A Method for Testing the Specificity of Inhibitors of Deoxyribonucleic Acid Synthesis in Growth Studies

Inhibitors of DNA synthesis are often used to infer the role of DNA synthesis in growth regulation (1, 11). The usefulness of such an inhibitor depends upon the extent to which it is both effective and specific. The requirement of effectiveness demands that the inhibitor truly prevent subsequent DNA synthesis. The requirement of specificity demands that essentially all the growth-inhibition result from inhibition of DNA synthesis; in other words, there should be an absence of "nonspecific growth-inhibition," which we here define as a reduction in growth that is not caused directly or indirectly by the inhibition of DNA synthesis. Whereas the requirement of effectiveness is often tested, the requirement of specificity is seldom if ever checked, probably owing to the lack of a suitable method. We here describe a method for testing the specificity of inhibitors of DNA synthesis. We also show how this test of specificity, when coupled with an independent test of effectiveness, permits conclusions concerning usefulness of chemicals as specific inhibitors of DNA synthesis in studies of growth. We test the specificity of inhibitors by examining their actions in two systems that do not require DNA synthesis. These systems are the germination of heavily irradiated lettuce seeds, which occur without detectable DNA synthesis (8), and the growth of wheat "gamma-plantlet" seedlings, which does not require DNA synthesis (5). Since both systems gave similar results, we present only the findings for wheat. Despite the heavy seed irradiation and because of the absence of many of the typical cytogenetic effects of ionizing radiations (6). gammaplantlet growth is remarkably normal in many respects, including sensitivity to growth regulators (5, 7, 8. 12). Since in gamma-plantlets there is no DNA synthesis to inhibit, any growth-inhibition that is caused by an inhibitor must be a nonspecific growth-inhibition. The inhibitors tested were FdUrd' (4, 10 11), hydroxyurea (1, 13). and phenethyl alcohol (2, 11). The wheat, Triticium vulgare Vill. (Triticuiin aestivurm L.)