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Chapter 18 : Hyphal Growth and Polarity
Category: Microbial Genetics and Molecular Biology; Fungi and Fungal Pathogenesis
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This chapter summarizes the progress achieved toward understanding the organization of fungal hyphae and the cellular systems involved in hyphal morphogenesis. Particular attention is paid to the mechanisms that have been implicated in the regulation of polarized growth and septum formation in filamentous fungi. Finally, the intriguing question of how morphogenetic regulatory systems may have evolved in the fungal kingdom is briefly addressed. Hyphal growth encompasses several different morphogenetic processes. Foremost among these is the establishment and maintenance of a stable axis of polarized growth. As a result, cell surface expansion and cell wall deposition are confined to a discrete location that ultimately becomes the hyphal tip. Genetic analyses demonstrate that Bni1 is absolutely essential for the establishment of hyphal polarity in A. gossypii. The importance of understanding the molecular mechanisms underlying polarized hyphal growth cannot be understated. The genetic tractability of filamentous fungi such as A. nidulans and N. crassa affords a tremendous opportunity to elucidate these mechanisms and to acquire insight that might be relevant to neurological disorders and other motor diseases. The use of increasingly sophisticated microscopy techniques has revealed the subcellular organization of hyphal tip cells and, in particular, emphasized the role of the Spitzenkörper in polarized hyphal growth. Future experiments that exploit genomic and proteomic tools will undoubtedly provide new insights that test the validity of this hypothesis and reveal the key symmetry-breaking event(s) that lead to polarized hyphal growth.
Organization of growing hyphae at the edge of a mycelial colony. Shown is the advancing edge of an N. crassa (strain FGSC9716) colony growing on Vogel’s minimal medium supplemented with histidine. Thick white arrows indicate dominant extending hyphae. Thin black arrows show examples of secondary lateral branches. Note that branch emergence is suppressed in the immediate vicinity of hyphal tips.
The organization of hyphal tip cells and subapical cells is shown through a schematic depiction of an extending hypha. Note the asymmetric organization of the hyphal tip cell, whereas the subapical cell remains uniformly organized until a new tip (i.e., the incipient branch) is formed. In hyphal tip cells, nuclei (nuc) exhibit a gradient of mitosis, with condensed mitotic nuclei located proximal to the tip. In addition, vacuoles (vac) and endomembranes (G/ER) are more fragmented near the tip. Finally, the tip also houses the Spitzenkörper (SPK) and the polarisome (pol). The enlarged depiction of the hyphal tip shows the exocyst (exo), microtubules (mTs), actin filaments (mFs), and actin patches (AcP). In the subapical cell, mitosis is blocked until a new branch emerges. At that time, nuclei proximal to the branch site resume mitosis. See the text for further details.
Model for the coordination of polarity establishment with growth by monomeric GTPases. (Left) Prior to receiving a strong growth signal (e.g., glucose), Ras is in an inactive GDP state that is unable to activate Rac1 or Cdc42. As a result, the latter GTPases (dots) are uniformly distributed. (Right) Upon reception of a strong growth signal, activated Ras (i.e., Ras-GTP) triggers the activation of Rac1 and Cdc42. Stochastic fluctuations in Rac1-GTP and/or Cdc42-GTP levels lead to local asymmetries in what was initially a uniform distribution. Feedback loops reinforce these asymmetries until a threshold is reached at a given site that then becomes the dominant polarity axis (black arrow). Both endocytic recycling of surface components and enhanced Rac1/Cdc42 GAP activity at nonpolarization sites likely play a key role in reinforcing the polarity axis.