Studies on the chemotherapy for plant virus diseases.

II. Screening tests of the new antibiotics by leaf-spraying method.

Abstract

Recently the writers have presented the evidence that the new antibiotics separated by the laboratory of Takeda Pharmaceutical Industries, Ltd. inhibit by 20-58per cent the TMV multiplication in detached tobacco leaf. These results have been obtained by leaf-dipping and -floating methods, presumably under the most favorable conditions. The subsequent investigations were designed to apply by leafspraying method the antibiotics which had been proved to be relatively effective in the previous screening tests.
As test plant, detached tobacco leaf-halves were used. Selected leaves of uniform size were inoculated to the lower side with TMV, and cut into halves. One of the halves was sprayed with an antibiotic solution, and another half was left for check. These halves were maintained in a Petri dish, dipping the cut edge in water, as illustrated in figure 1. The assay of TMV produced in leaf tissues was carried out by the chemical analysis of the virus protein, the procedure of which is shown in figure 2, in parallel with the local lesion method using bean leaves. The measured account of virus protein in the leaf showed a moderate correlation with the number of local lesions produced on bean leaves (Figure 3).
From the results of the virus protein analysis, 3 new antibiotics out of 18 tested were found to exhibit 20 per cent or more inhibition to virus multiplication, while higher figures of inhibition was obtained by the local lesion assay (Table 1). This finding raises the serious problem whether the local lesion method represents a real value of virus activity. In order to see the effect of antibiotics upon symptom expression, potted tomato seedlings inoculated with TMV, were sprayed with antibiotics. As represented in table 2, almost no visible effect of the antibiotics upon symptom exhibition was noted, nevertheless yet 20per cent or more virus inhibition was confirmed by the chemical analysis of the virus protein produced within the plant.
The authors' sincerely thanks are due to Dr. R.A. Gray, who kindly supplied noformicin to our laboratory.