Rhizopus is a saprophytic fungus of the class Zygomycetes (as it produces zygospores in the sexual reproductive phase). Its species usually occur on dead organic material.
The name Rhizopus was first given by Ehrenb in 1820.
Occurrence of Rhizopus
Rhizopus is a genus of about 14 species. The majority of the species are saprophytes. They prefer to grow on decaying fruits and vegetables.
Rhizopus forms fleecy, white, cottony mycelium on bread, cheese, jellies, etc. A few species (e.g., R. arrhizus) are parasitic on animals as well as humans, causing an opportunistic infection (the pathogen takes advantage of an opportunity not normally available, such as a weakened immune system) called mucormycosis or zygomycosis.
Rhizopus stolonifer is very common and often occurs on bread; hence, it is known as black bread mould.
- Some of the Rhizopus species are: R. stolonifer, R. delemar, R. arrhizus, R. microsporus, R. reflexus, etc.
Vegetative Structure of Rhizopus
The vegetative plant body is eucarpic, composed of white, cottony, much branched coenocytic mycelium. The mycelium ramifies over the surface of the substratum.
When the mycelium reaches the reproductive phase, the hyphae can be differentiated into three regions: rhizoids, stolons, and sporangiophores.
The stolons are the sub-aerial arching hyphae, which grow horizontally above the substratum. Some portions of the hyphae bend down and touch the substratum, forming the nodal regions.
The rhizoids arise in tufts from the nodal region of the stolon. They grow downward, inside the substratum.
Each rhizoid is a much-branched, slender, root-like hypha. Rhizoids anchor the mycelium to the substratum and help in the absorption of nutrients.
These are the reproductive hyphae, which bear sporangia terminally. Each sporangiophore is an erect, unbranched, elongated structure.
The sporangiophores grow upwardly from the stolon.
Read also- Peziza: Occurrence, Structure, Reproduction
Cell Structure of Hyphae
The hyphal cell wall is microfibrillar and consists mainly of chitin and chitosan. Other substances like polysaccharides, proteins, lipids, etc. are also present.
A cell membrane is present inside the cell wall, which surrounds the protoplast. The protoplast contains many cellular organelles, such as the mitochondria, endoplasmic reticulum, ribosomes, and vacuoles. Numerous nuclei are also present inside the protoplast.
Droplets of oils and glycogen are the reserve food materials found in the cytoplasm.
Reproduction in Rhizopus
Rhizopus reproduces in all three methods: vegetative, asexual, and sexual.
The vegetative reproduction in Rhizopus takes place by fragmentation.
The stolon can break into small fragments due to accidental breakage. Each fragment gives rise to a new mycelium in favourable conditions.
Asexual reproduction in Rhizopus occurs through the formation of sporangiospores and chlamydospores.
During favourable conditions, sporangiospores or aplanospores are formed inside the black-coloured spherical sporangium.
The sporangium develops singly at the tip of the erect, unbranched sporangiophore.
At the time of reproduction, one or more elongated hyphae (i.e., sporangiophores) develop aerially from the upper side of the rhizoidal node. After attaining a certain height, the nuclei and the cytoplasm of the sporangiophore push more and more towards the apical region. Consequently, the tip of the sporangiophore swells up to form a large, globular sporangium.
As the sporangium matures, many nuclei and much of the cytoplasm with nutrients are accumulated in the peripheral region just beneath the sporangial wall. The centre of the sporangium becomes highly vacuolated with fewer nuclei. This swollen, vacuolated portion is called the columella.
Meanwhile, a series of small vacuoles appear just above the columella, which become flattened and form a cavity. A wall then develops towards the inner side of the cavity, and it separates the columella from the peripheral region. With further development, the wall becomes dome-shaped.
The protoplasm of the peripheral region (i.e., sporoplasm) begins to divide into a large number of small multinucleate (5–10 nuclei) segments. Each segment becomes rounded, secretes a wall around it, and metamorphoses into a non-motile sporangiospore or aplanospore.
On maturation of the sporangiospores, the sporangial wall dries up, and the columella collapses like an inverted cup.
Consequently, the sporangial wall breaks up into many fragments, leaving a portion as a collar on the sporangiophore. The spores are exposed to the environment and finally disseminated by air currents.
Under favourable conditions (ideal moisture and temperature), each sporangiospore germinates into a germ tube, which later gives rise to a new Rhizopus mycelium.
The chlamydospores are formed during unfavourable conditions.
The old mycelium becomes transversely septate. The intercalary mycelium segments are enveloped by a thicker wall with sufficient food reserves. These thick-walled structures are called chlamydospores. The chlamydospores get separated from each other when the connecting mycelium dries up.
With the onset of favourable conditions, each chlamydospore germinates into a germ tube, which develops new mycelium on a suitable substratum.
In Rhizopus, sexual reproduction takes place by the process of gametangial copulation (fusion of multinucleate gametangia) during unfavourable conditions.
The species of Rhizopus may be homothallic (e.g., R. sexualis) or heterothallic (e.g., R. stolonifer). Heterothallic species are more common.
In heterothallic species, the hyphae taking part in sexual reproduction are of compatible strains, i.e., (+) and (-) strains. But in homothallic species, the hyphae are of the same strain, either (+) or (-) strains.
Initially, the two hyphae of compatible strains come close to each other, and each hypha produces a small copulating outgrowth called the progametangium.
The apical regions of the two progametangia now come into close contact. Many nuclei and much cytoplasm are accumulated in the apical regions, which swell up with dense protoplasm.
A septum then develops near the tip of each progametangium, separating it into two segments. The apical segment is called the gametangium, while the basal elongated segment is known as the suspensor.
Each gametangium contains multinucleate, dense protoplast. Such protoplast is known as coenogamete or aplanogamete.
Meanwhile, the common gametangial wall dissolves at the point of contact, and the protoplasts of both gametangia fuse to form a zygospore. Many nuclei of opposite gametangia fuse together to form diploid nuclei, while some remain unpaired. The unpaired nuclei degenerate gradually.
The zygospore enlarges considerably and secretes a thick, multilayered wall (usually five layers, two in the exosporium and three in the endosporium). Its surface becomes black and warty.
The zygospore then undergoes a period of rest.
Germination of Zygospore
The zygospore germinates after a considerable period of time.
Under suitable conditions, the exosporium (outer wall) of the zygospore bursts, and the endosporium (inner wall) comes out in the form of a tube called promycelium. With further development, the promycelium is differentiated into a spherical zygosporangium or germsporangium at its tip.
The sporangiospores (also called meiospores, as they develop from the diploid nucleus by reduction divisions) are produced inside the zygosporangium in the same manner as in asexual reproduction.
Each meiospore after liberation germinates into a germ tube, which produces new mycelium.
Sometimes gametangia fail to copulate. Such a single gametangium may develop directly (i.e., without fusion) into a thick walled spore, called an azygospore or parthenospore.
The parthenospore is haploid, and it resembles the typical zygospore in external structure. This phenomenon (parthenospore-producing) is called parthenogenesis.
Pathogenesis of Rhizopus
Some species of Rhizopus are opportunistic pathogens that cause infections in plants, animals, and humans.
Many fruits and vegetables, such as tomatoes, sweet potatoes, papayas, tobacco, and strawberries, are susceptible to R. stolonifer and R. arrhizus. The plant diseases are collectively known as storage rots (a number of postharvest diseases).
In humans, particularly in individuals with a weakened immune system, uncontrolled diabetes, etc., Rhizopus often causes a fatal disease called mucormycosis or zygomycosis. The animal infection by Rhizopus is also known as mucormycosis.
Taxonomic Position of Rhizopus