Growth in fungi refers to a positive change in size, volume and number of cell, and that lead to maturation, often over a period of time.
Most fungi are sessile filamentous organisms that grow by extending the tips of hyphae to form an expanding mycelial network. Tip growth, therefore, represents a form of cellular motility and hyphae must be able to coordinate this motility by extending and orientating the trajectory of hyphal extension
Not all fungi grow as hyphae. Some grow as discrete yeast cells which divide by fission or, more frequently, budding
The two fundamental properties of the vegetative fungal hypha _ the polarity of both growth and secretion of degradative enzymes _ have been known for over a century. Numerous studies have subsequently confirmed that ‘the key to the fungal hypha lies in the apex .
Ultrastructural studies have shown that many organelles within the growing hyphal tip are distributed in steep gradients, as would be expected of a cell growing in a polarized mode .This is visible even with the light microscope by careful observation .The cytoplasm of the extreme apex is occupied almost exclusively by secretory vesicles and microvesicles .
In the higher fungi (Asco- and Basidiomycota), the former are arranged as a spherical shell around the latter, and the entire formation is called the Spitzenkörper or ‘apical body .
The Spitzenkörper is an intracellular organelle associated with tip growth. It is composed of an aggregation of membrane-bound vesicles containing cell wall components. The spitzenkörper is part of the endomembrane system of fungi, holding and releasing vesicles it receives from the Golgi apparatus.
The Spitzenkörper may be seen in growing hyphae even with the light microscope. Hyphae of the Oomycota and some lower Eumycota (notably the Zygomycota) do not contain a recognizable Spitzenkörper, and the vesicles are instead distributed more loosely in the apical dome. By modifying certain parameters, it is even possible to generate the somewhat more pointed apex often found in hyphae of Oomycota and Zygomycota .
A little behind the apical dome, a region of intense biosynthetic activity and energy generation is indicated by parallel sheets of endoplasmic reticulum (ER) and an abundance of mitochondria . The first nuclei usually appear just behind the biosynthetic zone followed ultimately by a system of ever-enlarging vacuoles .These may fill almost the entire volume of mature hyphal regions, making them appear empty when viewed with the light microscope.
Mechanisms of mycelial growth
Hyphae grow at their tips. During tip growth, cell walls are extended by the external assembly and polymerization of cell wall components (The cell wall at the hyphal tip has viscoelastic properties and yields to the internal turgor pressure within the hypha. Further behind the tip the wall is rigidified and resistant to the turgor forces resulting from the osmotic flow of water into the hypha. Turgor pressure generated within the hypha therefore acts as the driving force for hyphal extension) and the internal production of new cell membrane. The vesicles which accumulated in Spitzenkörper region travel to the cell membrane via the cytoskeleton and release their contents outside the cell by the process of exocytosis, where it can then be transported to where it is needed. Vesicle membranes contribute to growth of the cell membrane while their contents form new cell wall. The spitzenkörper moves along the apex of the hyphal strand and generates apical growth and branching; the apical growth rate of the hyphal strand parallels and is regulated by the movement of the spitzenkörper. Hyphal extension at the apex requires synthesis and insertion of new wall material and new membranes in a way that does not weaken the tip. This highly organized process is supported by the continuous flow of vesicles generated within the cytoplasm behind the tip, and is coordinated with the growth and replication of all the other cytoplasmic organelles and their migration towards the extending apex.
38 000 secretory vesicles per minute fuse with the plasma membrane of a single growing hypha of Neurospora crassa. Microvesicles (chitosomes) probably arise from a discrete population of Golgi cisternae .
Diagrammatic representation of the possible organisation of wall growth at the hyphal apex. Only half of the hypha is shown. Vesicles (V) derived from the endoplasmic reticulum and Golgi body (G) are transported to the apex, probably by microtubule (M) -associated motor proteins. The vesicles could then be directed to the plasma membrane, perhaps by actin-associated motor proteins. The newly-formed wall at the extreme hyphal tip is thin and has few cross-linkages, but becomes increasingly cross-linked further back. In contrast, the actin cytoskeleton is highly developed at the extreme tip and might help to provide structural support, compensating for the lack of wall cross-linking at the tip. The concentration of actin progressively decreases behind the tip
Synthesis of the cell wall
The synthesis of chitin is mediated by specialized organelles termed chitosomes which inactive chitin synthases are delivered to the apical plasma membrane and become activated upon contact with the lipid bilayer . Microvesicles, visible especially in the core region of the Spitzenkörper, are likely to be the ultrastructural manifestation of chitosomes . In contrast, structural proteins and enzymes travel together in the larger secretory vesicles and are discharged into the environment when the vesicles fuse with the plasma membrane . Whereas most proteins are fully functional by the time they traverse the plasma membrane, the glucans are secreted by secretory vesicles as partly formed precursors and undergo further polymerization in the nascent cell wall, or they are synthesized entirely at the plasma membrane. Cross-linking of glucans with other components of the cell wall takes place after extrusion into the cell wall
Wessels (1997) suggested that hyphal growth occurs as the result of a continuously replenished supply of soft wall material at the apex, but there is good evidence that the softness of the apical cell wall is also influenced by the activity of wall-lytic enzymes such as chitinases or glucanases. Further, when certain Oomycota grow under conditions of hyperosmotic stress, their cell wall is measurably softer due to the secretion of an endo-b-(1,4)-glucanase, thus permitting continued growth when the turgor pressure is reduced or both cell wall material and synthetic as well as lytic enzymes are secreted together by the vesicles of the Spitzenkörper, the appearance, position and movement of this structure should influence the direction and speed of apical growth directly.
The properties of the cell wall depend in many ways on the environment in which the hypha grows. Thus, when Schizophyllum commune is grown in liquid submerged culture, a significant part of the β-glucan fraction may diffuse into the liquid medium before it is captured by the cell wall, giving rise to mucilage . In addition to causing problems when growing fungi in liquid culture for experimental purposes, mucilage may cause economic losses when released by Botrytis cinerea in grapes to be used for wine production. On the other hand, secreted polysaccharides, especially of Basidiomycota, may have interesting medicinal properties and are being promoted as anti-tumour medication both in conventional and in alternative medicine .
Diagrammatic representation of the internal scaffold model of tip growth in fungi proposed . Secretory vesicles and chitosomes are transported along microtubules from their subapical sites of synthesis to the growing apex. The Spitzenkorper forms around a cluster of actin filaments. An actin scaffold inside the extreme apex is linked to rivet-like integrin molecules which are anchored in the rigid subapical cell wall. The apex is further stabilized by spectrin molecules lining the cytoplasmic surface of the plasma membrane.
Secretion and membrane traffic
One of the most important ecological roles of fungi, that of decomposing dead plant matter, requires the secretion of large quantities of hydrolytic and oxidative enzymes into the environment. In liquid culture under optimized experimental conditions, certain fungi are capable of secreting more than 20 g of a single enzyme or enzyme group per litre culture broth within a few days’ growth.
The secretory route in fungi begins in the ER. Ribosomes loaded with a suitable messenger RNA dock on to the ER membrane and translate the polypeptide product which enters the ER lumen during its synthesis unless specific internal signal sequences cause it to be retained in the ER membrane. As soon as the protein is in contact with the ER lumen, oligosaccharide chains may be added onto selected amino acids. The transport of proteins from the ER to the Golgi system occurs via vesicular carriers , although continuous membrane flow is also possible . Membrane lipids seem to be recycled to the ER by a different mechanism relying on tubular continuities .
In the Golgi system, proteins are subjected to stepwise further modifications and proteins destined for the vacuolar system are separated from those bound for secretion Both destinations are probably reached by vesicular carriers, the secretory vesicles moving along microtubules to reach the growing hyphal apex which is the site for secretion of extracellular enzymes as well as new cell wall material. There is mounting evidence that fungi, like most eukaryotes, are capable of performing endocytosis by the inward budding of the plasma membrane at subapical locations. Endocytosis may be necessary to retrieve membrane material in excess of that which is required for extension at the growing apex, i.e. endocytosis and exocytosis may be coupled. The prime destination of endocytosed membrane material or vital stains is the vacuole .In fungi, large vacuoles represent the main element of the lytic system and are the sink not only for endocytosed material but also for Autophagocytosis, i.e. the sequestration and degradation of organelles or cytoplasm. Autophagocytosis is especially prominent under starvation conditions .
Growth of hyphae tip
Vesicles are thought to deliver the main wall-synthetic enzymes (chitin synthase and glucan synthase) to the tip, where they lodge in the plasma membrane as integral membrane proteins. Mannoproteins and other glycoproteins are transported in vesicles from the Endoplasmic reticulum - Golgi secretory system (because the glycosylation of proteins occurs only in the Golgi). Multivesicular bodies, whose functions are still unclear, may be carried as vesicular cargoes along microtubules. Enzyme activators and inhibitors also are thought to be involved in the orchestration of tip growth, but the substrates for wall synthesis arrive from metabolic reactions in the cytosol.
Branching of fungal hyphae:
1-Apical branching of fungal hyphae: The emergence of a branch from the hyphal tip is referred to as apical branching. This pattern of branching has been observed in a large number of fungi . In many of these fungi, apical branching presumably occurs in response to the abnormal accumulation of exocytic vesicles at the hyphal tip. This could conceivably be triggered by perturbations that slow extension of hyphal tips without interrupting the flow of exocytic vesicles through the cytoplasm. Because the supply of vesicles exceeds their capacity to be incorporated into the existing tip, they accumulate leading to the formation of a new tip . Instead, it seems likely that apical branching is a general response that enables continued growth under conditions that compromise organization of hyphal tips and thereby disrupts apical dominance.
2- Lateral branching.
Lateral branching would only occur when a potential site is far enough removed from the tip so as to escape the effects of these factors. Accordingly, the nature of these factors and their mode-of-action is of great interest . There appears to be two broad patterns of lateral branching; branches associated with septa, and random branching. In the former pattern, new branches emerge adjacent to septa , and it seems likely that some component(s) of the septum provides a spatial cue that specifies the position of the branch. In most cases of lateral branching, the branch emerges just behind the septum, which would be expected if the septum were serving as a barrier that impeded the tip-bound flow of exocytic vesicles and thus led to their local accumulation. Nevertheless, branches still emerge from these sites. It is tempting to speculate that a component involved in an early step in septum formation .