Role of Indole-3-acetic Acid and Gibberellin in the Control of Internodal Elongation in Avena Stem Segments

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Abstract
Exogenous application of IAA results in a significant suppression of the linear growth that is promoted by exogenous GA in Avena [cv. Victory] stem segments in a fashion similar to that previously noted in Avena leaf base segments. Treatment with the auxin transport inhibitors, methyl-2-chloro-9-hydroxyfluorene-(9)-carboxylate (CFM) or 2,3,5-triiodobenzoic acid (TIBA), alone promotes elongation growth of the stem segments over that of control growth. This effect may be due to the interference in the transport of native IAA by CFM and TIBA, thus removing the inhibitory effect of native IAA on GA-promoted growth in the internodal intercalary meristem. This results in a greater promotion of internodal growth by native GA. In the presence of (2-chloroethyl) trimethylammonium chloride (CCC), the growth-promoting effects of CFM and TIBA are decreased, and the antiauxin, PCIB (4-chloro-phenoxyisobutyric acid), has no growth-promoting effects whatsoever. These results indicate that the CFM and TIBA-promoted growth require the continuous presence of gibberellins. They further support the view that native IAA acts as a growth suppressor hormone in its regulation of GA-promoted internodal extension in Avena shoots.