Abscisic acid accumulation maintains maize primary root elongation at low water potentials by restricting ethylene production

Previous work showed that primary root elongation in maize (Zea mays L.) se edlings at low water potentials (psi(w)) requires the accumulation of absci sic acid (ABA) (R.E. Sharp, Y. Wu, G.S. Voetberg, I.N. Saab, M.E. LeNoble [ 1994] J Exp Bot 45: 1743-1751). The objective of the present study was to d etermine whether the inhibition of elongation in ABA-deficient roots is att ributable to ethylene. At a psi(w) of -1.6 MPa, inhibition of root elongati on in dark-grown seedlings treated with fluridone to impose ABA deficiency was largely prevented with two inhibitors of ethylene synthesis (aminooxyac etic acid and aminoethoxyvinylglycine) and one inhibitor of ethylene action (silver thiosulfate). The fluridone treatment caused an increase in the ra te of ethylene evolution from intact seedlings. This effect was completely prevented with aminooxyacetic acid and also when ABA was supplied at a conc entration that restored the ABA content of the root elongation zone and the root elongation rate. Consistent results were obtained when ABA deficiency was imposed using the vp5 mutant. Both fluridone-treated and vp5 roots exh ibited additional morphological symptoms of excess ethylene. The results de monstrate that an important role of ABA accumulation in the maintenance of root elongation at low psi(w), is to restrict ethylene production.