Migration des virus dans la plante. Revue bibliographique

Afin de générer une infection, un virus doit migrer dans sa plante hôte. Il existe 2 formes de migration des virus dans la plante : la migration à courte distance et la migration à longue distance. La migration à courte distance s'effectue de cellule à cellule. Au cours de ce processus, le virus migre des cellules infectées aux cellules adjacentes par les plasmodesmes. Plusieurs résultats indiquent que ce type de transport est sous le contrôle partiel du virus. En effet, chez différents phytovirus tels que le TMV, le TRV, le CPMV, l'AMV, le CaMV..., une ou plusieurs protéines, ayant une fonction de transport et dont les mécanismes d'action sont discutés, ont été caractérisées. La migration à longue distance qui s'effectue par les vaisseaux conducteurs est indispensable à l'établissement d'une infection systémique. Les mécanismes de la migration à longue distance restent l'objet de controverses car il n'est pas encore tout à fait établi si ce processus nécessite l'activité d'une fonction de transport ou si c'est un phénomène passif lié au transport des assimilats. Certains virus qui produisent normalement une infection subliminale ou qui restent localisés dans certains tissus se généralisent dans la plante lorsqu'ils sont co-inoculés avec d'autres qui sont eux systémiques. Apparemment, le virus assistant code pour une fonction qui manque au virus assisté dans cette plante, et cette fonction est probablement impliquée dans le transport. Cette revue présente une synthèse des différents aspects du transport des virus dans la plante de même que les mécanismes de la résistance des plantes à la migration des virus.The movement of viruses within the plant: a review. In order to generate a productive infection, a virus must move within its host plant. There are 2 forms of virus movement within a plant, namely, short distance and long distance. The short-distance movement is from cell to cell. During this process, the virus moves to an adjacent cell through the protoplasmic bridges, the plasmodesmata. Several results indicate that this form of spread is virus controlled. Indeed, proteins to which a movement function has been assigned have been identified in a number of plant viruses, such as TMV, TRV, CPMV, AMV and CaMV, to name a few. The long-distance movement takes place in the vascular tissues and it is a phenomenon essential in the establishment of a systemic infection. The mechanism of the long-distance spread is poorly understood. It is not yet clear whether movement in the vascular system requires the activity of a spread function or whether it is a passive process within the phloem elements. In certain combinations of viruses, one virus, which normally only establishes a subliminal or tissue-localized infection, invades other tissues along with the other virus. The "helper" virus apparently provides a function, presumably involved in spread, that the "helped" virus lacks in that plant. This review has sought to deal with the different aspects of virus spread within a plant and also with the mechanisms of plant resistance to virus movement

Evidence for two distinct subgroups of Alfalfa mosaic virus (AMV) from France and Italy and their relationships with other AMV strains

The nucleotide sequence of the putative coat protein open reading frame of seven previously uncharacterized AMV strains from Italy and France was determined and aligned with comparable sequences of other AMV strains (425 L, 425 M, YSMV, S, VRU, 15/64 and Dal. The data set of AMV sequences was used to determine phylogenetic relationships by both a stochastic (stationary Markov model) and a deterministic method (maximum-parsimony) of analysis. The topology of the trees obtained with the two methods was essentially the same showing that all AMV strains clustered in two monophyletic groups. Close clustering of Italian strains in subgroup I and of French strains in subgroup II seems to suggests the effect of geographic distinctiveness of evolutionary dynamics of these AMV strains. This separation did not correlate with differences in host range or symptoms (necrotic or non necrotic) induced in tomato but rather it reflected variations in the amino acid sequence of their CP, which might be related to structural properties of virus particles. A simple and rapid procedure based on the reverse transcriptase-polymerase chain reaction (RT-PCR) followed by ezymatic digestion (RFLP) was developed to identify and classify AMV isolates into the two subgroups. The method applied to a number of other AMV isolates from Italy and France supported their division in two distinct subgroups. This RT-PCR RFLP method may be useful way to investigate the dynamics of AMV populations in nature