Rhizopus

Habitat and Nutrition

· Rhizopus, cosmopolitan genus of more than 50 species in the family Mucoraceae.

· Ehrenb first named the term rhizopus in 1820.

· They are mostly terrestrial in habitat present in soil or on plants and animals.

· They grow as saprophytes on a wide variety of organic substance like on breads, fruits, jellies, syrups, leather, bread, peanuts, tobacco and vegetables.

· Most species are saprobes but some also found as parasites causing fungal infection.

· Rhizopus stolonifer or R. nigricans (the common bread mold, black bread mold, rhizopus rot) grow as white mycelia and black sporangia on bread. It causes storage rot diseases of fruits and vegetables.

· R.arrhizus causes zygomycosis in humans primarily in burn victims.

· Mucor mucedo, commonly known as the common pin mould,

Rhizopus stolonifer.

Vegetative structure:

· The vegetative plant body is eucarpic and thalloid.

§ Thallus is white cottony mycelium and are highly branched.

§ The cell wall is microfibrillar and composed of chitin-chitosan. The cytoplasm is multinucleated and consists of other cell organelles such as mitochondria, ER, golgi bodies, vacuoles and oil droplets.

§ The hyphae are tubular, branched, aseptate and cenocytic.

§ Mycelium is differen­tiated into nodes and internodes.

§ Three types of hyphae recognized in the mycelium: stolons, rhizoids, and sporangiophores.

· Stolon: It is aerial, grows horizontally forms an arch (intermodal region) and are less branched. It have node at certain point of contact with substratum.

· Rhizoids are repeatedly branched hyphae and are formed where the stolon touches the substratum at nodes. It penetrates the substratum and absorbs water and nourishment from host.

· Sporangiophores are the aerial, erect, unbranched reproductive hyphal branch. It arises in clusters from the upper side of each node. It bears sporangia terminally and are reproductive in function.

Reproduction

It reproduces by both asexual and sexual method.

Asexual Reproduction

1. Fragmentation: Due to accidental breakage the hyphae may break up into pieces and each unit is capable of growing as mother mycelium. (vegetative method)

2. Chlamydospores: During unfavourable conditions an intercalary segment of mature mycelium becomes septate and the protoplast of each cell forms a thick rounded walled structure called chlamydospores. They remain dormant until the favourable conditions return and then germinate to form a new mycelium.

3. Formation of Sporangiospores: Sporangiospores are formed terminally in sporangia of aerial mycelium called sporangiophores. They are formed under favourable conditions. Sporangiophores develop from the upper side of the rhizoidal node and are aerial, arise to certain height.

i. The nuclei and cytoplasm push more and more towards the apical side, consequently the apex of the aerial hyphae swells up and forms swollen tip.

ii. The swollen part enlarges and develops into a young sporangium.

iii. The cytoplasm of sporangium differentiates into the denser peripheral region with more nuclei (sporoplasm) and the central columella region with fewer nuclei and more vacuoles (columellaplasm)

iv. A septum is formed and completely separates two region.

v. Nucleus in sporoplasm divides rapidly and later shows protoplasmic cleavage and form small multinucleate pieces.

vi. Each piece rounds off secrets its own cell wall and transform into a globose, multinucleated and non-motile sporangiospores or Aplanospores

vii. At maturity the sporangium wall dries and the columella collapsed like a inverted cup releasing sporangiospores in atmosphere

viii. When the spore fall on suitable substratum it germinates by producing a germ tube and develops into new mycelium.

Sexual reproduction

§ Sexual reproduction takes place by gametangial copulation and most of the rhizopus species (R. stolonifer) are heterothallic and few are homothallic (R. sexualis).

§ When two mycelium of opposite strain come close to each other, each mycelium produce small out­growth, called progametangia and the hyphal branch is called zygophores.

§ The apical region of the two progametangia come in close contact and cytoplasm of each progametangium push more and more towards the apical region which swell up with dense protoplasm.

§ Now the apical region is separated by septa and the apical portion is called gametangia and basal region is known as suspensor.

§ Each gamentangia contain dense granular multinucleate protoplast and called aplenogamete or cenogamete.

§ Now the wall of two cenogamete/gametangia at the point of their contact dissolve (Plasmogamy) and then the nuclei of opposite strains form pair and fuse to form diploid nuclei (karyogamy) and ultimatley forms the zygospore.

· Zygospores enlarge and become thick-walled and resistant to adverse environmental conditions. Zygospore is a resting spore and germinates after a long period. During favorable condition, the inner wall of zygospore develops into promycellium and consists of region; germsporangiophore and germsporangium

§ Reduction division (meiosis) occurs during the formation of germsporanium and thus the germsporangium contain haploid nuclei, which gather cytoplasm and behaves as spore.

§ Each haploid spore are released and germinates to give new mycelium

Economic Importance

• R. oryzae is used in the production of cortisone and lactic acid and also used for biosorption of heavy metals in wastewater

• R. stolonifer is used to produce lactic acid, fumaric acid and cortisone

• R. delemar is used to produce biotin and fumaric acid

• Rhizopus microsporus is used to produce Tempeh, a traditional soybean fermented product of Asia.

• R. stolonifer also acts as parasite causing soft rot diseases in fruits and vegetables and therefore preventive fungicides are often sprayed postharvest, cold storage and transport to avoid crop losses from these diseases or slow their spread.