The origin of modern plant virology

Plant virology, born with Mayer's work, saw a first (embryonic) phase of development during two decades (1900-1920) with outstanding contributions from Dimitri Ivanovski, Martinus Beijerinck, Erwin Baur and Harry Allard. Between 1920 and 1930 a second phase saw the elaboration of surprising hypotheses concerning the enigmatic nature of viruses and experimental evidence of great stress was obtained. Revolutionary renewal began from the mid-1930s on the basis of a body of knowledge which was organically assembled into the first textbook of plant virology published by Kenneth Smith in 1933. In 1922, the geneticist Hermann Muller put forward the hypothesis that considered viruses as possible genes. The theory was resumed in an apparently independent way by Benjamin Duggar and Joanne Karrer Armstrong in 1923, who considered TMV a biocolloidal self-reproducing protein, like genes appeared to be. This hypothesis, even if neglected by virologists, anticipated by some decades the functional nature of viruses and represented the first conceptual response to virus enigma. Considerable experimental results were obtained by James Johnson, who showed that plants could be infected by different viruses and who introduced a first rational system of plant virus classification. Harold McKinney showed that TMV could mutate. Harold Storey, Kenneth Smith and Harry Severin demonstrated that several viruses could be transmitted by insects and supplied the first interpretation of the relationship between virus and insect. Mayme Dvorak and Helen Purdy obtained the first experimental evidence of the antigenic power of plant viruses. Virus purification, first tentatively accomplished with physical methods, was brilliantly performed by chemical means. Finally, Francis Holmes elaborated the first suitable test to estimate virus infectivity. The evolution of plant virology from an empirical discipline to a biological science took place thanks to the work of one group of American and English scientists who must be regarded as the fathers of modern plant virology.