLASSIFICATION, VEGETATIVE STRUCTURE, REPRODUCTION, ECONOMIC IMPORTANCE

            A. CLASSIFICATION:

Division – Mycota

  Sub-division – Eumycotina

     Class – Ascomycetes

        Sub-class – Euascomycetidae

           Series – Plectomycetes

              Order – Aspergillales

                  Family – Aspergilaceae

                     Genus – Penicillium

            Penicillium is commonly known as the green or blue moulds. They are cosmopolitan in distribution. Usually, the species of Penicillium grows as a saprophyte on decaying fruits (oranges, lemons, etc.) and vegetables. It also grows on the preserved fruits and jellies. Some species have been reported to grow on forest floors and a few of them grow on cultivated and manured ground. Wild species of Penicillium have been reported to grow upon damp wood. Some species cause fermentation in cheese. The species of Penicillium are obligate saprophytes, and can be cultured in laboratory. The mycelium exhibits blue, sky-blue or blue-green colour on the substratum.

             B. STRUCTURE OF THE VEGETATIVE BODY:- The vegetative body is a mycelium. Mycelium is well developed and are freely branched, and composed of colourless, slender, tubular branched and septate hyphae. The hyphae run in all direction on the substratum and are intertwined one another to form a loose network of hyphae. Some of the hyphae ramify superficially on the substratum. They produce aerial hyphae upon which fruiting bodies called conidiophores are developed.  Some hyphae penetrate the substratum and produce rhizoids which absorb nourishment. In few species, the mycelium often forms compact structure called sclerotium.

               C. REPRODUCTION:- Penicillium reproduces by vegetative, asexual and sexual methods of reproduction.

            1. Vegetative Reproduction:- Vegetative reproduction takes place by the most common method of fragmentation. In this case the hyphae break up into small segments or fragments. Each fragment grows by repeated division into a full-fledged mycelium. In some species, the mycelium forms compact resting bodies, called sclerotia. The sclerotia can survive during unfavourable conditions. On the onset of favourable conditions of growth, each sclerotium germinates into a new mycelium.

            2. Asexual Reproduction:- Asexual reproduction is accomplished by the most common and dominant method of reproduction called sporulation. There are two types of sporulation in Penicillium –

            Conidia formation – In this case, the ordinary hyphae produce a special type of aerial hyphae known as conidiophores.

            Each conidiophore is simple, erect, branched (in some species, unbranched), broom like structure. The branched conidiophore bears successive whorls of branches terminating in clusters of uninucleate, bottle shaped sterigmata.

            From the apex of each sterigmata conidia are borne in chain. Conidia are globose, ovoid, elliptical or pyriform, thick walled, smooth or rough, uninucleate and mostly green in colour. At maturity, conidia become detached from the chain and are carried by wind to different substrata. Under favourable condition, each conidium germinates by producing a germ tube, thus producing a new mycelium.

            Oidia formation – The mycelia, when made to grow immersed in a sugary solution, divide by additional septa into short uninucleate segments. These segments later become rounded and separates as thin-walled spore-like structures called the oidia or oidiospores. On a solid medium, each oidium germinates to produce a new mycelium.

            3. Sexual Reproduction:- Most species of Penicillium are homothallic. Few are heterothallic. In both homothallic and heterothallic species, sexual reproduction takes place by the production of gametangia. The female gametangia is known as ascogonia while the male gametangia are known as antheridia. In homothallic both antheridia and ascogonia are produced from the same thallus or from different thalli of the same mycelium.

            Ascogonium – Each ascogonium is a long, erect, multi-nucleate, unseptate, tubular structure arising laterally from any cell of the vegetative mycelium. At its upper end it may be curved like the handle of an umbrella. When young, the ascogonium is uni-nucleate. As it elongates, the single nucleus divides and redivides to give rise to definite number of daughter nuclei which is either 32 or 64.

            Antheridium – Each antheridium is a short, slender terminal club-shaped, uninucleate structure. It originates either from an adjacent cell or the same hypha which gives rise to the ascogonium or from a separate neighbouring hypha. It grows up coiling loosely around the ascogonium making several turns about it. The distal end of the male branch become tightly inflated and is eventually cut off as an anteridium by a septum.

            Plasmogamy – It is the union of two protoplasts which brings the compatible nuclei close together in the same cell.

            The tip of the antheridium comes in contact with the ascogonium. At the point of contact, the double wall dissolves and a pore is developed between the two cells. Through the common pore, the protoplast comes to the ascogonium. Next the pairing of the female nuclei takes place, the term which is called as autogamy. Each pair is called dikaryon. With the establishment of the dikaryons the haplophase ends and the diplophase starts in the life cycle.

            Formation of Ascogonenous hypha, Asci and Karyogamy – Plasmogamy or autogamy is followed by septation of the ascogonium. Each segment has a pair of nuclei (dikaryon). Meanwhile, sterile hyphae grow up around the sexual apparatus and form loosely interwoven hyphae called peridium, which afford protection to the structures developing within.

            As a result of plasmogamy or autogamy, one or more lateral outgrowths arise from some binucleate segments of the ascogonium. Each outgrowth is known as ascogenous initial, each of which develops into a branched ascogenous hypa composed of bi-nucleate cells. From the bi-nucleate cells of each ascogenous hypha, asci (singular: ascus) are developed terminally in short chains. The two nuclei eventually fuse inside the ascus. This is called karyogamy.

            Formation of Ascospores – Soon after karyogamy, the young ascus begins to enlarge. Its diploid nucleus undergoes three successive divisions. The first and second division is meiotic and the third is mitotic, thus resulting in 8 haploid nuclei. A small amount of cytoplasm gathers around each daughter nucleus which metamorphosed into ascospores.

            Discharge and Germination of Ascospores – At maturity, the wall of the ascus dissolves and ascospores are released by the decay of the other wall of the peridium.

            The liberated ascospores remain dormant when temperature and moisture conditions are low and air supply is restricted. On falling on a suitable substratum and under suitable condition each ascospore germinate to form new mycelium.

            D. ECONOMIC IMPORTANCE:- Penicillia are of tremendous economic importance –

           1. The best known antibiotic drug ‘Penicillin’ is obtained from P. notatum and P. chrysogenum. P. griseo-fulvum produces an antibiotic termed ‘griseofulvin’ an antifungal.

            2. Some species are used in the industrial processes such as in the production of organic acids like citric acid, gluconic acid, fumaric acid and glycerol.

            3. P. roquefortii and P. camembertii are used in the fermentation of cheese, which gives not only colour and texture but also impart distinctive odour and flavor.

            4. Penicillia also cause economic loss by damaging stored fruits. P. digitatum and P. expansum attack fruits and cause rot and decay of lemon, orange and other stored vegetables.

            5. Te fungi are also responsible for the damage of leather goods, fabrics and furniture. Under damp conditions, the wood of doors, windows and household furniture get spoiled by the attack of such fungi.

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