Endogenous abscisic acid as a key switch for natural variation in flooding-induced shoot elongation

Abstract

Elongation of leaves and stem is a key trait for survival of terrestrial plants during shallow but prolonged floods that completely submerge the shoot. However, natural floods at different locations vary strongly in duration and depth, and, therefore, populations from these locations are subjected to different selection pressure, leading to intraspecific variation. Here, we identified the signal transduction component that causes response variation in shoot elongation among two accessions of the wetland plant Rumex palustris. These accessions differed 2-fold in petiole elongation rates upon submergence, with fast elongation found in a population from a river floodplain and slow elongation in plants from a lake bank. Fast petiole elongation under water consumes carbohydrates and depends on the (inter)action of the plant hormones ethylene, abscisic acid, and gibberellic acid. We found that carbohydrate levels and dynamics in shoots did not differ between the fast and slow elongating plants, but that the level of ethylene-regulated abscisic acid in petioles, and hence gibberellic acid responsiveness of these petioles explained the difference in shoot elongation upon submergence. Since this is the exact signal transduction level that also explains the variation in flooding-induced shoot elongation among plant species (namely, R. palustris and Rumex acetosa), we suggest that natural selection results in similar modification of regulatory pathways within and between species.

Figure 1.

Ethylene controls variation in submergence-induced elongation of the third-oldest petiole of the fast and slow accessions of R. palustris. A, Petiole elongation rates under submerged and drained control conditions. Submergence started at t = 0. Growth rates were calculated every hour from length data monitored by linear variable displacement transducers. Black bars indicate 8-h dark periods. B, Petiole elongation rates over 3 d of drained control or submerged conditions without or with 1-MCP pretreatment to block ethylene perception. C, Petiole elongation rates under submerged and drained control conditions after pretreatment with 1-MCP. The submergence treatment started at t = 0. Growth rates were calculated every hour from length data monitored by linear variable displacement transducers. Black bars indicate 8-h dark periods. D, Petiole elongation in response to different exogenous ethylene concentrations (insert left: for comparison, petiole elongation rates over 3 d under drained and submerged conditions [black bar: fast accession; white bar: slow accession]). All data are mean ± se, n = 6 for A to C, n = 4 for D. For clarity reason, se is not shown in A and C. se varies from 2.9 × 10⁻³ mm h⁻¹ to 9.7 × 10⁻² mm h⁻¹ (A) or 1.3 × 10⁻³ mm h⁻¹ to 9.6 × 10⁻² mm h⁻¹ (C). Different letters indicate significant differences (Games-Howell test for B and Tukey test for D, P < 0.05). For statistics of the line charts in A and C, see Supplemental Table S1.

Figure 2.

Concentrations of soluble sugars (A and B), fructans (C and D), and starch (E and F) of the shoots (A, C, and E) and roots (B, D, and F) of the slow (white symbols) and fast (black symbols) accessions of R. palustris under submerged (circles) and drained control (triangles) conditions. The submergence treatment started at t = 0. Data are mean ± se, n = 4. Black bars indicate 8-h dark periods. For statistics, see Supplemental Table S2. DW, Dry weight.

Figure 3.

ABA is differentially regulated upon submergence in the fast and slow accessions of R. palustris. A, ABA concentration of the third-oldest petiole of the two accessions under drained control (t = 0 and t = 6 h) and submerged (6 h) conditions. DW, Dry weight. B, Relative transcript abundance of the R. palustris NCED1 gene in the third-oldest petiole of the two accessions after 6 h of submerged and drained control conditions. Values are measured with real-time reverse transcription-PCR with tubulin as internal standard, relative to the value at t = 0 and ²log transformed. C, Elongation rates of the third-oldest petiole of the two accessions upon exposure to different concentrations of ABA under submerged and drained control conditions. Data are mean ± se, n = 5 to 6 for A, n = 3 for B, n = 10 for C. Different letters indicate significant differences (Games-Howell test, P < 0.05). For statistics of the data in C, see Supplemental Table S3.

Figure 4.

Responses to exogenous GA in the third-oldest petiole of the fast and slow accession of R. palustris. A, Dose-response curves for petiole elongation in response to different applied concentrations of GA in submerged (subm.) and drained control plants of the two accessions. Data are mean ± se, n = 10. For statistics, see Supplemental Table S4. B, Petiole elongation rates of the two accessions pretreated with paclobutrazol (Paclo; GA biosynthesis inhibitor) and 1-MCP (ethylene perception inhibitor) under submerged conditions with and without GA (10 mL 100 μm) in the submergence water. C, Petiole elongation rates of the two accessions pretreated with paclobutrazol, 1-MCP, and fluridone (ABA biosynthesis inhibitor) under submerged conditions with and without GA (10 mL 100 μm) in the submergence water. B and C, Submergence treatment started at t = 0. Growth rates were calculated every hour from length data monitored by linear variable displacement transducers. Data are mean of six to seven biological replicates. For clarity reason, se is not shown in the figures. se varies from 4.9 × 10⁻⁴ mm h⁻¹ to 7.4 × 10⁻² mm h⁻¹ in B, and from 4.1 × 10⁻⁴ mm h⁻¹ to 9.4 × 10⁻² mm h⁻¹ in C. Black bars indicate 8-h dark periods.

Figure 5.

Schematic presentation of the signaling pathway in which submergence induces enhanced petiole elongation in Rumex accessions. Submergence causes accumulation of ethylene inside plant tissues. These elevated ethylene levels induce reduction of ABA biosynthesis and a stimulation of ABA catabolism (Benschop et al., 2005) and lead to a lower endogenous ABA concentration. This stimulates GA signaling and ultimately enhances petiole elongation.