Long-term submergence-induced elongation in Rumex palustris requires abscisic acid-dependent biosynthesis of gibberellin1

Abstract

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 GA1 in elongating petioles. We have examined how GA biosynthesis was regulated in submerged plants. Therefore, cDNAs encoding GA-biosynthetic enzymes GA 20-oxidase and GA 3-oxidase, and the GA-deactivating enzyme GA 2-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 GA1 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 GA1. Abscisic acid had no effect on the tissue responsiveness to GA.

Figure 1.

Leaf elongation rate of R. palustris growing in air (white circles) or submerged at t = 0 (black circles). Means of three replicates with ses. Black bars at the x axis represent dark periods. Insert: Relative growth of petioles and leaf blades after 48 h in air (white bars) or submergence (black bars). Means of 12 replicates with ses.

Figure 2.

Concentrations of GA₅₃, GA₁₉, GA₂₀, GA₁, and GA₈ in petioles of R. palustris during submergence. White circles: controls in air; black circles: submerged at t = 0 h. Means of four replicates with ses.

Figure 3.

Relative transcription of RpGA20ox1, RpGA3ox1, and RpGA2ox1 mRNA in petioles of R. palustris during submergence measured with real-time RT-PCR. White circles: controls in air; black circles: submerged at t = 0. Expression of mRNA was quantified relative to the value obtained at t = 0. Means of four replicates with ses.

Figure 4.

Concentrations of GA₅₃, GA₁₉, GA₂₀, GA₁, and GA₈ in petioles during submergence. White bar: controls in air; black bar: submerged (Sub) at t = 0 h; gray bar: submerged at t = 0 h in the presence of 10 μm ABA. Means of four replicates with ses.

Figure 5.

Relative transcription of RpGA20ox1, RpGA3ox1, and RpGA2ox1 mRNA in petioles during submergence in the presence or absence of 10 μm ABA. White bar: controls in air; black bar: submerged at t = 0 h; gray bar: submerged at t = 0 h in the presence of 10 μm ABA. Transcription was quantified relative to the value obtained at t = 0. Means of four replicates with ses.

Figure 6.

Effect of different concentrations of GA (0–100 μm) and ABA (0 μm [black circle] or 10 μm [white circle]) on petiole elongation in submerged R. palustris pretreated with 10 mL 100 μm paclobutrazol 96 h prior to submergence. The length of the second youngest petiole was measured before and after 48 h of treatment and the increase in length was calculated as the difference. Means of 12 replicates with ses.

Figure 7.

Effect of paclobutrazol (paclo), GA, and ABA on submergence-induced leaf elongation in R. palustris measured using linear displacement transducers. Paclobutrazol was administered 96 h prior to submergence. Plants were submerged at t = 0 h. A, Plants were submerged in water (black circles) or in water containing 1 μm GA (white circles), 10 μm GA (black triangles), or 100 μm GA (white triangles). Average se was 0.06 mm h⁻¹ (four replicates). B, Plants were submerged in water after pretreatment with 10 mL 0.1% ethanol (black circles), or 10 mL 1 μm (white circles), 10 μm (black triangles), or 100 μm (white triangles) paclobutrazol. Typical ses were 0.06 mm h⁻¹ (four replicates). C, Plants remained in air (white circles) or were submerged (Sub) in water (black circles), or in water containing 0.3 μm ABA (white triangles) or 3 μm ABA (black triangles). Average ses were 0.05 mm h⁻¹ (six replicates).