Another quarter century of great progress in understanding the biological properties of plant viruses

Summary The last quarter century saw a massive application of new molecular and cell biological, and molecular genetical, techniques in research on plant viruses. Hundreds of complete virus genomic nucleotide sequences were analysed, their constituent genes and control elements identified, and similarities and differences revealed in genome organisation, so justifying the modern taxonomic classification of the viruses. Numerous virus replication systems were described and some viral replicases isolated. Especially good progress was made in understanding cell‐to‐cell and long‐distance virus transport within plants; in cloning dominant and recessive plant genes controlling virus resistances, and identifying the cognate viral avirulence factors; in unraveling mechanisms of virus transmission by invertebrate and plasmodiophorid vectors; and in showing through these advances how viruses utilise and subvert endogenous eukaryotic processes. The discovery of gene silencing, and of viral silencing‐suppressor proteins, transformed ideas on how virus replication is controlled, and explained the phenomena of recovery from disease, cross‐protection between virus strains and synergy between unrelated viruses. Transgenic, virus‐resistant plants were created, tested successfully in field conditions and a few commercialised. Factors underlying the appearance of new disease epidemics were identified. Genetic recombination was reported and found to make an important contribution to generating virus variation, and some major virus evolutionary pathways were recognised by comparative genome analysis. The scene is set for further major advances in the coming decades.