Hyphal morphogenesis in Aspergillus nidulans

The formation of hyphae that grow solely by apical extension is a defining feature of filamentous fungi. Hyphal morphogenesis involves several key steps, including the establishment and maintenance of a stable polarity axis, as well as cell division via the deposition of septa. Several filamentous fungi have been employed in attempts to decipher the mechanisms underlying these steps. Amongst these fungi, Aspergillus nidulans has proven to be a particularly valuable model. The genetic tractability of this fungus coupled with the availability of sophisticated post-genomics resources has enabled the identification and characterization of numerous genes involved in hyphal morphogenesis. Here, we summarize current progress towards understanding the function of these genes and the mechanisms involved in polarized hyphal growth and septation in A. nidulans. We also highlight important areas for future investigation. a 2009 The British Mycological Society. Published by Elsevier Ltd. All rights reserved. 1. Overview permit comparison to analogous processes in animals and Despite their apparent simplicity, fungal hyphae are remarkable structures that allow filamentous fungi to colonize a diverse array of habitats. The characteristic feature of a hypha is the localization of growth to the extreme tip, leading to the formation of an elongated tube capable of impressive extension rates. The formation of apical and lateral branches increases the surface area colonized by a hyphal network. The partitioning of hyphae into cellular units by cross-walls known as septa permits compartmentalization of functions and is thought to play a key role in supporting the development of reproductive structures that bear spores. A deeper understanding of the molecular basis of hyphal morphogenesis is important at two levels. First, it would yield meaningful insight that could be exploited to allow better control of fungal growth, whether limiting the growth of a pathogen or optimizing the growth of an industrial strain that produces valuable compounds. Second, it would 38; fax: þ1 402 472 3139. (S. D. Harris). ritish Mycological Society plants. This might be particularly relevant to other highly polarized cell types in these kingdoms, including neurons and pollen tubes, with a view towards the elucidation of common principles underlying this unique mode of growth. Accordingly, there is increasing interest in the identification and characterization of functions required for the establishment and maintenance of hyphal polarity, the formation of branches, and septation. One of the fungi that has proven to be a veritable ‘workhorse’ in this effort is Aspergillus nidulans, which is a widely recognized model fungus known for its genetic tractability and ease of manipulation. In this review, we summarize progress towards understanding the molecular basis of hyphal morphogenesis in A. nidulans. 2. A. nidulans as a model organism A. nidulans (teleomorph Emericella nidulans) is an ascomycete fungus that belongs to the class Eurotiomycetes and the order . Published by Elsevier Ltd. All rights reserved. Hyphal morphogenesis 21 Eurotiales. Over the past w50 y, the seminal efforts of a long list of notable research scientists have elevated A. nidulans to the status of a model organism. Befitting this status, numerous methods have been developed to facilitate the efficient analysis of gene function in A. nidulans. Foremost amongst these is the ability to use classical genetic approaches to identify and characterize interesting sets of mutants (Todd et al. 2007a,b), including conditional mutations that affect essential functions. Additional methods, such as PCR-mediated gene replacement and heterokaryon rescue, permit the targeted analysis of specific genes, including those whose deletion might be lethal (Osmani et al. 2006; Szewczyk et al. 2006). Finally, a diverse collection of fluorescent reagents and probes (i.e., GFP-based markers) enable the real-time imaging of several important proteins in growing hyphae. Collectively, through the use of these methods, numerous A. nidulans proteins have been functionally implicated in some aspect of hyphal morphogenesis (Harris et al., 2009). In many cases, these proteins were selected based on their homology to proteins known to be involved in the polarized morphogenesis of other organisms, primarily the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. In other examples, the proteins were identified as a result of unbiased genetic screens that focused on mutants that exhibit defects in polarity establishment, polarity maintenance, septum formation, or nuclear division. Notably, these screens often lead to the identification of proteins with no previously suspected role in hyphal morphogenesis. 3. Features of hyphal morphogenesis in A. nidulans Like most filamentous fungi, A. nidulans initiates a new round of growth through the process of spore germination. The events underlying the germination of asexual conidiospores leading to the growth of a mature hypha have been characterized extensively (Harris 1997; d’Enfert 1997; Osherov and May 2001; Momany 2002; Harris 2006). It is presumed that a similar sequence of events accompanies the germination of sexual ascospores, though this has not been investigated in any detail. In A. nidulans, the first step in spore germination is the breaking of dormancy, which is accompanied by spore rehydration, initiation of translation, resumption of metabolic activity, and isotropic expansion of the cell surface. The next step is the establishment of a polarity axis upon which subsequent cell surface expansion and cell wall deposition are directed. The stabilization of this axis results in the maintenance of polarity and enables the formation of a germ tube that ultimately matures into a hypha. Hyphae are populated by multiple nuclei due to a series of parasynchronous nuclear divisions (note that conidiospores are uninucleate, whereas ascospores are binucleate). Nuclear division is coordinated with growth such that each division is coupled to a doubling of cell mass, and the entire process is referred to as the duplication cycle (Fiddy and Trinci 1976; Harris 1997). Once hyphae reach a certain volume, which appears to vary depending on growth conditions, they are partitioned by the formation of the first septum. Notably, septation is coordinated with nuclear division and the first septum typically forms nears the basal end of a hypha near the junction with the conidiospore. Following the first septation event, each passage through the duplication cycle is terminated by the formation of septa in the hyphal tip compartment. On the other hand, sub-apical compartments enter a period of mitotic quiescence that is eventually broken by the formation of a branch that generates a new hypha. Branch formation requires the establishment and maintenance of a new polarity axis, and likely recapitulates many of the events involved in spore germination. For the remainder of this review, we will focus on specific features of hyphal morphogenesis in A. nidulans, with emphasis placed on what is known about the underlying mechanisms.