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The apical hook of dark-grown dicotyledonous seedlings is a protective structure resulting from an inhibition of cell elongation on the inner portion of the hook. This differential growth response is mediated by ethylene. Expression of the gene encoding 1-aminocyclopropane-1-carboxylate oxidase (ACO), the terminal enzyme in ethylene biosynthesis, is induced by ethylene via a positive feedback loop. Therefore, the ACO transcript can serve as a molecular marker for both ethylene formation and ethylene responsiveness. We examined the distribution of ACO mRNA of pea, Ps-ACO1 , and of ACO enzyme activity in the apical hook of etiolated pea seedlings. In situ hybridization showed that cells on the inner, concave side of pea hooks accumulated more Ps-ACO1 mRNA than did cells on the outer, convex side. The dis- tribution of ACO enzyme activity followed the same pattern. A direct correlation was observed between the cellular distribution of Ps-ACO1 mRNA, ACO enzyme activity, and the inhibition of cell elongation. Pea seedlings treated with a saturating concentration of ethylene still accumulated higher levels of the Ps-ACO1 transcript on the inner side of the apical hook, demonstrating an increased responsiveness to ethylene in this tissue. These results indicate that an asym- metrically distributed component of the ethylene signal transduction pathway mediates hook formation. Based on existing genetic evidence, we propose that this component is downstream from the serine/threonine protein kinase CTR1.

Asymmetric Responsiveness to Ethylene Mediates Cell Elongation in the Apical Hook of Peas