Mycology: Fungal Growth

Fungal Growth

Apical growth is characteristic of fungi, distinguishing them from other organisms.
Fungal growth occurs as a continuous tube extending at the extreme tip via localized synthesis of wall components.
The hyphal apex can:
- Swell into a balloon-like structure (spore or yeast cell).
- Taper to penetrate inert gold film or host plant walls using turgor pressure alone.

Hyphal extension is restricted to the hyphal apex, resulting in polarized growth.
The cell wall at the hyphal tip exhibits viscoelastic properties, yielding to internal turgor pressure.
Further back from the tip, the wall becomes rigidified and resistant to turgor forces caused by osmotic water influx.
Turgor pressure within the hypha drives hyphal extension.

Hyphal extension requires synthesis and insertion of new wall material and membranes at the apex without weakening it.
This process is highly organized and supported by continuous vesicle flow from the cytoplasm behind the tip.
It is coordinated with the growth, replication, and migration of cytoplasmic organelles towards the extending apex.

Catalyzes chitin chain synthesis, crucial for fungal wall growth.
Chitin is formed in situ at the apex, rather than delivered via vesicles.
Exists in two forms:
- Inactive zymogen in chitosomes.
- Integral membrane protein.
The zymogen form, inserted into the membrane, requires activation by a protease (likely delivered via vesicles).
The substrate is delivered from the cytosol to the inner face of the chitin synthase enzyme (anchored in the membrane).
Chitin chains are synthesized and extruded from the membrane's outer face.

The other major enzyme involved in wall growth, catalyzing β-1,3-glucan chain synthesis, which often comprises the bulk of the fungal wall.
Like chitin synthase, it arrives in vesicles and inserts into the plasma membrane at the apex.
The substrate is a sugar nucleotide (UDP-glucose) supplied from the cytosol.
Regulation differs from chitin synthase.
The enzyme consists of two subunits:
- One subunit (on the membrane outer face) contains the catalytic site.
- The other is a guanosine triphosphate (GTP) binding protein.
Activation occurs when GTP arrives at the cytoplasmic face, leading to glucan chain synthesis and extrusion into the wall.

Mannoproteins and other glycoproteins constitute a smaller portion of the hyphal wall but are more common in yeasts and yeast-like phases of dimorphic fungi.
These glycosylated proteins are pre-formed in the endoplasmic reticulum–Golgi complex and delivered to the apex in vesicles.

Various types of cross-linkages form between major wall polymers after their insertion, progressing from the hyphal tip.
Glucans can be extracted after treating walls with chitinase to degrade chitin, indicating covalent bonds between chitin and glucans.
Amino acids, such as lysine in Schizophyllum commune, may be involved in glucan–chitin linkages.
Chitin chains associate via hydrogen bonding to form microfibrils, leading to a progressively more cross-linked and rigidified structure.

There are conflicting views regarding the necessity of wall lytic enzymes for apical growth.
One view suggests that existing wall softening is required for new component insertion, implying a balance between wall lysis and synthesis.
Chitinase, cellulase (in Oomycota), and β-1,3-glucanase activities are found in hyphal wall fractions, potentially in a latent form.

Wessels (1990) proposed a steady-state model where wall-lytic enzymes are unnecessary, explaining other features of tip growth.
The newly formed wall at the extreme tip is viscoelastic, allowing wall polymers to flow outwards and backwards during new component addition.
The wall rigidifies progressively via extra bond formation behind the tip.

Some fungal spores, like uredospores of rust fungi, have a fixed germination point (germ pore) with a thinner wall.
Zoospores of Chytridiomycota, Oomycota, and plasmodiophorids also have fixed germination points, settling and adhering to surfaces to locate the germ-tube outgrowth.

Many spores can germinate from any point on the cell periphery, following a common pattern.

Initially, the spore swells by hydration, followed by further swelling via an active metabolic process where new wall materials are incorporated over most of the cell surface.

A germ-tube (young hypha) emerges from a localized point, and subsequent wall growth is localized to this region.

The production of spores from germinating spores with minimal intervening growth.
Occurs naturally in some fungi, especially in water films under nutrient-limited conditions.
Reported for saprotrophs on leaf surfaces (e.g., Cladosporium, Alternaria spp.).
Observed in leaf-infecting pathogens (e.g., Septoria nodorum), vascular wilt pathogens (Fusarium oxysporum), and the rhizosphere fungus Idriella bolleyi (a biological control agent of root pathogens).
These fungi form normal hyphae in nutrient-rich conditions, suggesting microcycling behavior in nutrient-poor conditions aids dispersal to more favorable environments.

A tropism is defined as a directional growth response of an organism to an external stimulus.
A directional growth response of an organism to an external stimulus.
Some fungal spores exhibit marked tropism, e.g., the yeast-like fungus Geotrichum candidum, a common cause of dairy product spoilage.
Cylindrical spores typically germinate from one or the other pole, influenced by neighboring spores.

Tropism is a bending response that orientates a hypha towards a nutrient source or away from a potential inhibitor.

Despite the need for nutrients, true fungi somatic hyphae do not exhibit nutrient seeking behavior.
Only Oomycota display this behavior in a strain-specific manner.
Some Saprolegnia or Achlya spp. strains orientate towards amino acid mixtures, while others show no response.