The tonoplast-localized sucrose transporter in Populus (PtaSUT4) regulates whole-plant water relations, responses to water stress, and photosynthesis

Abstract

The Populus sucrose (Suc) transporter 4 (PtaSUT4), like its orthologs in other plant taxa, is tonoplast localized and thought to mediate Suc export from the vacuole into the cytosol. In source leaves of Populus, SUT4 is the predominantly expressed gene family member, with transcript levels several times higher than those of plasma membrane SUTs. A hypothesis is advanced that SUT4-mediated tonoplast sucrose fluxes contribute to the regulation of osmotic gradients between cellular compartments, with the potential to mediate both sink provisioning and drought tolerance in Populus. Here, we describe the effects of PtaSUT4-RNA interference (RNAi) on sucrose levels and raffinose family oligosaccharides (RFO) induction, photosynthesis, and water uptake, retention and loss during acute and chronic drought stresses. Under normal water-replete growing conditions, SUT4-RNAi plants had generally higher shoot water contents than wild-type plants. In response to soil drying during a short-term, acute drought, RNAi plants exhibited reduced rates of water uptake and delayed wilting relative to wild-type plants. SUT4-RNAi plants had larger leaf areas and lower photosynthesis rates than wild-type plants under well-watered, but not under chronic water-limiting conditions. Moreover, the magnitude of shoot water content, height growth, and photosynthesis responses to contrasting soil moisture regimes was greater in RNAi than wild-type plants. The concentrations of stress-responsive RFOs increased in wild-type plants but were unaffected in SUT4-RNAi plants under chronically dry conditions. We discuss a model in which the subcellular compartmentalization of sucrose mediated by PtaSUT4 is regulated in response to both sink demand and plant water status in Populus.

Figure 1. Populus water uptake during acute drought stress.

Differences in plant water uptake were estimated by comparing the rates of soil water loss. Data is shown for two experimental trials (A) and (B). Data points represent means ± SD. Concurrent soil moisture changes in plant-free pots (o) are included in (B). Each experimental trial included eight wild-type and six SUT4-RNAi plants. (C) Onset of wilt symptoms averaged from the two experimental trials. **p≤0.01 as determined by Student’s t-test.

Figure 2. Effects of water availability on Populus stem growth and water concentrations.

(A) height growth rate, (B) stem diameter growth rate, (C) wood water concentrations (g/g×100), and (D) bark water concentration (g/g×100). Light and dark bars represent high and low soil moisture, respectively. For (A) and (B), asterisk denotes statistical significance between high and low soil moisture within a line. For (C) and (D), asterisks and p-values represent pairwise comparisons between high soil moisture groups delineated by the arrows (i.e., wild type vs. one of the transgenic lines). Bars represent means ± SD of 6–8 replicate plants. *p≤0.05, **0.001<p≤0.01, ***p≤0.001 as determined by Student’s t-test.

Figure 3. Leaf gas exchange properties of Populus wild-type and RNAi plants.

Light response curves for (A) Photosynthetic CO₂ fixation, and (B) Leaf transpiration (solid lines) and stomatal conductance (dashed lines). A fully expanded source leaf (LPI-10) was used for the measurements. Data points are means ± SD of 19 WT, 10 line G, and 12 line F plants. * p≤0.05 for each transgenic line compared to wild type as determined by Student’s t-test.

Figure 4. Effects of water availability on sucrose and RFO metabolism in source leaves.

Metabolite concentrations (mg/g DW in A-F or µg/g DW in G-H) were determined using standard curves of authentic standards. Light and dark bars represent plants maintained under high or low soil moisture, respectively. Bars are means ± SD of 6–8 replicates each. Bars with different letters are statistically different at α = 0.05 based on Student’s t post-hoc test of LSMeans models using JMP 9.0 (SAS Institute, Cary, NC).