Hyphal Growth and Polarity

Abstract

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 Spitzenkorper 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.