Monday, May 20, 2024
General Agriculture

Life Cycles and Alternation of Generations in Phytophthora and Rhizopus

In this article, we shall discuss a detailed study of reproduction and alternation of generations in Phytophthora and Rhizopus.

1. Phytophthora

This fungus belongs to the Division Oomycota. There are about 75 species in this genus, most of which live as parasites on flowering plants. The species

Phytophthora infestans are of great economic importance. It causes a serious potato disease called potato blight or late blight of potatoes.


The mycelium of Phytophthora is profusely branched and consists of aseptate, hyaline, and coenocytic hyphae. The hyphae ramify in the intercellular spaces of the host tissues.

The mycelium produces haustoria which penetrates the host cell wall and enter the cells to draw nourishment. The haustoria may be simple or branched. Phytophthora reproduces both asexually and sexually.

Asexual Reproduction

In warm and humid weather it normally reproduces asexually. During this stage, a tuft of slender, branched hyphae usually arises from the internal mycelium. They come out through the stomata or pierce through the epidermal cell on the lower surface of the leaf.

In tubers they come out through the injured portions of the skin. These aerial hyphae are hyaline and branched. They bear a sporangium at their tip.

You have learnt earlier that the hyphae-bearing sporagia or conidia are called sporangiophores or conidiophores respectively. The sporangia are thin-walled, hyaline and lemon-shaped and have a beak-like projection or papilla at their tips.

The mature sporangia can easily be separated from the sporangiophore. The sporangiophore is branched. It bears nodular swellings which denote the point of detachment of sporangia.

Wind, rain drops or contact with neighbouring leaves detach and scatter the ripe sporangia on neighbouring potato plants. They may fall on the ground and get spread into the soil. The sporangia lose their viability if they fail to germinate within a few hours.

When the sporangia fall on the leaf of a host plant, they germinate. Moisture and temperature are the determinants for germination. In the presence of water and low temperatures (upto 12°C) the sporangiun behaves as a zoosporangium. The protoplast divides into 5-8 uninucleated daughter protoplasts which transform into zoospores.

Read Also: Fungal Morphology and Forms

Life Cycles and Alternation of Generations in Phytophthora and Rhizopus

The zoospores are uniform and biflagellate of the two flagella one is of the whiplash type and the other is of the tinsel type. The zoospores are set free through the apical papilla into a vesicle in some species. The vesicle soon bursts open to liberate the zoospores.

The liberated zoospores swim for some time, and later settle on a substratum losing the flagella and germinate. During germination, the zoospore puts out a short hypha called appressorium.

The appressoria help to fix the fungus on the surface of the host leaf. From the appresorium, a narrow, peg- like infection hypha develops which forces its way into the host leaf.

At temperatures up to 24°C, and low relative humidity the sporangium germinates directly behaving like a conidium. It germinates producing a germ tube or a short hypha, which enters into the host leaf.

The sporangia, which are washed into the soil, germinate and infect the tubers. As a result the tubers rot by harvest time or during storage. Under favourable conditions a number of asexual generations may be produced in one growing season. This results in rapid propagation of the fungus to spread the disease.

Sexual Reproduction

Sexual reproduction is of the oogamous type. The male sex organs are antheridia and the female oogonia. They arise at the tips of short lateral branches as antheridial and cogonial initials respectively. Phytophthora infestans is heterothallic.

The antheridium is a club-shaped structure with one or two nuclei to begin with. Later the nuclei divide and produce about 12 nuclei. At the time of the fertilization only one functional nucleus persists and the others degenerate.

The oogonium develops on a neighbouring hypha of the antheridial branch. It grows across the antheridium and swells to form a pear-shaped or spherical structure. It contains dense cytoplasm and many nuclei (about 40).

The protoplast of the oogonium becomes differentiated into an outer multinucleate periplasm and a central uninucleate ooplasm. The central nucleus divides into two and one of them disappears. The surviving nucleus functions as the egg nucleus. The nuclei of the periplasm later degenerate.

The oogonial wall bulges out at a certain point to make a receptive spot. The oogonial wall disintegrates at this spot. Through this opening the antheridium pushes a short fertiIization tube. The fertilization tube penetrates the periplasm and reaches the ooplasm.

Here it opens and delivers the male nucleus along with the surrounding cytoplasm. The male and female nuclei fuse, thus bringing out fertilization.

The fertilized egg secretes a thick wall around itself and becomes the oospore. When the conditions are favourable the oospore germinates. It is believed that meiosis takes place during germination.

The germination of the oospore takes place after the decay of the host tissue. A germ tube develops from the oospore and may directly develop into a mycelium or oospore may bear a terminal sporangium. Inside the sporangium zoospores are produced which after liberation develop into new mycelia.

ln the life cycle of Phytophthora there is an asexual cycle which may repeat during favourable conditions. The sexual cycle takes place prior to the onset of unfavorable conditions forming a resting spore. These cycles normally alternate with each other.

2. Rhizopus

Rhizopus isa member of Division Zygomycota. It is commoly called bread mould since it is frequently found growing on stale bread. It is a saprophytic fungus. It also grows on decaying fruits, vegetables and other food materials.

Rhizopus stolonifer sometimes grows as a facultative parasite on strawberries causing a transit disease called ‘leak’ and also causes ‘soft rot’ disease of sweet potatoes, yam and cassava tubers.

The mycelium is a white cotton-like fluffy mass with numerous, slender, branched hyphae. The mycelium has three types of hyphae: i) rhizoidal ii) stolons and iii) sporangiophores.

The rhizoidal hyphae are a cluster of brown, slender and branched rooting hyphae which arise from the lower surface of the stolon at certain points which are the apparent nodal points. These hyphae help in anchorage and in the absorption of water and nourishment from the substratum.

The aerial hyphae which grow horizontally over the surface of the substratum are called stolons. These hyphae are comparatively large, and slightly arched. The stolons grow rapidly in all directions, completely filling the surface of the substratum.

The third kind of hyphae called sporangiophores develop during the reproductive phase. The sporangiophores arise from the apparent nodal regions, opposite to the rhizoidal hyphae in a cluster. They grow vertically bearing sporangia at their tips.

Asexual Reproduction

Rhizopus reproduces asexually by multinucleate, non-motile spores which are produced in small, round, black sporangia. The sporangia are borne terminally, and singly on unbranched sporangiophores.

A mature sporangium is differentiated into two regions, a central less dense, vacuolated region with fewer’ nuclei called columella and a peripheral dense region with many nuclei called sporoferous region. The protoplast in the columella is continuous with that of the sporangiophore.

The soporiferous region undergoes cleavage to form a number of muItinucleate segment. These segments round off and secrete walls around them to become sporangiospores. These are unicellular, multinucleate, non-motile aplanospores, globose or oval in shape.

As the spores mature the sporangium bursts open liberating the spore mass. A part of the wall remains as a collar-like fringe at the base of the sporangium.

The spores are dispersed away by the wind. Falling on a suitable substratum, under suitable conditions a spore germinates producing a short germ tube that grows further and branches profusely to produce three types of hypae.

Life Cycles and Alternation of Generations in Phytophthora and Rhizopus

Under unfavorable conditions Rhizopus produces chlamydospores. As you learned they are thick-walled spores with accumulated reserve food. They are produced intercalarily.

They help to tide over unfavorable conditions during which time the mycelium perishes. With the return of favorable conditions they germinate and produce normal mycelium.

Sexual Reproduction

Towards the end of the growing season, Rhizopus reproduces sexually. Sexual reproduction is of the conjugation type. Here the two gametangia fuse. You learnt above that such a union of protoplasts is called gametangial copulation.

Some species of Rhizopus are homothallic while others are heterothallic. In heterothallic species the mycelia belong to two mating types or strains one plus and the other minus.

During sexual reproduction, the hyphae of the two mating types (+ and -) called zygophores are attracted towards each other.

They produce copulating branches called progametangia which meet at their tips. The tips of the progametangia enlarge due to the accumulation of cytoplasm and nuclei, and are cut off from the basal portion by cross walls.

The terminal portion is call suspensor. The entire gametangium transforms into an aplanogamete. The two gametangia may be of the same size or one of them slightly smaller than the other.

When the gametangia mature the intervening, waIls dissolve and the two gametes and their nuclei fuse producing a zygospore. The zygospore increases in size and secretes a thick two-layered wall around it.

The outer layer is dark and warty. It is called extine or exospore. The inner layer is thick and is called an intine or endospore. As the zygospore increases in size, the wall of the fusion cell containing the zygospore ruptures and it is set free.

Prior to germination of the zygospore, the diploid nuclei divide meiotically producing numerous haploid nuclei. During meiosis segregation of strains takes place. The zygospore during germination absorbs water and swells.

Read Also: Types of Algae Reproduction and Life Cycle of Algae

As a result, the outer wall extine breaks open. The inner wall intine with the inner contents grows out as a germ tube or promycelium. The promyceliurn is of limited growth and produces a terminal sporangium.

It is called zygosporangium or germ sporangium. Inside the sporangium numerous, non-motile germ spores called meiospores are produced. They are liberated at maturity which develop into new mycelia.

The life cycle of Rhizopus consists of two phases, asexual and sexual. The asexual phase consists of myelium, sporangiophores, sporangia and the sporangiospores. This phase in the life cycle serves to propagate the haploid phase of the fungus during favourable conditions.

The sexual phase, consists of mycelia of the plus and minus strains, the progametangia, gametangia, aplanogametes, zygospore, promycelium, germ sporangium and the germ spores. Among these, the zygospore is the only diploid structure.

All others are hapoid. Such a sexual cycle is called haplontic characterized by zygotic meiosis and haploid mycelium as the only adult fungi.

Rhizopus exhibits heterothallism wherein the mycelia of a single species are morphologically similar but physiologically different. There is no apparent distinction between male and female mycelia except in their sexual behaviour.

Such a distinction is designated by the terms plus and minus. This was first discovered by Blakeslee in 1904. This is the first indication of the origin of dioecious condition of sexual phase in an organism.

In summary, reproduction in fungi occurs by vegetative asexual and sexual methods. Vegetative reproduction occurs by budding, fission, framentation, formation of oidia and rhizomorphs. Asexual reproduction occurs more frequently.

Sexual reproduction involves plasmogamy, karyogamy and meiosis. Phytophthora infestans and Rhizopus are some of the examples of fungi treated in this article.

Phytophthora infestans causes serious potato disease. The coenocytic hyphae ramify in the host tissue and draw nutrition through haustoria. Asexual reproduction takes place by the formation of sporangia.

Depending upon the condition of temperature and humidity the sporangia may produce zoospores or germinate directly. Sexual reproduction is of oogamous type.

In Rhizopus, the mycelium has rhizoidal hyphae, stolons, and the hyphae- bearing sporangiophores. The sporangiophores bear non-motile aplanospores, the asexual reproductive bodies.

Sexual reproduction occurs by the fusion of gametangia of opposite strains forming zygosporangia which bear numerous non-motile zygospores.

Read Also: Building Resilient Food Systems: The Role of Agroecology


Benadine Nonye is an agricultural consultant and a writer with over 12 years of professional experience in the agriculture industry. - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education - PhD Student in Agricultural Economics and Environmental Policy... Visit My Websites On: 1. - Your Comprehensive Practical Agricultural Knowledge and Farmer’s Guide Website! 2. - For Effective Environmental Management through Proper Waste Management and Recycling Practices! Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4Profits TV and WealthInWastes TV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

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