Advances in understanding hyphal morphogenesis: Ontogeny, phylogeny and cellular localization of chitin synthases

Abstract

Hyphal morphogenesis is largely determined by the mode the cell wall is synthesized. One of the main structural components of the cell wall is the chitin microfibril, whose synthesis is catalyzed at the cell surface by an organized but not fully understood complex of chitin-synthesizing enzymes. Genetic studies have identified several chitin synthase genes ( chs) among different fungi. In each given species, several chitin synthases (CHS) may be present. These have been assigned to different classes (I–VII) on the basis of characteristic amino acid sequences. A revised phylogenetic scheme of fungal CHS is presented but there was no apparent correlation between CHS class and a specific cell function or cell cycle stage. The availability of methodology to make genetic fusions between CHS and green fluorescent protein (GFP) and to follow them in living cells with high-resolution confocal microscopy and widefield fluorescence microscopy has made it possible to study the location and dynamics of different CHS in several fungi. Among these, Neurospora crassa was recently used to analyse the spatial distribution and role of chitin synthases in hyphal tip growth. Here we summarise recent advances in this area with particular emphasis on N. crassa. CHS-3, CHS-6 and more recently CHS-1 are abundantly present in the distal regions of the hypha and contained in membranous structures of different shapes from spheres to elongated tubes; as the GFP–CHS tagged structures advance towards the tip, they begin to disintegrate. In the subapical region GFP–CHS was not found in large organelles; it only occurred as fine punctuate fluorescence. These minute structures are probably chitosomes. Finally, at the tip there is always a conspicuous accumulation of GFP–CHS in the Spitzenkörper core where microvesicles are known to accumulate. The collective evidence points to CHS travelling to its destination at the hyphal apex via a secretory route distinct from the conventional ER–Golgi route. The accumulation of CHS microvesicles at the Spk reinforces the view that this structure plays a pivotal role in cell wall growth and hyphal morphogenesis.