Plant Virus Biodiversity and Ecology

T he International Committee for the Taxonomy of Viruses recognized a global total of about 2,000 species of viruses as of 2005 [1]. However, data on viruses that infect terrestrial organisms, including plants, are sparse, and recent sequencing of samples from marine environments for viruses suggests that 2,000 is a gross underestimate of the total number of viral species on earth [2]. In addition to the paucity of data on viral diversity, little is known about the level of mixed virus infections or temporal patterns of virus accumulation in native plants. For both social and scientifi c reasons, it is important to extend our knowledge of viruses and their hosts beyond those combinations currently known to cause disease to those that may cause disease in the future (i.e., emerging threats). In addition, we need to gain a greater understanding of the potential for mutualistic interactions between viruses and their hosts. That is, we traditionally view viruses as parasites, which is not surprising given their prominent role as pathogens, but this is not always the case. Consider, for example, polydnavirus–braconid wasp interactions, in which the virus carries the essential genes required to suppress the immune system of the lepidopteran hosts of the wasp [3]. This is a frontier that is largely unexplored. The Plant Virus Biodiversity and Ecology (PVBE) project has been initiated to survey the biodiversity of viruses affecting vascular plants, including their endophytic fungi, in The Nature Conservancy's Tallgrass Prairie Preserve of Oklahoma, home to over 700 plant species. Plants are an ideal starting point for studies on virus ecology since they are immobile and can be readily resampled. Viral screening is done using double-stranded RNA analysis, cloning, sequencing, and microarray analysis. The PVBE effort is not directed toward studying economically important or symptomatic plants, both of which are already heavily studied. Information in Figure 1, for example, summarizes the initial plant source of viruses cataloged in the Viral Identifi cation Data Exchange database [4]. Most of the available virus information we have is derived from cultivated crop species, yet these species only comprise a minute fraction of all plant species. And much of this information is derived from symptomatic hosts, yet only a small fraction of viruses probably cause disease [5]. Furthermore, much of the data gathered on viruses come from monocultures, which favors a low diversity. Thus, the data gathered so far have not only been heavily …