The Effect of 2-thiouracil on the Growth of Cells in the Root

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Abstract
Observations have been made with either intact seedling roots or segments of roots of Pea (Vicia faba Meteor) and Maize (Zea mais American yellow horsetooth). Thiouracil, uracil and orotic acid were employed—mostly in concentrations ca. I-5 × 10-3 M. With intact roots, two effects of thiouracil must be distinguished: the first, an immediate stimulation on length increase, and the second (shown with high con-centration), a marked inhibition which in the conditions of the present series of experiments becomes well defined after about I2 hours. A change in growth-rate immediately after a treatment is applied may be taken as an effect on cell expansion. When a second delayed effect is observed, and particularly when it is different from an initial effect, this may be taken as an effect on division. Accordingly the effects observed with the intact root suggest that thiouracil may have a marked inhibiting effect on division and an equally marked stimulating effect on expansion. With the intact root the final strong inhibition is partially relieved when uracil is supplied simultaneously. When supplied alone at the same concentration uracil itself has a slightly inhibiting effect. Uracil, while having no stimulating effect on division, nevertheless mitigates the strong inhibiting effect that thiouracil has on this process. It is probable that thouracil exerts a stimulating effect as a result of incorporation into RNA. The fact that in the intact root uracil may reverse the effect suggests as much. The position with segments is markedly different from that with intact roots. In the latter case the stimulation is reversed by uracil, with the isolated segment it is not. With 2.0-4.0 mm pea segments a non-reversible stimulation with thiouracil is observed. With 1.0-3.0 mm segments, in which thiouracil alone has no effect, the two compounds together give a marked stimulation. In this system when thiouracil alone is available, a stimulation in the basal tissue is compensated by an inhibition in the apical. When uracil is simultaneously applied the inhibition is removed and some of the stimulation due to the thiouracil persists. The inhibition in the youngest tissue may be interpreted in the same terms as the stimulation that is due to a prolongation of the period of growth. A discussion is given of the interpretation of the experimental results. Thiou-racil affects not only the time but also the rate of extension, and for the influence on rate no interpretation can at present be offered.