The Uredinales : Cytology, Biochemistry, and Molecular Biology

Abstract

Fungi belonging to the order Uredinales are commonly referred to as rust fungi. All members of the Uredinales are parasitic on plants, often causing dramatic losses in various important crop plants (Alexopoulos et al. 1996). Together with the powdery mildew fungi and the downy mildew-causing oomycetes, rust fungi form an extremely successful group of parasites, the obligate biotrophs. The term obligate biotrophic characterizes a specific lifestyle in which the pathogen is absolutely dependent on a living host to complete its life cycle. In turn, the host plant as a whole usually suffers only limited damage over an extended period of time (Staples 2000). By contrast, necrotrophic parasites kill their hosts quickly after infection and subsequently thrive on the dead plant material (Staples 2001). Hemibiotrophic fungi, such as Colletotrichum spp., are characterized by a more or less extended biotrophic phase before switching to necrotrophic growth and killing their host (Perfect and Green 2001). In order to separate the true obligate biotrophic pathogens from hemibiotrophs and necrotrophs we suggest the following six criteria:1. Obligate biotrophs are not culturable in vitro (at least not to a point representing the parasitic phase)2. They form highly differentiated infection structures (variations of the normally tubular cell shape, which are necessary for pathogenesis)3. They have limited secretory activity4. They establish a narrow contact zone separating fungal and plant plasma membranes5. They engage in a long-term suppression of host defense responses6. They form haustoria (specialized hyphae that penetrate host cells).The peculiarities of the lifestyle of obligate biotrophs, paired with their huge economic impact, make rust fungi a versatile field of study at both the fundamental and the applied level. This chapter on Uredinales can by no means cover the complete literature on rust fungi. It is intended to summarize key references, review articles, and books to provide the interested reader witha gateway to more specialized literature on most aspects of research involving rust fungi. Readers new to the field are encouraged to consult the excellent textbooks by Alexopoulos et al. (1996) and Webster and Weber (2007) to gain easier access into the exciting field of mycology in general and obligate biotrophic plant parasites like the rust fungi in particular.