The susceptibility of Rhoeo discolor to infection by tobacco mosaic virus.

Gordon and Smith (1) reported that leaves of Rhoeo discolor, although insusceptible to infection by whole tobacco mosaic virus, become infected when inoculated with the virus ribonucleic acid. If established, this would be perhaps the strongest possible circumstantial evidence that infection by tobacco mosaic virus necessarily entails the ribonucleic acid separating from the protein and that some plants resist infection because they do not contain systems able to disrupt the virus particles. The result of the experiment described by Gordon and Smith, however, was not fully conclusive, because the inoculum with tvhich they demonstrated infection contained 20 mg per liter, whereas the apparently ineffcctivc virus inoculum was 0.5 mg per liter, which is only 0.00125 as much nucleic acid. Infection of such hosts as tobacco, in which the virus readily spreads from cell to cell and reaches a high concentration, is easily demonstrated with di1ut.e inocula, but this is not necessarily so with a host in which spread is limited and the virus content per cell is small. Detecting virus multiplication in such plants will depend more on the number of initial infection sites, which in turn will tlcpend on the concentration of virus in the inoculum.