The role of abscisic acid in ethylene-induced elongation

Rumex palustris responds to submergence with an upward bending of the petioles (hyponastic growth) followed by a strong enhancement of elongation of the entire petiole. These two growth responses together help the plant to regain contact with the water surface, so that gas exchange to the submerged tissue can be restored. The underwater escape response is initiated by ethylene, which accumulates inside the submerged tissue (Voesenek et al., 1993). Almost immediately after submergence, ethylene induces a reduction in the concentration of abscisic acid (ABA, Chapter 2), via an increase in ABA catabolism and a depression of ABA biosynthesis. Externally applied ABA can almost fully inhibit underwater growth, while an artificial reduction in ABA enhances the elongation response by shortening the distinct lag phase between the start of submergence and the start of faster elongation growth. Submergence of the submergence-intolerant R. acetosa, a species related to R. palustris but incapable of enhanced submergence-induced elongation, does not lead to a decrease in ABA (Chapter 2). ABA is still a negative regulator of enhanced growth even in R. acetosa, as fluridone treatment permits enhanced underwater extension growth in submerged R. acetosa. Air-grown R. acetosa does not show enhanced elongation upon fluridone treatment. Since pre-treatment with the ethylene action inhibitor 1-MCP does not prevent underwater elongation in fluridone-treated R. acetosa, there must be a submergence signal other than ethylene that stimulates the petioles of R. acetosa to extend rapidly. Whatever the signal eventually proves to be, it is seemingly inactive in R. palustris, where submergence-induced elongation can be inhibited fully by 1-MCP. In R. palustris, submergence induces an up-regulation of the concentration of GA1. This up-regulation results predominantly from increased transcription of RpGA3ox1, a gibberellin biosynthesis gene the product of which catalyses the final enzymatic step to form GA1 (Chapter 4). Submergence induces an acidification of the apoplast of petiole cells, mediated by an increase in H+ efflux activity (Chapter 5). This acidification process facilitates the action of cell wall-loosening proteins, such as the expansin RpExp1. The submergence-induced apoplastic acidification is found to be independent of ABA as it takes place prior to a decrease in ABA concentration, and externally applied ABA fails to inhibit the acidification (Chapter 2 and 5). Thus, in the signal transduction pathway of submergence-induced elongation, apoplastic acidification and RpEXP1 expression are placed downstream of ethylene, but parallel to ABA (Fig. 1). Hyponastic growth Hyponastic growth is an important component of the underwater escape mechanism in R. palustris. The response is regulated by ABA. Application of this hormone to submerged plants inhibits hyponastic growth, while pre-treatment with fluridone induces an enhanced hyponastic response in submerged or ethylene-treated plants (Chapter 3). Arabidopsis thaliana responds to ethylene (5 µL L-1) with a hyponastic growth response (Millenaar et al., unpublished). This process is thought to consist of enhanced cell extension on the abaxial side of the petiole. The magnitude of this response varies strongly Columbia-0 (Col), showing strong hyponastic growth, and Landsberg erecta (Ler) showing little hyponasty. Aba negatively influence petiole angles. ABA and fluridone were less effective in altering hyponasty in Ler than in Col. Ethylene did not decrease ABA in petioles of ethylene-treated Arabidopsis. However, ethylene did increase the transcript abundance of ABI1, a negative regulator of ABA signal transduction