A PRIMER ON PLANT PATHOGENESIS In common with all other terrestrial angiosperms, Arabidopsis provides ecological niches for an array of microorganisms. They may inhabit the aerial parts of the plant or be confined to the roots; they may live within the plant or on its outer surfaces; and they may have detrimental, beneficial, or neutral effects. In this chapter we consider the relationships between Arabidopsis and its pathogens; that is, microorganisms causing overt symptoms of disease. Plant pathogens are in no sense a biologically homogeneous assemblage of organisms, and their diversity is well illustrated by the fungi and bacteria, listed in Tables 1 and 2, respectively, capable of parasitic growth on Arabidopsis. Despite this biological diversity, there is well-founded optimism that studies of Arabidopsis as a host to a variety of pathogens will facilitate a deeper understanding of common processes in microbial pathogenesis of plants. Modes of Parasitism Microbial plant parasites obtain their nutrients either “biotrophically” from living cells or “necrotrophically” from cells which they have killed (Lewis 1973). Examples of the former include fungi, such as powdery and downy mildews, and mollicutes, such as mycoplasma-like organisms which are obligately biotrophic and have not thus far been cultured on any synthetic medium. These obligate biotrophs invade and extensively colonize susceptible plants in such a way that host cells either remain alive or die only after the pathogen has grown on to exploit other living cells. Many of the most destructive plant pathogenic fungi (e.g., Botrytis spp. and Pythium spp.) and...

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