Plant viruses and the recent discovery of unforeseen basic cellular processes

Given their small genome size, the biological cycle of plant viruses is tig htly integrated with the cellular processes of their host plants, so that s tudies of the viral biology will often provide insights into basic cellular processes. in the last decade, two such unforeseen mechanisms were discove red. One concerns intercellular communications: for their movement in infec ted plants, viruses use channels (plasmodesmata, phloem) also used by the p lant 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 expre ssion (post-transcriptional gene silencing, PTGS) now appears to be widespr ead in pluricellular organisms. Besides their general interest, these new r esults modify drastically our vision of interactions between plant and viru ses and raise numerous new research questions. (C) 2001 Academie des scienc es/Editions scientifiques et medicales Elsevier SAS.