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DEVELOPMENT AND HORMONE ACTION

Auxin from the Developing Inflorescence Is Required for the Biosynthesis of Active Gibberellins in Barley Stems¹

School of Plant Science, University of Tasmania, G.P.O. Box 252–55, Hobart, Tasmania 7001, Australia (C.M.W., J.J.S., J.J.R.); and Commonwealth Scientific and Industrial Research Organization Plant Industry, G.P.O. Box 1600, Canberra, Australian Capital Territory 2601, Australia (P.M.C.)

Multiple gibberellins (GAs) were quantified in the stems of intact, decapitated, and decapitated auxin-treated barley (Hordeum vulgare) plants. Removal of the developing inflorescence reduced the endogenous levels of indole-3-acetic acid (IAA), GA₁, and GA₃ and increased the level of GA₂₉ in internodal and nodal tissues below the site of excision. Application of IAA to the excised stump restored GA levels to normal in almost all cases. The conversion of [¹⁴C]GA₂₀ to bioactive [¹⁴C]GA₁ and of [¹⁴C]GA₅ to bioactive [¹⁴C]GA₃ was reduced by decapitation, and IAA application was able to restore conversion rates back to the levels found in intact plants. The amount of mRNA for the principal vegetative 3-oxidase (converting GA₂₀ to GA₁, and GA₅ to GA₃) was decreased in decapitated plants and restored by IAA application. The results indicate that the inflorescence of barley is a source of IAA that is transported basipetally into the internodes and nodes where bioactive GA₁ and GA₃ are biosynthesized. Thus, IAA is required for normal GA biosynthesis in stems, acting at multiple steps in the latter part of the pathway.

¹ This work was supported by the University of Tasmania Institutional Research Grants Scheme.

^* Corresponding author; e-mail John.Ross{at}utas.edu.au; fax 03–62262698.

Received July 17, 2003; returned for revision August 18, 2003; accepted October 29, 2003.

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