Rhizopus Systematic position: Division: Eumycota Sub-Division: Zygomycotina Class: Zygomycetes Order: Mucorales Family: Mucoraceae Genus: Rhizopus Occurrence: Rhizopus occurs worldwide in soil, on decaying fruits, dung and vegetation as saprophytes. Rhizopus occurs very frequently on moist bread and is therefore commonly called “bread mould”. It is so frequent contaminant of laboratory cultures of bacteria and fungi that is considered a weed of laboratory. Some Rhizopus species behaves parasitically, causing rot of sweet potato or fruit rot of apple, strawberry and tomato. Some Rhizopus species cause “mucormycosis” in domestic animals and few are reported from human lesions. Laboratory culture: Rhizopus species are saprophytes, the fungus can be grown on dead organic material such as bread and butter by keeping them in dark and damp atmosphere. By first exposing a moistened piece of bread in a Petri-dish for about 24 hours at room temperature and then covering it for a few days, Rhizopus appears in the form of white tuft of mycelium. Somatic Structure: Young mycelim of Rhizopus consists of many wellbranched, white, tubular or filamentous hyphae, which are multinucleate and without cross walls, i.e., “coenocytic”. Hyphae are alike and provide a cottony appearance to mycelium. But later on the mycelium soon enters the reproductive phase and becomes differentiated into three different types of hyphae, i. e., a) Rhizoids, b) Stolons and c) Sporangiophores (Fig). Rhizoids are the repeatedly branched hyphae that penetrate the substratum. From each node of the stolon they arise in the form of a cluster towards lower side. They absorb water and nourishment from the substratum. Rhizoids are sometimes called holdfast. Stolons are the hyphae that grow horizontally above the substratum for some distance and then bend down into the substratum. The part bending down functions as a node and forms a tuft of rhizoids. The hyphae of the stolon are therefore aerial and unbranched. Sporangiophores are the erect, aerial, unbranched and negatively geotropic hyphae, which grown upward in tufts at the point where the stolons from rhizoids. They are reproductive in function. Each sporangiophore bears a terminal sporangium. Internal structure of hypha: The hyphae are tubular structures and the hyphal wall is composed of fungal chitin. The ultrastructure of the hyphal wall shows that it is microfibrillar and microfibrills run parallel to the surface. It is lined internally by thin plasma membrane. The protoplast is granular and encloses many nuclei, glycogen, oil droplets, ribosomes, endoplasmic reticulum, mitochondria and many small vacuoles. The protoplasm is very dense in the hyphal tips. Nutrition: Certain enzymes are secreted by the rhizoidal hyphae. These enzymes convert starch into soluble carbohydrates, which are readily absorbed by the fungus. Utilization of some inorganic and organic nitrogenous compounds also helps Rhizopus in synthesizing some proteins. Reproduction: Reproduction takes place by vegetative, asexual and sexual reproduction methods. a) Asexual reproduction: It takes place by sporanigospores and rarely by chlamydospores. the formation of Sporangiophores are unicellular, multilayered, non-motile spores, produced in round black bodies called sporangia. Because of their non-motile nature they are also called aplanospores. A sporangium develops singly and terminally on the sporangiophores. The sporangiophores are erect positively geotrophic structures, which develop in tufts from the mycelium. At a stage when a number of such pin-head-like black-colored sporangia are present (fig), the entire mycelium appears blackish and hence popular name blackmold is also given to the fungus. Development of sporangium starts by the swelling of the tip of the sporangiophore into a knob-like vesicle. The cytoplasm along with many nuclei from the sporangiophore flows into the swollen vesicle. This increases the size of the vesicle. The swollen portion represents the young sporangium. The protoplasmic contents of the young sporangium soon become differentiated into two zones, i.e., outer peripheral multinucleate dense region and the central less dense region with comparatively fewer nuclei. Two portions get separated by a layer of vacuoles. Fusion and flattening of these vacuoles results in the formation of a cleft in between two zones. A wall is then secreted in this region of cleft. This wall thus finally differentiates outer sporangiferous zone and central columella in the young sporangium. The columella therefore remains in continuity with that of the protoplast of sporangiophore. Cleavage in the peripheral sporangiferous zone results in the formation of many multinucleate segments. These segments secrete wall around each of them and metamorphose into unicellular, globose or oval, multinucleate, non-motile sporangiophores, called aplanospores. Germination of sporangiophores starts soon after the dispersal by the dehiscence of sporangium and developed into new thallus by producing germ tube. Sexual Reproduction in Rhizopus: Rhizopus reproduces sexually by the process of conjugation or gametangial copulation, which results in the formation of zygospores. A majority of the species Rhizopus (Rhizopus stolonifer) are heterothallic oberseved by Blakeslee (1904) known as heterothallism, whereas some species are homothallic (R.sexualis). In heterothallic conditions zygopsore is formed by the fusion two different hypha/mycelia/thalli designated as (+) and (-). In the homothallic species the zygospores are formed in the mycelium derived from a single sporangiophore or within a same/single mycelium. During gametangial copulation in homothallic or heterothallic two copulating horizontal braches are produced by the hyphae belongs to same mycelium as in case homothallic or from two different hyphae belongs to two separate mycelium designated as strains (+) and (-), respectively. The two copulating branches are called progametangia. These progammetangia of adhere together by their tips. They begin to enlarge because of the flow of the cytoplasm and nuclei into them. Later tip of each progammentangia is soon cut by septum. The small terminal cell so formed is called a gametangium. The gametangium has densely grandular multincleat protoplast, whereas the susepensor has more vculated protoplast. The protoplasm of each gametangium constitutes the aplanogamete. The size of the both the gametangia and the number of nuclei there in increase. The gametangia of the fusing pairs are generally equal in size but they may also unequal. A large pore develops in the adjoining wall of the two gametangia, which allows both the gametangial protoplasts to fuse and form a zygospore (aplanospores). The zygospore becomes surrounded by thick, black, warty wall. The zygospore wall is made up to two layers, of which the outer dark, thick and warty layer is called exine, and the inner thin layer is called intine. Germination of Zygospore: After resting period, due crack in its cell wall layers inner thin intine comes out in the form of hypha-like germ tube, which is also called promycelium. Meanwhile diploid nucleus of divides meiotically resulting into a number of haploid nuclei in the protoplasm of germinating zygospore. The young germ tube functions as a sporangiophore and develops a germsporangium as its tips and due disintegration of cell of germosporangium or meiospores later meiospore germinates by producing germ tube into new mycelium. Economic Importance of Rhizopus. 1) Majority of species belongs to the Rhizopus genera are saprophytic causes biodeterioration but few of them are phytopathogens on crops (Rhizopus oryzae on rice) 2) Few species Rhizopus are pathogenic on human beings and animals in the form mucaromysis. 3) Rhizopus is used bioremediation process. 4) Rhizopus is biotransformation of steroids. 5) Rhizopus is used in production commercial enzymes and organic acids. Nutrition: Along with chitin microfibrils the cell wall also contains polysaccharides (glycogen and mannan), lipids, phosphate and proteins. Plasmalemma is the limiting membrane of the cytoplasm. It is characteristics in having a series of shallow invaginations or elongated structure or pits within cytoplasm known as “Lomasomes“. Inner to the plasma membrane are present almost all membrane bound cell inclusions in the cytoplasm i. e., mitochondria, Golgi apparatus, ribosomes, endoplasmic reticulum, lipid granules in the form of sphaerosomes and eukaryotic nucleus. Mature yeast cells enclose a large, well-developed, centrally located vacuole surrounded of single vacuolar membrane, called tonoplast. The cytoplasm contains various kinds of inclusions. Examples of stored foods are lipid globules, granules of glycogen, oils and the carbohydrate trehalose, proteinaceous material and volutin. The glycogen may occur in vacuoles. Vacuole filled with water, lipid granules and granules of polymetaphosphate. Nucleus of mature yeast cells contain chromatin of nuclear material. Reproduction in Saccharomyces: Sexual reproduction life-cycles in Saccaromyces: Saccharomyces exhibit three different types of life-cycle patterns. a) Haplobiontic life-cycle: It is observed in homothallic Schizosaccharomyces octosporus. It is a haploid and each haploid somatic cell is a potential gametangium. At the time of sexual reproduction two such small cells come together due attraction by harmones and send a protuberance towards each other. The protuberances of both these cells come in contact with each other and the wall at the point of contact dissolves to form a common passage called conjugation tube or conjugation canal. The nucleus of the gametangia moves into this tube. Two nuclei fuse (karyogamy) in this region conjugation tube and form a diploid zygotic nucleus. The cytoplasm of the two gamentangial cells mixed to form common zygote cell. It now functions as an ascus. The zygotic nucleus divides first meiotically to form four haploid nuclei and them mitotically to form eight nuclei. All these nuclei organize themselves into ascospores. The ascospores rupture the ascus wall, come out and develop into individual uninucleate, haploid somatic cells. Thus the life-cycle is completed. The in life cycle haploid stages are more or dominated therefore it is called as haplobiontic life cycle. b) Diplobiontic life cycle: This kind of life cycle is observed in Saccharomyces ludwigii. In this yeast four ascospores formed in an ascus, do not rupture the ascus wall. Ascospores start to behave as “gametangia” Fusion (plasmogamy and karyogamy) of two ascospores of the opposite mating types takes place within ascus. This result in the formation of two diploid zygotic cells within the ascus. Each such diploid cell starts germination within the ascus. A germ tube develops, which ruptures the ascus wall. The germ tube becomes multinuclear and functions as diploid sporut mycelium. From the cells of this sprout mycelium develop some diploid buds. The diploid bus so formed get detached from the parent sprout mycelium by the formation of a septum at the base and function as dipoid sprout cells. Under suitable conditions this sprout cell functions as an ascus. Its diploid nucleus undergoes meiosis, and four haploid ascospores are formed within the ascus. Out of these four ascospores two belong to mating type (A1) and other two to the mating type (A2). Thus in this type of life-cycle diploid stage is dominated more whereas haploid phase is represented only by the ascospores, therefore is it called as diplobiontic life cycle. Diplobiontic life cycle in Saccharomyces ludwigii C) Haplo-diplobiontic life cycle: It is observed in Saccharomyces cerevisiae. It is heterothallic species. Out of the four (4) haploid ascospores liberated from an ascus, two carry one mating type (α) and other two carry other mating type (a). Both these mating types develop into independent cells of their respective mating types. During this life cycle two different mating types comes in contact with each other in response to hormones. Cells elongate and enlarge towards each other which bring about two opposite mating types by fusion through plasmogamy and karyogamy results in the formation of diploid zygote. Under favorable conditions these zygote undergoes budding and keeps on forming diploid cells for several generations. Under unfavorable conditions the diploid cells start to function as asci. The diploid nucleus undergoes meiosis to produced four haploid daughter nuclei are formed. Among these haploid ascospores rupture the ascus wall, come out and develop into fresh haploid cells of Saccharomyces cerevisiae. Two of these cells belong to mating type (α) and the remaining two to the mating type (a). These four haploid nuclei accumulate cytoplasm around themselves and change into ascospore. These haploid ascospores rupture the ascus wall, come ut and develop into haploid cells of Saccharomyces cerevisiae. Two of these cells belong to mating type (a) and the remaining two to the mating type (α). In this life-cycle of heterothallic strains of Saccharomyces cerevisiae indicates the existence of independent haploid and diploid phases of equal importance therefore it is called as haplo-diplobiontic life-cycle. Haplo-Diplobiontic life cycle in Saccharomyces cerevisae Economic importance of Saccharomyces (Yeast): 1. Systematic Position: Class: Deuteromycetes Order: Moniliales Family: Tuberculariaceae Genus: Fusarium Vegetative Structure of Fusarium: Mycelium is branched, sepate, hyaline or coloured, inter-or intracellular and uninucleate to multinucleate. Hyphae invade the tracheids and vessels of xylem, ramify there, produce toxic substances and block them completely. As a result the plants wilt and die. Economic Importance of Fusarium: 1) Fusarium oxysporum causes the most important vascular wilt diseases. It has several specialised forms known as form specialis (f. sp.) that infect a variety of host plant . 2) Some species of Fusarium produce mycotoxins − Fumonisins and trichothecenes. It is said that these toxins may cause oesophageal cancer. Some of these toxins are said to have been used as biological war fare agents in Vietnam and Afghanistan. 3) Few Fusarium species like F. solani, F. Verticilloides, F. proliferatum, F. solani infect human nails (dry rot of nails) and eye. Fusarium contaminated wheat flour when eaten, immune system is weakened (neutropenia). 4) F. graninearum has been used in U.K. to produce a high quality mycoprotein that can be fabricated into a number of meatless food. F. venenatum is produced industrially for use as human food quom in Europe and north America.
