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Fungi: Vegetative Structures & Reproduction

Vegetative structure

Lower fungi: Coenocytic

Higher fungi

  • Septate, thalloid (can be one celled e.g. Yeast) and branched.
  • Color: Hyaline (colorless), sub-hyaline (light color), brown.

Special vegetative structure

  • Appressorium: Colletotrichum
  • Chalmydospore: Fusarium
  • Sclerotium: Sclerotium rolfsii
  • Hyphopodium : Meliola
  • Haustoria: Erysiphe sp.

N.B. Facultative and Deuteromyces: i+ii+iii examples

Obligate (only in living cell) and Ascomyces: iv+v examples

(i) Appressorium

Latin ‘apprimere’ means ‘to press against’.

An appressorium is a specialized cell typical of many fungal plant pathogens that is used to infect host plants. It is a flattened, hyphal “pressing” organ, from which a minute infection peg grows and enters the host.

Require optimum humidity and temp.

Process of formation

Following spore attachment and germination on the host surface, the emerging germ tube (first mycelia) perceives physical cues such as surface hardness and hydrophobicity, as well as chemical signals including wax monomers that trigger appressorium formation. Appressorium formation begins when the tip of the germ tube ceases polar growth, hooks, and begins to swell. The contents of the spore are then mobilized into the developing appressorium, a septum develops at the neck of the appressorium, and the germ tube and spore collapse and die. As the appressorium matures, it becomes firmly attached to the plant surface and a dense layer of melanin is laid down in the appressorium wall, except across a pore at the plant interface. Turgor pressure increases inside the appressorium and a penetration hyphae emerges at the pore, which is driven through the plant cuticle into the underlying epidermal cells.

Figure: Appressoria source here

Figure: Formation of appressorium source here

(ii) Chlamydospore

A chlamydospore is the thick-walled large resting spore of several kinds of fungi, including Ascomycota such as Candida, Basidiomycota such as Panus. It is the life-stage which survives in unfavorable conditions, such as dry or hot seasons.

Chlamydospores are usually dark-coloured, spherical, and have a smooth (non-ornamented) surface. They are multicellular, with cells connected by pores in the septae between cells.

Can be seen by naked eyes (?).

Figure: Chlamydospores of the yeast Candida albicans source here 

(iii) Sclerotium

A sclerotium (plural sclerotia) is a compact mass of hardened fungal mycelium containing food reserves. One role of sclerotia is to survive environmental extremes.

It is a hard structure made up of pseudoparenchymatous tissue.

Formation

In fungi, there are three stages in the development of sclerotia:

  1. Initial aggregation of hyphae;
  2. Increase in size due to the growth and branching of hyphae;
  3. Maturation with the formation of an outer coating that isolates the sclerotia from the

surrounding environment, with the progressive dehydration of the hyphae and accumulation of reserve substances and pigments.

Figure: Sclerotia (white structured bodies) in a colony. Source here

Figure: T.S of a sclerotium. Source here

(iv) Hyphopodium

A specialized hyphal branch, composed of one or two usually lobed cells, serving for attachment to the host and for the absorption of food. It is the characteristic of group Meliolales in which mucronate hyphopodia function as condiogenous cells and capitate hyphopodia give rise to haustoria.

Figure: Hyphopodium source here

(v) Haustoria

Haustoria is an absorbing organ originating on a hyphae of a parasite that penetrates the host plant’s cell wall and siphon nutrients from the space between the cell wall and plasma membrane but do not penetrate the membrane itself. Larger (usually botanical, not fungal) haustoria do this at the tissue level. It is found most often in obligate parasites.hy

Figure: Haustoria (the knob like structures) source here

Reproduction

Asexual reproduction

  • Conidia
  • Mycelial fragmentation
  • Zoospore (lower fungi)
  • Chlamydospore
  • Sclerotium

Zoospore

In the simple fungi, sporangiospores may be motile ornon-motile. The motile sporangiospores are called zoospores. Zoospore contains one or two flagella. E.g. Saprolegnia, Phytopthora.

2 types of zoospores:

  1. Primary
  2. Secondary zoospore:
  • Kidney shaped.
  • g. Phytopthora

Sexual reproduction

It involves the formation and fusion of gametes. Sexual reproduction found in all groups of fungi except deuteromycetes or fungi imperfecti. Sexual reproduction has three distinct phases i.e. plasmogamy (protoplasmic fusion), karyogamy (fusion of nuclei) and meiosis (reduction division of zygote).

  1. Planogametic copulation: Fusion of flagellate gametes.
  2. Gametangial contact: Fertilization tube (to transfer nucleus).
  3. Gametangial copulation
  4. Somatogamy: +,- strain of mycelia.
  5. Spermatization: Spermatia, receptive hyphae.

 

1. Planogametic copulation

This is simplest type of sexual reproduction. In this process fusion of two gametes of opposite sex or strains takes place where one or both of the fusing gametes are motile (flagellated). It results in the formation of a diploid zygote.

This process is usually of three types:

  • Isogamy

In this process fusing gametes are morphologically similar and motile but physiologically dissimilar. These gametes are produced by different parents e.g. Synchytrium.

  • Heterogamy

When the fusing gametes are morphologically as well as physiologically different, the process is known as heterogamy. Heterogamous reproduction is of two types: anisogamy and oogamy.

  1. Anisogamy: Anisogamy consists of the fusion of two motile gametes where the male gamete is small and more active than the female gamete, e.g., Allomyces.
  2. Oogamy: In oogamy the motile male gamete (antherozooid) fuses with the large, non-motile female gamete (egg or ovum), Monoblepharis, Synchytrium etc.

Anisogamy and oogamy produce oospore after fertilization which is always diploid.

2. Gametangial contact

In this process two gametangia of opposite sex come in contact with one another. The male gametangium (antheridium) transfers male nucleus or gamete into the female gametangium (oogonium) either through a pore at the point of contact or through a fertilization tube especially developed for the purpose by the male gametangium. The two gametangia do not fuse.

E.g. Phytophthora, Sphaerothera, Albugo, Pythium, Saprolegnia etc.

3. Gametangial copulation

In involves the fusion of entire contents of two gametangia to form a common cell called zygote or zygospore, e.g., Mucor, Rhizopus.

4. Somatogamy

This takes place in fungi where formation of gametes is absent. In such fungi, anastomoses takes place between hyphae and their somatic cells fuse to produce dikaryotic cells, e.g, Agaricus, Peniophora etc.

5. Spermatization

Some fungi produce many minute, spore-like, single-celled structures called spermatia (nonmotile gametes). These structures are transferred through agencies like water, wind and insects to either special receptive hyphae or trichogyne of ascogonium. The contents migrate into receptive structure. Thus dikaryotic condition is established, e.g. Puccinia.

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About Abulais Shomrat

Abulais Shomrat
Currently in 4th year (Hons) in Department of Botany, University of Dhaka. Planning to have multiple careers one by one but promised to be with 'Plantlet' as long as it's primary stage remains unfinished.

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