Life Cycle of Albugo

Introduction

Albugo is a plant-parasitic oomycetes genus. This genus has an incomplete taxonomy, although numerous species are plant pathogens. Albugo is one of three genera of the Albuginaceae family that have been identified so far.

Albugo candida, commonly known as Cystopus candidus, is the most common cause of white rust in Brassicaceae/Cruciferae members. The Asteraceae, Convolvulaceae, and Chenopodiaceae families are also susceptible to this fungus’ attack.

In this article, we’ll study the life cycle and mechanisms of reproduction of Albugo candida, a Phycomycetes species.

Table of Contents

• Classification, Occurrence and Habitat of Albugo Fungi
• Signs and Symptoms of Albugo Fungi
• Life Cycle of Albugo Fungi
• Reproductive Mechanism of Albugo
• Economic Significance of Albugo
• Frequently Asked Questions (FAQs)

Classification, Occurrence and Habitat of Albugo Fungi

• Kingdom: Fungi
• Phylum: Oomycota
• Class: Oomycetes
• Order: Peronosporales
• Family: Albuginaceae
• Genus: Albugo
• Species: Albugo candida

Occurrence

Albugo is an obligatory parasite that feeds on the flowers of a variety of blooming plants. It has a total of 25 species. Albugo candida is a widespread fungus that causes “white rust” in members of the Cruciferae family.

Habitat

In India, there are roughly 18 species of Albugo that attack crucifers, such as mustard, turnips, radish, cauliflower, cabbage, and other vegetables. It has, however, been found on some members of the Asteraceae family (Composite, Convolvulaceae and Chenopodiaceae).

Non-septate coenocytic hyphae grow in the intercellular gaps of the host to form the vegetative plant body. The hyphae have short, globular, or knob-shaped haustoria. They get inside the cells of the host and collect food from there.

Signs and Symptoms of Albugo Fungi

• This infection attacks all areas of the plant body that are above ground. The stomata are the entry points for infection.
• Shiny white irregular spots appear on leaves or stems as a result of the disease. These areas become powdery in the later phases. Flowers and fruits become distorted. The condition also causes hypertrophy (an increase in the size of cells and organs).

Life Cycle of Albugo Fungi

Mycelium of Albugo

It has branching, aseptate, and coenocytic hyphae that are well developed. The hyphae are found in the intercellular gaps of the vulnerable host tissue, where they reside and multiply.

It’s made up of a lot of nuclei, oil globules, and glycogen. Mitochondria, endoplasmic reticulum, and ribosomes can all be observed on electron micrographs.

Lomasomes are formed by the cytoplasmic membrane adhering to the hyphal wall. In actively growing hyphae, septa are repressed, yet they appear to segregate reproductive structures and seal off wounded sections.

The mycelium of the fungus grows rapidly. Within the host, the hyphae branch and ramify, targeting the tissues near the infection site. Albugo and Peronospora can coexist on the same host, especially Capsella bursa-pastoris. The smaller diameter of Albugo‘s hyphae and the more abundant vesicular haustoria distinguish it from Peronospora.

The obligate parasite’s intercellular hyphae form intracellular haustoria in the host’s mesophyll cells. At the point where the hyphal wall is closely pushed against the mesophyll cell wall, the haustorium emerges as a lateral outgrowth.

The penetration matrix, an electron-dense amorphous substance, is frequently deposited at the point of contact between the host and the hypha cell walls, which is referred to as the penetration site.

The haustorial mother cell’s crescent-shaped protrusion, known as the haustorial initial, penetrates the host cell wall at the penetrating site and extends into the lumen of the mesophyll cell to form a haustorium. The vascularised host plasma membrane forms its boundary.

The haustorium is visible under a light microscope as a tiny, spherical structure made up of two parts: the haustorial stalk or neck and the terminal haustorial head or body.

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Fungus Life Cycle

Reproductive Mechanism of Albugo

Fungus Albugo reproduces both asexually and sexually.

Asexual Reproduction

• Conidia, condiosporangia, and zoosporangia are used for asexual reproduction. They are generated by sporangiophores. Mycelium develops and branches quickly in suitable conditions.
• It creates a dense mat-like growth just beneath the epidermis of the host after reaching maturity, and some of the hyphae begin to behave as sporangiophores or conidiophores. There are around a dozen nuclei and abundant cytoplasm in these sporangiophores.
• Later, the sporangiophore’s apical portion swells, and confinement forms below the enlarged end, resulting in the formation of the first sporangium. A second sporangium grows from the tip of the first one, immediately below it. Above each sporangiophore, a chain of sporangia or conidia is formed in basipetal succession.
• Due to the disintegration of the separating disc, the sporangia are released as they reach maturity. When sporangia are carried away by the wind or rainwater and land on an appropriate host, it germinates within 2 or 3 hours.
• The sporangia germinate to produce zoospores, which germinate and enter the host through stomata when they come into contact with it.

Sexual Reproduction

• It takes place at the end of the growing season. The mycelium penetrates into the host’s deeper tissues. Its type of sexual reproduction is oogamous in nature. The male sex organ is known as antheridium, whereas the female sex organ is known as oogonium.
• Antheridium is a club-shaped elongated structure. It is multinucleate; in some situations, only one nucleus is functioning, while in others, many are.
• Oogonium is a multinucleate spherical organism with 65 to 115 nuclei. The cytoplasm contains an even distribution of nuclei.
• The contents of the oogonium become organised into an exterior peripheral region of periplasm and an interior dense centre region of ooplasm or oosphere or the egg as the oogonium matures. All nuclei, with the exception of a single functioning nucleus, dissolve at maturity.
• The coenocentrum is a highly stained mass of cytoplasm that forms in the middle of the ooplasm before fertilisation. The functioning female nucleus is drawn to it and attaches itself to a spot nearby.
• At the site of contact with the antheridium, the oogonium forms a papilla-like outgrowth. The receptive papilla fades away soon, and the antheridium produces a fertilisation tube.
• It passes through the receptive papilla, the oogonial wall, and the periplasm before reaching the ooplasm. It has only one male nucleus. It ruptures at the tip, releasing the male nucleus close to the female nucleus. The male nucleus eventually merges with the female nucleus (karyogamy).
• The oosphere and the fusion nucleus result in the formation of the oospore. When the oospore reaches maturity, it produces a two to three-layered wall. The exospore is represented by the thick, warty, or tuberculated outer layer. The endospore is the thin inner layer of the spore.
• The zygotic nucleus divides repeatedly with the secretion of the wall, resulting in roughly 32 nuclei. Meiosis is the first division. The oospore rests for a long time at this stage until unfavourable conditions have subsided. Meanwhile, the oospore is freed as the host tissues dissolve. The oospore germinates after a long period of dormancy. Its nuclei divide mitotically, resulting in a huge number of nuclei.
• Each nucleus is enveloped by a small portion of the cytoplasm. Protoplasm is segmented, and each segment eventually rounds up and transforms into a zoomeiospore or zoospore. The endospore emerges as a thin vesicle once the exospore is broken. The zoospores spread out into the thin vesicle, which eventually dies, releasing the zoospores. After swimming for a while, the zoospores encyst and germinate via a germ tube, reinfecting the host plant.

Economic Significance of Albugo

The fungus is the cause of the crucifer “white rust” disease, which causes major yield losses in rapeseed, turnips, and mustard. The thick-walled oospores make up the majority of the deformed and hypertrophied inflorescences, or “stage heads.”

Another species, A. ipomoeae-panduratae parasitises sweet potatoes, and A. bliti thrives like a parasite on various members of the Amaranthaceae family. White rust is a minor crucifer disease in terms of economic significance.

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