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Plant viruses, discovered over a century ago when the science of virology was born (for review, see Creager, 2002), are obligate parasites on their hosts. Through their life cycle, from virus accumulation to intracellular, local, and systemic movement, viruses utilize plant proteins, normally involved in host-specific activities, for their own purposes. Although the first identification of a host protein interacting with plant viral RNA took place more than 25 years ago (for review, see Buck, 1999; Waigmann et al., 2004), the true complexity of this interaction between plant viruses and their hosts to allow virus accumulation and spread is just now becoming understood.
In addition, the ability of the host to defend itself against virus replication and spread is now known to be much more complex than was thought not long ago. During the early 1990s, the first findings were published suggesting that a plant host defense system targeting viral RNA with extreme sequence specificity existed (e.g. de Haan et al., 1992; Lindbo and Dougherty, 1992). Initially, these observations were not fully understood to represent an RNA-mediated host defense system now referred to as the RNA interference (RNAi), but with time were well differentiated from the better studied host and transgene defense systems mediated through proteins (e.g. the hypersensitive reaction of Nicotiana tabacum cv Xanthi NN against tobacco mosaic virus [TMV] and coat protein-mediated resistance; Beachy, 1999; Marathe et al., 2002). In the last few years, plant molecular virologists and biologists have moved with increasing speed to document the incredibly complex interactions between virus and host factors necessary to allow or defeat virus infections in the presence of RNAi (e.g. Baulcombe, 2004). Thus, plant viruses, besides their traditional role as causative agents of numerous plant diseases, represent molecular tools to examine and dissect diverse basic cellular processes in plants, ranging from intracellular transport and nucleocytoplasmic shuttling (Lazarowitz and Beachy, 1999; Oparka, 2004) to intercellular transport (Waigmann et al., 2004) to gene silencing (Moissiard and Voinnet, 2004).
This focus issue reports new insights into how viruses may utilize host factors to accumulate and move intracellularly to position for intercellular movement (Chen et al., 2005; Ju et al., 2005; Liu et al., 2005). Also, information further illuminating the "give and take" between virus and host factors...