Growth and Developmental Control in the Model and Pathogenic Aspergilli

Abstract

The filamentous fungi comprise a ubiquitous group of heterotrophic organisms living as saprophytes, parasites, or symbionts. The basis for fungal vegetative growth is the continued and coordinated expansion of a series of fungal cell tips into a linear or complex structure. Fungi differentiate into a variety of structures including spores, which are the effective means of genome protection, survival, and propagation. Spores are also the primary agent for infecting host organisms for many human- and plant-pathogenic fungi. Asexual sporulation is a prevalent mode of reproduction for a diverse group of fungi, which results in the production of vast numbers of mitotically derived spores (reviewed in reference 2). The genus Aspergillus represents the most widespread fungi in our environment, which all reproduce asexually by forming long chains of conidiospores (or conidia) radiating from a central structure known as a conidiophore (Fig. 1) (reviewed in reference 3). The impact of various Aspergillus species on humans runs the range from “good” to “bad.” For instance, several species such as Aspergillus oryzae and Aspergillus niger are used in industry for enzyme production and food processing. In contrast, Aspergillus flavus and Aspergillus parasiticus can produce the most potent naturally present carcinogen, aflatoxin, which can contaminate various foods and feeds (reviewed in reference 43). Moreover, the opportunistic human pathogen Aspergillus fumigatus has become the most prevalent airborne fungal pathogen, causing severe and usually fatal invasive aspergillosis in immunocompromised patients (reviewed in reference 22). Aspergillus nidulans has served as an excellent model system for studying various biological questions, primarily due to the ease of genetic analysis through meiotic (sexual) recombination and the development of sophisticated molecular tools (32). These properties have provided a better understanding of the mechanisms controlling growth, development, secondary metabolism, and other aspects of cell biology in filamentous fungi (3, 35, 50, 57). The availability of the genome sequences of several aspergilli facilitates comparative genomic, genetic, and functional studies. In particular, knowledge and information obtained from a model fungus can be effectively tested in less genetically tractable aspergilli. Recent studies of a number of Aspergillus species have proven that a model fungus can provide a useful framework for understanding the biology of agriculturally and/or medically relevant aspergilli (6, 7, 38, 41, 49, 57). For instance, the novel nuclear protein LaeA has been shown to function as a global regulator of secondary metabolism as well as a regulator of morphogenetic virulence factors in the genus Aspergillus (6, 7). This review summarizes our current understanding of the genetic mechanisms controlling asexual development (conidiation) and vegetative growth in the model (A. nidulans) and pathogenic (A. fumigatus) aspergilli.