Long-term submergence-induced elongation in Rumex palustris requires abscisic acid-dependent biosynthesis of gibberellin

Rumex palustris (polygonceae) responds to complete submergence with enhanced elongation of its youngest petioles. This process requires the presence of gibberellin (GA) and is associated with an increase in the concentration of GA 1 in elongating petioles. We have examined how GA biosynthesis was regulated in submerged plants. Therefore, cDNAs encoding GA-biosynthetic enzymes GA20-oxidase and GA3-oxidase, and the GA-deactivating enzyme GA2-oxidase were cloned from R. palustris and the kinetics of transcription of the corresponding genes was determined during a 24 h submergence period. The submergence-induced elongation response could be separated into several phases: (1) during the first phase of 4 h, petiole elongation was insensitive to GA; (2) from 4 to 6 h onward growth was limited by GA; and (3) from 15 h onward underwater elongation was dependent, but not limited by GA. Submergence induced an increase of GA 1 concentration, as well as enhanced transcript levels of RpGA3ox1. Exogenous abscisic acid repressed the transcript levels of RpGA20ox1 and RpGA3ox1 and thus inhibited the submergence-induced increase in GA(1). Abscisic acid had no effect on the tissue responsiveness to GA.