Introduction: Rhizopus is a type of fungus which belongs to the class Zygomycetes because of the fact that it produces zygospore in its sexual reproductive phase. It also refers to bread, black or pin mould. These can reproduce by vegetative, asexual and sexual means followed by fragmentation, sporangiospores and zygospores.
Rhizopus is cosmopolitan in distribution. It grows on a variety of substrates like fruits, vegetables, bread, jellies etc. There are around 8-10 known species of Rhizopus. The name Rhizopus was first named by Ehrenb in 1820.
- Kingdom: Fungi
- Division: Mycota
- Sub-division: Eumycotina
- Class: Zygomycetes
- Order: Mucorales
- Family: Mucoraceae
- Genus: Rhizopus
Features of Rhizopus
- The mode of nutrition is saprobic and parasitic in Rhizopus where a majority of them are saprophytic and few are parasitic viz. R.artocarp, R.arrhizus etc.
- In Rhizopus, the mycelium is coenocytic, tubular, multinucleated, vacuolated and consist of cytoplasmic materials like Golgi body, mitochondria etc.
- The reserve food material is in the form of glycogen and oil droplets.
- The cell wall of the thallus in non-cellulose and made of chitin.
- The hyphae of Rhizopus differentiates into three distinctive parts namely stolon (intermodal region), rhizoids (nodal region) and sporangiophores.
- For the growth of Rhizopus, the most important factor is moisture or water availability.
- Its digestion is extracellular.
- These are the opportunistic fungi which can commonly cause “Zygomycosis or Mucormycosis”.
Structure of Rhizopus
The morphology of Rhizopus consist of the following features:
Hyphae: Rhizopus consists of two kinds of hyphae namely vegetative and reproductive hyphae. Vegetative hyphae differentiate into two types namely Stolon and rhizoids
Stolon: It is the intermodal region which also refers to Runner. A stolon is the aerial hyphae which grow horizontally and is found attached to the substratum. It is aseptate, branched and lacks cross walls.
Rhizoids: It is the nodal region which forms when the stolon touches the surface of the substratum. Rhizoid is the much-branched structure that forms under the substratum. Its main function is to invade all the nutrients from the substratum.
Reproductive hyphae consist of sporangiophores which grow vertically from the stolon. Sporangiophores are unbranched, elongated, columellate and give rise to the reproductive structures refers to sporangiospores.
- Cell- wall: The cell wall of hyphae is composed of chitin chitosan, lipids, proteins etc.
- The protoplasm: It consists of many nuclei, mitochondria, endoplasmic reticulum and other cytoplasmic inclusions like the ribosome, oil droplets, vacuoles etc.
- Columella: It arises from the sporangiophore which is generally dome or umbrella-shaped. The surface of columella can be smooth or rough. The columella is hygroscopic in nature which the main function is water absorption.
- Sporangium: It connects with the columella and a long sporangiophore. The shape of the sporangium is spherical or globose in shape. It carries the reproductive structures refers as sporangiospores. The size of the sporangium is 0.2mm.
- Sporangiospores: These are the asexual spores which are globose to ovoid and unicellular. The colour of sporangiospores is hyaline to brown.
General culture characteristics
Growth: The growth of mycelium is very fast- growing.
Appearance: Dense cottony colonies
Colour: The colour of mycelium is generally white and after sporulation turns grey or golden brown
Reproduction in Rhizopus
It reproduces by vegetative, asexual and sexual methods
Vegetative reproduction of Rhizopus
Sometimes the thallus of the Rhizopus breaks accidently or some other factors into few fragments where each fragment give rise to a new thallus on favourable conditions.
Asexual reproduction of Rhizopus
The asexual reproduction can take place through both sporangiospores and chlamydospores.
Asexual reproduction through sporangiospores:
It includes the following steps:
- First, the vegetative hypha grows to form sporangiophore that is long and slender.
- Then the sporangiophore develops and enlarges at the apical region to form columella which is of variable shape and size.
- This columella gives rise to the large, round sporangium by pushing cytoplasmic material to the peripheral wall.
- After that, differentiation of sporangia takes place wherein between sporangia and columella, spore sac is present which carries the sporangiospores. The structure of Sporangiospores is oval, unicellular, multinucleate and non-motile.
- These sporangiospores multiply their number in the spore sac and in extreme conditions, it ruptures the sporangial wall and releases out of the wall.
- In the environment, the sporangiospores remains in a dormant state or in a resting state.
- When favourable conditions return, these sporangiospores form germ tube and undergo germination to form a vegetative hypha.
Asexual reproduction through chlamydospores:
It includes the following steps:
- During unfavourable condition like insufficient food material and water, the protoplasm is surrounded by a thick and nutrient-rich wall.
- This chlamydospore then detaches from the vegetative hyphae and remains in the resting phase.
- When this chlamydospore gets moisture then it forms a germ tube which then germinates into a new thallus.
Sexual reproduction of Rhizopus
In Rhizopus, the sexual reproduction occurs by the method of “Gametangial conjugation”. It includes the following steps:
- First, the (+) and (-) thallus comes in contact with each other.
- Then conjugation occurs between the positive and negative thalli by the means of outgrowth developed by positive and negative thalli.
- The septum develops between the progametangium and plasmogamy occurs that results in the formation of “Coenogametes”.
- Then karyogamy occurs by which the gametes of both the (+) and (-) thallus fuse with each other to form “Zygote”.
- The zygote undergoes the maturation phase and surrounds by a thick-walled structure called as “Zygospore” which covers by an outer layer and Inner layer.
- The zygospore undergoes a resting phase for some time. On favourable conditions, zygospore forms germ tube and germinates by forming a new vegetative body.
Therefore, a Rhizopus reproduces through vegetative, asexual and sexual means to form differentiated hyphae.