The potyvirus group [named after its type member, potato virus Y (PVY)] is the largest of the 34 plant virus groups and families currently recognized (Ward & Shukla, 1991). It contains at least 180 definitive and possible members (or 30% of all known plant viruses) which cause significant losses in agricultural, pasture, horticultural and ornamental crops (Ward & Shukla, 1991). These viruses are unique in the diversity of inclusion bodies that are formed during the infection cycle (see Lesemann, 1988). A feature shared by all potyviruses is the induction of characteristic pinwheel or scroll-shaped inclusion bodies in the cytoplasm of the infected cells (Edwardson, 1974). These cylindrical inclusion (CI) bodies are formed by a virus-encoded protein and can be considered as the most important phenotypic criterion for assigning viruses to the poty-virus group (Milne, 1988; Shukla et al., 1989; Ward & Shukla, 1991).

Plant viruses are biotrophic pathogens that need living tissue for their multiplication and thus, in the infection–defence equilibrium, they do not normally cause plant death. In some instances virus infection may have no apparent pathological effect or may even provide a selective advantage to the host, but in many cases it causes the symptomatic phenotypes of disease. These pathological phenotypes are the result of interference and/or competition for a substantial amount of host resources, which can disrupt host physiology to cause disease. This interference/competition affects a number of genes, which seems to be greater the more severe the symptoms that they cause. Induced or repressed genes belong to a broad range of cellular processes, such as hormonal regulation, cell cycle control and endogenous transport of macromolecules, among others. In addition, recent evidence indicates the existence of interplay between plant development and antiviral defence processes, and that interference among the common points of their signalling pathways can trigger pathological manifestations. This review provides an update on the latest advances in understanding how viruses affect substantial cellular processes, and how plant antiviral defences contribute to pathological phenotypes.

Genome structure and sequence are notably conserved between members of the familyPotyviridae. However, some genomic regions of these viruses, such as that encoding the P1 protein, show strikingly high variability. In this study, some partially conserved motifs were identified upstream of the quite well-conserved protease domain located near the P1 C terminus. The irregular distribution of these motifs suggests that the potyviral P1 proteins have undergone complex evolutionary diversification. Evidence was found of recombination events in the P1 N-terminal region, similar to those reported in potyviruses of the bean common mosaic virus subgroup, but also affecting other potyviruses. Moreover, intergeneric recombination events affecting potyviruses and ipomoviruses were also observed. Evidence that these recombination events could be linked to host adaptation is provided. Specific sequence features and differences in net charge help to classify the P1 proteins of members of the familyPotyviridaeinto two groups: those from potyviruses and rymoviruses and those from tritimoviruses. The ipomovirusCucumber vein yellowing virushas two P1 copies arranged in tandem, the most N-terminal one being of the potyvirus type and the other being of the tritimovirus type. These findings suggest that both recombination and gene duplication have contributed to P1 evolution and helped to facilitate successful adaptation of members of the familyPotyviridaeto a wide range of host species.