Key Characteristics and Importance of Rhizopus
Rhizopus is a fungus genus that includes both saprophytic and parasitic species. They prefer moist or humid environments. They can be found on organic foods such as vegetables, fruits, bread, jellies, and other jams. Coenocytic (multinucleated) and branched hyphae make up the vegetative structure. They're utilised to make a variety of chemicals and alcoholic beverages. Some rhizopus species cause plant diseases and can also infect humans, causing mucormycosis.
It is the genus of common saprophytic fungi, which grows on plants and specialised parasites on plants. They have the ability to grow on wide organic substances including fruits and vegetables, bread, leather, etc. They are multicellular in nature and some rhizopus causes fungal infection and they cause fatal disease. They grow in filamentous, branching hyphae that generally lack cross-walls, i.e, they are coenocytic. They reproduce by spore formation both by asexual and sexual mode of reproduction as sporangiospores are produced inside a spherical structure, the sporangium. Sporangia are supported by a large apophysis atop a long stalk, the sporangiophore. In sexual reproduction, a dark zygospore is produced at the point where two compatible mycelia fuse. After germination, zygospores produced colonies that are genetically different from their parents.
Classification of Rhizopus
Kingdom: Fungi
Phylum: Zygomycota
Order: Mucorales
Family: Mucoraceae
Genus: Rhizopus
Features of Rhizopus
They have a branching body known as mycelia.
The majority of rhizopus are saprobic in nature i.e decomposer.
They mainly feed on dead organic matter or organisms.
They reproduce by spore formation.
They are used for industrial purposes like in the making of biotin, alcoholic beverages, etc.
Morphological Features of Rhizopus
They are dark greyish-brown in colour.
The size of rhizoids is 10mm high.
Sporangia are 100 micrometre in diameter.
Structure of Rhizopus
Mostly they are fast growing in nature and mainly have a cottony appearance.
The Body of a rhizopus contains branched mycelium and they are coenocytic in nature. They also consist of branched mycelium and mycelium is coenocytic in nature and composed of three types of hyphae:, stolon, rhizoids, and sporangiophores.
Stolon is present in the internodal region, it is aerial, forms an arch, and touches the substratum forming nodal region.
Rhizoids are formed where the stolon touches the substratum at nodes. They have branching, anchor the mycelium to the substratum, and also absorb food.
Apart from this, there is a reproductive hyphae consisting of sporangiophores which grows vertically from stolon. They are unbranched, elongated, columellate and they also give rise to a reproductive structure known as sporangiospores. Given below are the different parts of hyphae:
Cell-Wall: Cell wall is composed of chitin, chitosan, lipids, proteins, etc.
Protoplasm: Protoplasm consists of nuclei, mitochondria, endoplasmic reticulum, and various other cytoplasmic inclusions like ribosomes, oil droplets, etc.
Columella: It mainly arises from u-shaped sporangiophore and they are mainly hygroscopic in nature and their main function is the absorption of water.
Sporangium: It acts as a connecting link between columella and sporangiophore. They are mainly spherical or globose in shape. They also carry a reproductive structure known as sporangiospores.
Sporangiospores: They are known to be asexual spores and mainly unicellular in nature.
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Common Example of Rhizopus
Some common examples of rhizopus species are:
Rhizopus Arrhizus: They help in the production of alcoholic beverages.
Rhizopus Zygospores: they are used to make tempeh.
Rhizopus Microsporus: It is used to ferment soybean products.
Rhizopus Stolonifer: It is also known as black bread mould. They are also used in the production of chemicals, e.g., cortisone, fumaric acid, etc. Causes fruit rot disease.
Life Cycle of Rhizopus
Rhizopus reproduces by three methods. They are vegetative, asxeaul, and sexual mode.
1. Vegetative Reproduction
Under this mode of reproduction, small fragments are formed on the body surface of rhizopus and due to accidental breakage, the stolon may break up into two or more than two small units and each unit or part is capable of growing as a mother mycelium.
2. Asexual Mode of Reproduction
It takes place in two ways either by the formation of sporangiospore or by the formation of chlamydospore.
A. By Sporangiospore Formation
Under favourable conditions, non-motile spores such as sporangiospores are formed inside the sporangium. Sporangium starts developing singly at the apex of sporangiophore and they further develop into tufts from the upper side of the node opposite to rhizoidal hyphae. These hyphae continue to grow up to a certain height. Their nuclei and cytoplasm moved towards the apical region due to this enlargement in hyphae taking place and this enlarged part is known as sporangium. With time, the protoplast of sporangium is differentiated into a thick dense layer of multinucleated cytoplasm and this differentiation occurs towards the peripheral region and this wall is called as sporoplasm and the remaining vacuolated portion with few nuclei towards the centre is known as columella plasm. After this, a series of vacuoles appear between sporoplasm and columella plasm and these vacuoles become flattened and form a cleavage cavity. This leads to the development of septum and they become dome shaped and force their way into the sporangium. After the maturation process, the wall of sporangium dries and collapses to form a cup-shaped irregular surface. This sporangial wall gets raptured into different fragments leaving a small portion as a collar on the sporangiophore. The powdery remains of spores are scattered in the atmosphere.
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B. Chlamydospores
This type of axeual reproduction occurs by formation of zoospores, aplanospores, hypnospores, or a palmella stage. During unfavourable conditions like lack of food material and water, the protoplasm gets surrounded by thick and nutrient-rich walls. Then, this chlamydospore gets detached from vegetative hyphae and they remain in the resting phase and when this chlamydomonas gets enough moisture, they undergo the formation of germ tube which leads to the formation of the new thallus.
3. Sexual Mode of Reproduction
Sexual mode of reproduction in rhizopus occurs in various steps, they are as follows:
Under this, positive and negative thallus comes in contact with each other.
After coming in contact, conjugation between male and female thalli occurs by the means of the outgrowth of positive and negative thalli.
Due to this, conjugation septum is developed between progametangia and plasmogamy. This leads to the formation of coenogametes.
Due to the conjugation process of gametes, karyogamy occurs which leads to the formation of zygote.
Further zygote undergoes maturation process and gets surrounded by thick-walled structure zygospore, which covers both outer as well as inner layer.
For some time, zygospores undergo a resting phase and under favourable condition, zygospore forms a germ tube and germinates into a new vegetative body.
FAQs on Rhizopus: Classification, Features, and Life Cycle
1. What is Rhizopus and where is it commonly found?
Rhizopus is a genus of saprophytic fungi, meaning it feeds on dead and decaying organic matter. It is most frequently found in moist environments on substrates rich in carbohydrates. Its common habitats include stale bread, decaying fruits and vegetables, soil, and animal dung. Because of its frequent appearance on bread, it is widely known as bread mould.
2. What is the typical structure of a Rhizopus mycelium?
The body of Rhizopus consists of a network of thread-like filaments called hyphae, which collectively form the mycelium. The structure is composed of three distinct types of hyphae:
- Stolons: These are horizontal hyphae that grow across the surface of the substrate, helping the fungus to spread.
- Rhizoids: These are root-like, branching hyphae that penetrate the substrate to anchor the mycelium and absorb nutrients.
- Sporangiophores: These are vertically growing hyphae that bear a reproductive structure called the sporangium at their tip.
3. How does spore formation occur in Rhizopus?
Spore formation in Rhizopus is the primary method of asexual reproduction. It occurs when the tip of a sporangiophore swells to form a spherical structure called a sporangium. Inside the sporangium, the cytoplasm divides, and each part develops into a tiny, light-weight spore. Once mature, the wall of the sporangium ruptures, releasing hundreds of these spores into the air to be dispersed by wind.
4. What is the ecological and economic importance of Rhizopus?
Ecologically, Rhizopus acts as a crucial decomposer, breaking down complex organic matter and recycling nutrients in the ecosystem. Economically, it has both negative and positive impacts. It is a common cause of food spoilage. However, several species are used industrially to produce organic acids like fumaric acid and lactic acid, as well as for the fermentation of soybeans to create foods like tempeh.
5. What disease can be caused by Rhizopus in humans?
While generally harmless, certain species of Rhizopus can be opportunistic pathogens, particularly in individuals with weakened immune systems. They can cause a serious and often fatal fungal infection known as mucormycosis (also sometimes called zygomycosis). This infection typically affects the sinuses, lungs, or skin.
6. Why is the mycelium of Rhizopus described as coenocytic?
The mycelium of Rhizopus is described as coenocytic because its hyphae are aseptate, meaning they lack internal cross-walls or septa. This results in a continuous, tube-like filament containing multiple nuclei suspended within a shared cytoplasm. This structure allows for the rapid and efficient transport of nutrients and cellular components throughout the fungal body.
7. How does asexual reproduction in Rhizopus differ from its sexual reproduction?
Asexual and sexual reproduction in Rhizopus differ in conditions, process, and outcome. Asexual reproduction occurs under favourable conditions via spore formation from a single parent, producing genetically identical offspring. In contrast, sexual reproduction is a response to unfavourable conditions. It involves the fusion of hyphae from two compatible mating strains to form a thick-walled, dormant zygospore that can withstand harsh environments and produces genetically diverse offspring upon germination.
8. What is the key difference between the rhizoids of Rhizopus and the roots of a plant?
The key difference lies in their structural complexity and origin. Rhizoids in Rhizopus are simple, thread-like filaments that serve for anchorage and absorption but are not true tissues. They are part of the undifferentiated fungal body (thallus). In contrast, plant roots are complex multicellular organs with specialized tissues like xylem and phloem for transport and a distinct, organized anatomical structure. Thus, rhizoids and roots are analogous (similar function) but not homologous (different structure and evolutionary origin).
