Effects of Cytokinins on Growth and Auxin in Coleoptiles of Derooted Avena Seedlings

It is known that removal of the endosperm (deseeding) reduces subsequent auxin production in the oat coleoptile tip and the "regeneration of the physiological tip" (auxin production) in the decapitated coleoptile as determined by the quantities of auxin which are secreted into agar blocks and assayed by the Avena curvature test (10). This decrease in auxin production is associated with parallel decreases in growth rate and geotropic responses of the coleoptile. However, the capacity of the coleoptile to respond to unilaterally applied auxin (the curvature response) is increased in deseeded plants as compared with intact controls, mainly owing to an increase in the time of active bending. Van Overbeek (11) found that removal of the roots of the Avena seedling brought about a similar decrease in auxin production and an increase in the curvature response of the coleoptile to unilaterally applied auxin. Subsequently Went (13, 14) showed that, in tomatoes, shoot growth is dependent upon hormone-like substances supplied by the root. Mothes and co-workers (3, 8) have demonstrated that substances from the roots are required for the maintenance of chlorophyll, protein, and RNA levels in the leaves, and that in detached leaves these factors can be replaced by cytokinins. The above observations, together with the evidence for cytokinins in the transpiration stream of many plants (5-7) have led Mothes and others to conclude that the roots supply cytokinins needed for growth and normal metabolism in the shoot. Earlier work in this laboratory has shown that tobacco tissue cultures will grow without exogenous auxin if supplied with high levels of cytokinins, and we have now found that the tobacco cultures synthesize the auxin they require for growth. In the light of this effect of cytokinin on auxin biosynthesis, it appeared possible that the application of cytokinin would restore normal auxin production and growth rate in the coleoptiles of derooted seedlings. Results of experiments to test this premise are presented below.