Studies on Phytoalexins: The Relationship between Actinomycin D and Ribonucleic Acid Synthesis during the Induction of Phaseollin in the French Bean (Phaseolus vulgaris L.).

Actinomycin D stimulated phaseollin production in endocarp tissues of the French bean (Phaseolus vulgaris L.), maximum production being obtained with 25 to 30 micrograms per milliliter of antibiotic. Under these conditions, net incorporation of (3)H-uridine into total cell ribonucleic acid was inhibited by more than 80% over a 6-hour induction period. If allowance was made for a 2-hour lag in the action of actinomycin D, inhibition of incorporation was greater than 95%. Contrary to other reports, no evidence was obtained of an increased formation of any specific ribonucleic acid fraction. Actinomycin D applied in the cold (4 C) was not found to be effective in stimulating phaseollin production. When applied in this way, actinomycin D did not affect induction of phaseollin by a fungal peptide, Monilicolin A, although ribonucleic acid synthesis was inhibited by more than 95%. It is suggested that the induced formation of phytoalexins may not be dependent on increased ribonucleic acid synthesis as has previously been claimed.Other experiments indicated that the apparent effects of actinomycin D on ribonucleic acid synthesis could be influenced by the choice of precursor used to label ribonucleic acid and by the order of addition of precursor and antibiotic to the plant tissue. These effects were only observed in the period immediately following application of actinomycin D. It is suggested that such effects could critically influence the results obtained in short term experiments and may explain some differences in reported action of actinomycin D from different laboratories.