The effect of auxin on the incorporation of [(3)H]thymidine into the DNA of pea epicotyls.

Preincubation of apical segments of etiolated peas (Pisum sativum L.) in indole-3-acetic acid (IAA) results in an inhibition of the incorporation of [(3)H] thymidine ([(3)H]TdR) into DNA. Preincubation in IAA for 4 h led to an inhibition of [(3)H]TdR incorporation only at the highest concentration of IAA tested (10(-4) M). A 20-h preincubation in various concentrations of IAA resulted in a bimodal dose response curve. High concentrations of IAA (10(-4) M) inhibited incorporation by ca. 50%, as did concentrations of about 10(-6)M, but 10(-5) M IAA did not inhibit this incorporation. The absorption of [(3)H]TdR was not affected by preincubation in IAA for either 4 or 20 h. When the apical segments were cut into two portions, the hook with the shoot apex, and the portion remaining below the apical hook, preincubation in IAA for 20 h gave different results for the upper and lower portions of the apical segments. In the lower portion, concentrations of about 5×10(-6) M gave a slight increase in [(3)H]TdR incorporation and 10(-4) M IAA inhibited DNA synthesis. In the upper portion, IAA pretreatment for 20 h resulted in a bimodal dose response curve which was very similar to that found initially for the entire apical segment. Thus the effect of pretreatment of apical segments with IAA depends upon the physiological status of the tissue. The rapidly expanding cells in the lower portion of the apical segments respond to IAA differently than the cells of the upper portion which are principally quiescent.