Recovery. An enigmatic and neglected form of plant resistance to viruses

This short history relates the main events of a phenomenon called "recovery", characterised by the disappearance of symptoms from leaves of plant affected with an initially severe virus disease and by immunity to reinoculation with the same virus. It is a subject first disclosed in 1916 from a rather strange disease of tomato but that was confirmed for a certainty between the 1920s and 1930s in most suitable virus-host combinations. Several authoritative virologists gave different interpretations of this phenomenon so that, for at least two decades, there was on this subject some confusion. The work of Conway Price and Carlyle Bennett directed the problems towards a right understanding of the phenomenon, that had its final legitimation in the 1960s. The mechanism of recovery was long matter of hypotheses, which, however, did not gave solving responses as for neither the disappearance of symptoms nor the presence of virus in recovered tissues. Only in the 1990s, owing to the discovery of the "gene silencing system" some molecular virologists proved that recovery is a consequence of a mechanism operating at the transcript level due to RNA silencing. Today, recovery appears to be an occasional phenomenon shown by a little number of virus-host combinations, but it is not possible to rule out that in the ancient past it was a rather diffuse phenomenon allowing virus to colonise plant without damage.