Interaction between auxin, cytokinin and ethylene in the regulation of root growth in Pisum sativum L

Abstract: The effects of exogenous auxins (indole-3-acetic acid (IAA), indole-3-butyric acid (IBA) and 2,4-dichlorophenoxyacetic acid (2,4-D)), cytokinins (benzylaminopurine (BAP) and zeatin) and ethylene, applied as the precursor 1 -aminocyclopropane- 1-carboxylic acid (ACC), on the root growth of pea (Pisumsativum L.) seedlings were investigated. Ethylene production was determined in the excised root tips of intact seedlings that had been growing in the test solutions for specified time periods. Treatment of roots with auxins, cytokinins or ACC inhibited root elongation and caused swelling (increased diameter) of the part of the roots formed in the presence of the regulator. Both auxins and cytokinins are known to stimulate ethylene production in plant materials. Over the last 20 years there have been many proponents of the hypothesis that the inhibition of root elongation caused by auxin is mediated by an auxin-induced increasein ethylene production in the roots. In an investigation with IAA at concentrations causing partial inhibition, three lines of evidence indicated that the hypothesis was not valid for intact roots: (1) IAA caused only a small, transient increase in ethylene production, but the inhibition of root growth was longlasting; (2) ethylene produced from added ACC only caused a similar degree of inhibition in cases where its production was increased several times and lasted throughout the growth period; (3) inhibitors of ethylene biosynthesis (cobalt ions) and action (silver ions) counteracted the effects of added ACC but notof IAA. In addition, ethylene and auxins interacted synergistically in inducing elongation and swelling of the roots. The cytokinin BAP caused a more prolonged increase in ethylene production than the auxins, and the effects of this regulator on elongation and swelling could be reversed, especially by cobalt ions.Ethylene produced from ACC decreased the endogenous level of IAA, whereas BAP increased the IAA amount per root tip. The degree to which BAP inhibited elongation depended on the concentration of calcium in the growth medium.