It is generally accepted that addition of a high molecular weight polycation such as poly-L-ornithine or poly-L-lysine to the inoculum is an essential requirement for a high frequency of adsorption and endocytic uptake of plant viruses by isolated mesophyll protoplasts. This was basically similar to an endocytic uptake of polystyrene spheres by the leaf protoplasts, whereas a high molecular weight polyanion, dextran sulfate, reduced adsorption and endocytic uptake of them.
In order to see if such an endocytic uptake of viruses is common in the mechanical inoculation of leaf tissues, primary leaves of bean were rubbed with mixture of tobacco masaic virus (TMV) and poly-L-lysine or dextran sulfate. Addition of poly-L-lysine to the inoculum solution greatly reduced the number of local lesions formed on the inoculated leaves. On the other hand, addition of dextran sulfate to the inoculum increased the number of local lesions. A function of poly-L-lysine is to induce aggregation of TMV by neutralization of the negative charge on virus particles. Aggregation of inoculum viruses reduced efficiency of infection in the mechanical inoculation. TMV aggregates induced by poly-L-lysine were dissolved by addition of the equal volume of dextran sulfate to poly-L-lysine. However, tenfold volume of dextran sulfate was required for full recovery of efficiency of infection of the inoculum. Since the effects of high molecular weight polycation and polyanion in the mechanical inoculation of leaf tissues were apparently opposed to those in the protoplast system, it is difficult that endocytic uptake is regarded as a common mechanism of virus entry in the mechanical inoculation of leaf tissues.
The pH of the inoculum solution is usually around pH 7.0 in the mechanical inoculation. The number of local lesions formed on the inoculated leaves was reduced when hydrogen-ion concentrations in the inoculum solution increased. On the other hand, scanning electron microscopy revealed that the adsorption of inoculum TMV onto the leaf surface was increased by high hydrogen-ion concentrations in the inoculum solution. Thus, a negative correlation was found between the efficiency of infection of the inoculum and the adsorption of TMV particles onto the inoculated leaf surface. Presumably, the virus particles adsorbed on the leaf surface are abortive for establishment of infection.

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