Plant viruses and the recent discovery of unforeseen basic cellular processes.

Given their small genome size, the biological cycle of plant viruses is tightly integrated with the cellular processes of their host plants, so that studies of the viral biology will often provide insights into basic cellular processes. In the last decade, two such unforeseen mechanisms were discovered. One concerns intercellular communications: for their movement in infected plants, viruses use channels (plasmodesmata, phloem) also used by the plant to exchange information-rich molecules (proteins, RNAs) between cells. The second phenomenon concerns the existence, in plants, of an anti-viral defence mechanism based on the specific degradation of RNA molecules in the cytoplasm. This same mechanism, also allowing the regulation of gene expression (post-transcriptional gene silencing, PTGS) now appears to be widespread in pluricellular organisms. Besides their general interest, these new results modify drastically our vision of interactions between plant and viruses and raise numerous new research questions.