Advances in understanding plant viruses and virus diseases

Plant viruses have had an impact on the science of virology and on plant pa thology ever since the virus concept was discovered with Tobacco mosaic vir us at the end of the nineteenth century. In this review, we highlight those discoveries. We have divided plant virus research into a "Classical Discov ery Period" from 1883-1951 in which the findings were very descriptive; an "Early Molecular Era" from 1952 to about 1983, in which information was dev eloped that described further properties of the viruses, aided by the devel opment of a number of salient techniques; and the "Recent Period" from 1983 to the present, when techniques have been developed to modify plant virus genomes, to detect nonstructural gene products, to determine the functions of viral gene products, and to transform plants to elicit novel forms of re sistance to viral diseases. In this period, plant virology has played a sig nificant role in formulating an understanding of the mechanisms of gene sil encing and recombination, plasmodesmatal function, systemic acquired resist ance, and in developing methods for pathogen detection. We also attempt to predict the direction plant virology will take in the future.