Hyphal ontogeny in Neurospora crassa: a model organism for all seasons

Abstract

Filamentous fungi have proven to be a better-suited model system than unicellular yeasts in analyses of cellular processes such as polarized growth, exocytosis, endocytosis, and cytoskeleton-based organelle traffic. For example, the filamentous fungus Neurospora crassa develops a variety of cellular forms. Studying the molecular basis of these forms has led to a better, yet incipient, understanding of polarized growth. Polarity factors as well as Rho GTPases, septins, and a localized delivery of vesicles are the central elements described so far that participate in the shift from isotropic to polarized growth. The growth of the cell wall by apical biosynthesis and remodeling of polysaccharide components is a key process in hyphal morphogenesis. The coordinated action of motor proteins and Rab GTPases mediates the vesicular journey along the hyphae toward the apex, where the exocyst mediates vesicle fusion with the plasma membrane. Cytoplasmic microtubules and actin microfilaments serve as tracks for the transport of vesicular carriers as well as organelles in the tubular cell, contributing to polarization. In addition to exocytosis, endocytosis is required to set and maintain the apical polarity of the cell. Here, we summarize some of the most recent breakthroughs in hyphal morphogenesis and apical growth in N. crassa and the emerging questions that we believe should be addressed.

Keywords: apical growth; filamentous fungi; microtubule; polarity.

Figure 1.. Representation of the localization pattern of proteins participating in hyphal morphogenesis in Neurospora crassa.

( A) Spatial distribution of the polarisome components (SPA-2, BUD-6, and BNI-1), Rho GTPases (RAC, CDC-42, and CDC-24), and septins (CDC-3, CDC-10, CDC-11, CDC-12, and ASP-1) during early developmental stages. ( B) Spatial distribution of the polarisome, Rho GTPases, GEFs (CDC-24 and RGF-1), Rab GTPases (YPT-1, YPT-31, and SEC-4), septins (CDC-3, CDC10, CDC11, CDC12, and ASP-1), actin-binding proteins (TPM-1, LIFE ACT, FIM-1, CRN-1, ARP3, MYO-1, and MYO-5), exocyst components (EXO-70, EXO-84, SEC-3, SEC-5, SEC-6, SEC-8, and SEC-15), and cell wall biosynthetic enzymes (FKS-1, GS-1, and CHS-1 to CHS-7) participating in apical extension of mature hyphae. Each protein displays one or two color tags corresponding to a specific distribution pattern.

Figure 2.. Apical distribution of the cell wall biosynthetic nanomachinery.

FKS-1 and CHS are transmembrane proteins that take precursors from the cytoplasm to incorporate them into a growing chain of β-1,3-glucan or chitin, respectively. The polysaccharide chains are extruded to the cell wall and remodeled by glycosylphosphatidylinositol (GPI) glycosyltransferases that hydrolyze the chains and transfer the cleaved residues to another chain of β-1,3-glucan or chitin. Polysaccharide chains are cross-linked to cell wall glycoproteins.

Figure 3.. Depiction of a Neurospora crassa hyphal apex and subapex with some of the components participating in polarized growth.

Vesicles move along microtubules or actin microfilaments, helped by motor proteins, to reach the Spitzenkörper (Spk). There, vesicles accumulate prior to fusing to the plasma membrane via exocytosis. At the subapex, an actin collar mediates endocytosis and recycling of important polarity factors. Some representative organelles are shown.