ASYMMETRIC RESPONSIVENESS TO ETHYLENE MEDIATES CELL ELONGATION IN THEAPICAL HOOK OF PEAS

The apical hook of dark-grown dicotyledonous seedlings is a protective structure resulting from an inhibition of cell elongation on the inne r portion of the hook. This differential growth response is mediated b y ethylene, Expression of the gene encoding 1-aminocyclopropane-1-carb oxylate oxidase (ACO), the terminal enzyme in ethylene biosynthesis, i s induced by ethylene via a positive feedback loop. Therefore, the ACO transcript can serve as a molecular marker for both ethylene formatio n and ethylene responsiveness. We examined the distribution of ACO mRN A of pea, Ps-ACO1, and of ACO enzyme activity in the apical hook of et iolated pea seedlings. In situ hybridization showed that cells on the inner, concave side of pea hooks accumulated more Ps-ACO1 mRNA than di d cells on the outer, convex side. The distribution of ACO enzyme acti vity followed the same pattern. A direct correlation was observed betw een the cellular distribution of Ps-ACO1 mRNA, ACO enzyme activity, an d the inhibition of cell elongation. Pea seedlings treated with a satu rating concentration of ethylene still accumulated higher levels of th e Ps-ACO1 transcript on the inner side of the apical hook, demonstrati ng an increased responsiveness to ethylene in this tissue. These resul ts indicate that an asymmetrically distributed component of the ethyle ne signal transduction pathway mediates hook formation. Based on exist ing genetic evidence, we propose that this component is downstream fro m the serine/threonine protein kinase CTR1.