Rhizopus, Diagram, Structure, Classification, and Reproduction

Rhizopus: Rhizopus is a fungal genus known for its saprophytic and parasitic nature. These fungi, which prefer damp or moist conditions, are commonly found in organic matter such as fruits, vegetables, bread, and jellies. Rhizopus' vegetative structure is defined by coenocytic (multinucleated) and branched hyphae.

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Rhizopus Classification

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Rhizopus Species

• Rhizopus stolonifer (black bread mold): This species is the most prevalent within the genus and acts as a saprophyte, decomposing dead organic matter. It is also employed in industry to produce fumaric acid, a compound used in food and pharmaceuticals.
• Rhizopus oryzae (synonym: Rhizopus arrhizus ): Used in the production of tempeh, a fermented soybean cake popular in Indonesia, Rhizopus oryzae can also lead to mucormycosis, a severe fungal infection.
• Rhizopus microsporus: Commonly associated with mucormycosis, this species is also utilised in industry to manufacture cortisone, a type of steroid medication.
• Rhizopus delemar: This species is involved in the production of fumaric acid and biotin, a vitamin.

Connective Tissue

Rhizopus Characteristics

1. Morphology: Rhizopus species display a cottony or woolly appearance due to their mycelium, composed of branching hyphae.
2. Habitat: Found on decaying organic matter like bread, fruits, vegetables, and soil.
3. Reproduction : Asexual reproduction via sporangia formation, which releases sporangiospores for dispersal and germination.
4. Rhizoids: Root-like structures anchoring the fungus and aiding in nutrient absorption.
5. Sexual reproduction: Under specific conditions, sexual reproduction occurs, forming zygospores from fused gametangia.
6. Economic importance: Some species are crucial in food production (e.g., tempeh fermentation), while others can be pathogenic to plants, animals, and humans.
7. Optimal growth conditions: Thrive in warm, humid environments with temperatures between 20-30°C.
8. Spore dispersal: Sporangiospores disperse through air currents, water, or physical contact, facilitating colonization of new substrates.

Sexual Reproduction in Flowering Plants

Rhizopus Structure

1. Stolon: Aerial and arching, stolon is present in the internodal region, touching the substratum to form nodal regions.
2. Rhizoids: Branching structures that anchor the mycelium to the substratum and absorb food, forming where stolons touch the substratum at nodes.
3. Sporangiophores: Vertical, unbranched hyphae that grow from stolons and produce a reproductive structure called sporangiospores.

• Cell Wall: Composed of chitin, chitosan, lipids, and proteins.
• Protoplasm: Contains nuclei, mitochondria, endoplasmic reticulum, and various cytoplasmic inclusions like ribosomes and oil droplets.
• Columella: A hygroscopic structure arising from u-shaped sporangiophores, primarily responsible for water absorption.
• Sporangium: A spherical or globose structure that connects the columella and sporangiophore, containing sporangiospores.
• Sporangiospores: Unicellular asexual spores.

Rhizopus Reproduction

1. Vegetative Reproduction:

• Occurs through fragmentation.
• Each stolon fragment develops independently, forming a complete mycelium.

2. Asexual Reproduction:

• Terminal formation in sporangia of aerial mycelium (sporangiophores).
• Sporangiophores develop from the upper side of the rhizoidal node.
• Apical swelling forms sporangium, with nuclei and cytoplasm moving apically.
• Differentiation of sporangium cytoplasm into the denser peripheral region with more nuclei and central columella region with fewer nuclei and more vacuoles.
• Sporangiospores develop inside sporangium, being multinucleated and non-motile.
• The sporangium wall ruptures after maturation, releasing sporangiospores as powdery masses.
• Each spore germinates into new mycelium under suitable conditions.

• Formed during unfavorable conditions.
• An intercalary segment of mycelium develops due to septae formation and protoplasm accumulation.
• Thick-walled chlamydospore detaches from mycelium once it dries.
• Chlamydospores remain dormant until favourable conditions return, then germinate to form new mycelium.

3. Sexual Reproduction:

• Involves the fusion of two compatible hyphae.
• Most Rhizopus species (e.g., R. stolonifera) are heterothallic, having different mycelia for + and - mating strains, while R. sexualis is homothallic.

• Compatible hyphae develop small outgrowths known as progametangia in both mycelia.
• Nuclei and cytoplasm move towards the apical region, and progametangia make contact.
• The apical region is separated from the rest of the hyphae by septae formation, forming gametangia.
• Gametangia conjugate to form a multinucleated structure.
• Plasmogamy is followed by karyogamy, forming a diploid (2n) zygote known as a zygospore, with the rest of the unpaired nuclei degenerating.
• Zygospores enlarge, becoming thick-walled and resistant to adverse environmental conditions. Upon receiving favourable conditions, zygospores germinate.
• The inner wall of the zygospore develops into promycelium, forming germ sporangiophore, with germ sporangium formed apically.
• Meiosis occurs, forming haploid meiospores, which come out after the rupture of the germ sporangium wall, developing into new mycelia.

Rhizopus Hyphae

1. Coenocytic and Branching: Rhizopus hyphae lack internal walls, making them coenocytic. They also branch out, forming a mycelium.
2. Three Types of Hyphae:

• Stolons: Aerial hyphae acting as runners, extending outward and curving upwards.
• Rhizoids: Root-like structures produced by stolons, anchoring the fungus and absorbing nutrients.
• Sporangiophores: Upright, unbranched hyphae that produce sporangia at their tips for reproduction.

Rhizopus Economic Importance

1. Food Fermentation: Rhizopus species, notably Rhizopus oligosporus, are crucial in fermenting foods like tempeh. This Indonesian delicacy is made from fermented soybeans, enhancing their nutritional value and flavour.
2. Enzyme Production: Rhizopus species produce enzymes like amylases and lipases. These enzymes find application in various industries, such as starch breakdown in food and lipid degradation in detergents.
3. Organic Acid Synthesis: Certain Rhizopus species produce organic acids like citric acid, widely used in the food and beverage industry as an acidulant, flavouring agent, and preservative.
4. Biological Control: Some Rhizopus species act as biological control agents against plant pathogens, inhibiting the growth of harmful fungi and protecting crops.
5. Bioremediation: Rhizopus species are studied for their ability to degrade pollutants like pesticides and hydrocarbons, making them valuable in cleaning up contaminated environments.
6. Research: Rhizopus serves as a model organism in genetics and molecular biology research due to its rapid growth and ease of cultivation, aiding in the study of various biological processes.