Hyphal Growth

Abstract

Hyphae are tubular structures that constitute the basic unit of a fungal mycelium. It is the efficient growth of hyphae that plays a crucial role in fungal colonisation of a substrate. Hyphal growth is driven by localised cell surface expansion and cell wall deposition at the hyphal tip. Spatial and temporal regulatory mechanisms enforce this growth pattern by controlling the recruitment of the morphogenetic machinery, which consists of the cytoskeletal and vesicle trafficking elements that mobilise the precursors needed for growth. In addition to the establishment and maintenance of hyphal polarity, the ability to regulate the termination of polarised growth is likely to be important because it facilitates transitions from hyphal growth to development or pathogenesis. Hyphal growth presumably evolved as an adaptation that enabled filamentous fungi to efficiently colonise terrestrial habitats.

Key Concepts:

  • Hyphal growth is the defining feature of filamentous fungi.

  • The morphogenetic machinery consists of those elements of the cytoskeleton and vesicle trafficking components that are needed to drive extension of the hyphal tip.

  • The Spitzenkörper is a vesicle supply and transit centre located at the hyphal tip that is required for optimal rates of tip extension.

  • The spatial separation of exocytosis and endocytosis within the hyphal tip complex enables rapid tip extension.

  • The direction of hyphal extension is regulated by a microtubule‐based marking system that conveys positional information to the morphogenetic machinery.

  • Reactive oxygen species and calcium appear to serve as signals that regulate tip growth and the emergence of branches.

  • Duplication cycle describes the events that encompass the doubling of cell mass and nuclear numbers during hyphal growth.

  • The termination of polarised hyphal growth is often a key prerequisite for development and pathogenesis.

  • The hyphal mode of growth appears to have independently evolved on multiple occasions during evolution.

Keywords: colony growth; hyphal extension; hyphal tip cell; morphogenetic machinery; mycelium; polarised growth; septum formation

Figure 1.

Morphogenetic events during the early stages of hyphal growth. (a) Dormant spore. (b) Swollen spore that has undergone isotropic expansion. (c) Establishment of a polarity axis. (d) Germ tube emergence. (e) Germ tube extension. (f) Septum formation. Maximal extension rates are typically achieved following septation. (g) Emergence of a lateral branch.

Figure 2.

Organisation of the cytoskeleton in fungal hyphae. Mature Aspergillus nidulans hyphae were imaged to detect microtubules (a) and actin (b). Microtubules were detected in living hyphae using a GFP‐tubA fusion protein. Actin was detected in fixed hyphae using a monoclonal anti‐actin antibody. Note that microtubules extend the length of the hypha, whereas actin localises to the septation site (arrow) and the hyphal tip (arrowhead). Bar, 3 μm.

Figure 3.

Organisation of hyphal tip cells and sub‐apical cells. Schematic depiction of an extending hypha. Note the asymmetric organisation of the hyphal tip cell. Nuclei (blue) exhibit a gradient of mitosis, with condensed mitotic nuclei located proximal to the tip. In addition, vacuoles (open red circles) and endomembranes (green) are more fragmented near the tip. Finally, the tip also houses the Spitzenkörper (stippled black circle) and the polarisome (yellow). The enlarged depiction of the hyphal tip shows the exocyst (orange), microtubules (green lines), actin filaments (red lines) and actin patches (filled red circles). Adapted from Harris SD ().

Figure 4.

A microtubule‐based marking system that recruits the morphogenetic machinery to hyphal tips. Kinesin‐dependent transport utilises microtubules (green) to deliver TeaA to the hyphal tip. TeaR is tethered to the membrane at the hyphal tip, and serves as a ‘receptor’ for TeaA. Upon interaction with TeaR, TeaA recruits the formin SepA, which in turn nucleates the formation of actin filaments (purple).

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Further Reading

Berepiki A, Lichius A and Read ND (2011) Actin organization and dynamics in filamentous fungi. Nature Reviews Microbiology 9: 876–887.

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Seiler S and Justa‐Schuch D (2010) Conserved components, but distinct mechanisms for the placement and assembly of the cell division machinery in unicellular and filamentous ascomycetes. Molecular Microbiology 78: 1058–1076.

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Harris, Steven D(Sep 2012) Hyphal Growth. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000367.pub2]