Architecture and development of the Neurospora crassa hypha – a model cell for polarized growth

Abstract

Neurospora crassa has been at the forefront of biological research from the early days of biochemical genetics to current progress being made in understanding gene and genetic network function. Here, we discuss recent developments in analysis of the fundamental form of fungal growth, development and proliferation – the hypha. Understanding the establishment and maintenance of polarity, hyphal elongation, septation, branching and differentiation are at the core of current research. The advances in the identification and functional dissection of regulatory as well as structural components of the hypha provide an expanding basis for elucidation of fundamental attributes of the fungal cell. The availability and continuous development of various molecular and microscopic tools, as utilized by an active and co-supportive research community, promises to yield additional important new discoveries on the biology of fungi.

Introduction

Our understanding of the morphogenesis of filamentous fungi is progressing rapidly (with >15 000 publications in just the last 5 y). The wealth of genetic information, availability of mutants and the progress made in live imaging techniques, coupled with biochemical analysis, have significantly contributed to the progress made in understanding one of the most characteristic and fundamental forms of fungal growth, development and proliferation – the hypha. Along with neurons and pollen tubes, hyphae are the most highly polarized cell forms known (Palanivelu and Preuss, 2000, Borkovich et al., 2004, Harris, 2006, Ischebeck et al., 2010). On the one hand, much has been discovered about the role and function of hyphal elements that are shared with many other eukaryotic cell types, albeit in the context of a syncytium. On the other hand, many structures and functions unique to filamentous fungi have now been identified and analyzed. Thus, the accumulating information, along with the technological advances enhancing our capabilities of probing and analyzing both existing and new directions, make the compilation of this review timely. Featuring N. crassa, we intend this review to serve as an updated resource and a source of ideas for future studies on the fungal filament. Furthermore, the increased interest in fungal pathogens of humans, animals and plants, along with the use of filamentous fungi in biotechnology and bioprospecting warrants the in-depth understanding of the hyphal filament as the fundamental unit in these organisms.

Neurospora crassa has been an excellent model organism for eukaryotic genetics and biochemistry and one of the workhorses for fungal cell biology research. While Saccharomyces cerevisiae is often referred to as a good representative of the Fifth Kingdom, it has become increasingly apparent that - despite its virtues - the yeast cell represents only a minor fraction of the fungal kingdom in many morphological and biochemical aspects. Most fungi have a highly branched filamentous morphology and occupy a much broader spectrum of habitats. The rapid (∼4 mm hr⁻¹) filamentous growth habit of N. crassa is the result of a strongly polarized mechanism culminating in the biogenesis of the tubular cell wall. Seven decades of pioneering research on the biology of the hypha performed with this model organism (Beadle and Tatum, 1945, Garnjobst and Tatum, 1967, Collinge and Trinci, 1974, Vollmer and Yanofsky, 1986, Metzenberg and Glass, 1990, Yarden et al., 1992, Plamann et al., 1994, Steinberg and Schliwa, 1995, Seiler et al., 1997, Riquelme et al., 1998, Davis, 2000, Perkins et al., 2001, Seiler and Plamann, 2003, Gavric and Griffiths, 2003), have proven N. crassa to be a rewarding model fungus for experimental work – work that continues today in more than 30 laboratories around the world. Extensive work has been performed, utilizing N. crassa, on genome defence, DNA repair and recombination, on light and circadian regulation as well as on mitochondrial protein import and biogenesis, but because of the scope of this article we refer readers interested in this subject to recent reviews and genome-wide studies that describe the relevant findings and address the challenges in these fields (Galagan and Selker, 2004, Ninomiya et al., 2004, Neupert and Herrmann, 2007, Jinhu and Yi, 2010, Chen et al., 2010, Vitalini et al., 2006, Smith et al., 2010, Chen et al., 2009, Borkovich et al., 2004).

The entire community of fungal biologists has benefited from useful resources derived from the Functional Genomics and Systems Biology Project, a project promoted by members of the Neurospora community that culminated in the publication of the N. crassa genome draft sequence (Galagan et al., 2003, Borkovich et al., 2004, Dunlap et al., 2007). Some valuable tools include a collection of single-gene deletion mutants (Colot et al. 2006), as well as expression and tiling microarrays (Greenwald et al., 2010, Hutchison et al., 2009, Kasuga and Glass, 2008), and single nucleotide polymorphism data for widely used strains (Lambreghts et al. 2009). The publication of the first high-quality draft genome of a filamentous fungus was just the beginning. In fact, ∼200 fungal genome sequences will soon be available (for details see http://fungalgenomes.org/wiki/Fungal_Genome_Links). This number will greatly increase in the near future as high-throughput sequencing allows affordable sequencing and de novo assembly of fungal genomes (Nowrousian 2010). In addition to genetics-based developments and tools, techniques that have progressed our abilities to study the cell biology of N. crassa have also evolved. Fluorescent protein (FP) labelling was successfully developed for N. crassa separately by two different labs in 2001 and 2002 (Freitag et al., 2001, Fuchs et al., 2002), and made widely available to the Neurospora community in 2004 (Freitag et al. 2004). Currently, an ever-increasing number of strains with fluorescently labelled proteins (Table 1) are readily available from the Fungal Genetics Stock Center (http://www.fgsc.net/).

By providing a critical and current evaluation of research on one of the most advanced model systems useful to all researchers studying filamentous fungi, we hope to stress opportunities for future research directions and identify important challenges.

The ability to form polarized cell types is not only a fundamental property of filamentous fungi, but is also one of the key attributes that contributes to their success in inhabiting beneficial niches and/or avoiding detrimental ones. As such, the development of hyphae is one of the bases for fungal proliferation. In many cases, hyphal development can be a prerequisite for the formation of additional cell types that, along with hyphae, are involved in growth, development and propagation.

Amongst at least 28 distinct morphological cell types described in Neurospora crassa (Bistis et al. 2003), more than six can be designated as hyphae. These forms of hyphae encompass both asexual and sexual development of this fungus. The hyphal cell types described include (for more details see Bistis et al. 2003): Leading hypha (wide, fast growing with subapical branching; Robertson 1965); Trunk hypha (in the colony interior); Fusion hypha and conidial anastomosis tubes hypha (bridge between hypha and between conidia; Glass et al., 2004, Roca et al., 2005); Aerial hyphae (growing away from the medium and required for macroconidiation); Enveloping (or ascogonial investing) hyphae (engulf the ascogonium; Read, 1983, Read, 1994); Trichogyne (exhibits a positive tropism towards cells of opposite mating type; Bistis 1981); Ascogenous hyphae (contain nuclei of both mating types; Raju, 1980, Raju, 1992).

Many structural and regulatory elements required for, or involved in, hyphal development have been identified over the years, via classical genetics analysis of morphological mutants, molecular and biochemical approaches, and advanced microscopy.