Metabolite ratios of source leaves (LPI 15) of wild-type and transgenic <i>Populus.</i>

1<p>Pairwise comparisons were conducted using Student’s <i>t-</i>test.</p

Gas exchange in source leaves (LPI 15) of wild-type and transgenic <i>Populus</i> from the chronic drought experiment.

1<p>Determined at saturating light intensity of 1500 µmol/m²/sec.</p>2<p>WUE = A_max/Transpiration.</p>3<p>Chlorophyll concentrations determined using equations presented in Porra et al. 1989.</p>4<p>Specific leaf area calculations were used to present the Chl data per unit leaf area.</p>5<p>Determined by Student’s <i>t-</i>test.</p

Leaf gas exchange properties of <i>Populus</i> wild-type and RNAi plants.

<p>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. * <i>p</i>≤0.05 for each transgenic line compared to wild type as determined by Student’s <i>t</i>-test.</p

The Tonoplast-Localized Sucrose Transporter in <em>Populus</em> (PtaSUT4) Regulates Whole-Plant Water Relations, Responses to Water Stress, and Photosynthesis

<div><p>The <em>Populus</em> 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 <em>Populus</em>, <em>SUT4</em> 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 <em>Populus</em>. Here, we describe the effects of <em>PtaSUT4-</em>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 <em>Populus</em>.</p> </div

Biomass data for two independent cohorts of wild-type and transgenic plants used in acute drought experiments.

<p>Values are means ± SD. Bold values represent <i>p</i>≤0.05 and italicized values represent 0.05<<i>p</i><0.1, as determined by Student's <i>t</i>-test.</p

Area, water content, and specific leaf area of source leaves (LPI 10) from wild-type and transgenic <i>Populus</i> under contrasting long-term soil moisture regimes.

<p>Values are means ± SD of n = 6–8 replicates per treatment group. Bold values are <i>p<</i>0.05, italics are 0.05<<i>p</i><0.1 as determined by Student’s <i>t-</i>test.</p

Effects of water availability on <i>Populus</i> stem growth and water concentrations.

<p>(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 <i>p</i>-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. *<i>p</i>≤0.05, **0.001<<i>p</i>≤0.01, ***<i>p</i>≤0.001 as determined by Student’s <i>t</i>-test.</p

<i>Populus</i> water uptake during acute drought stress.

<p>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. **<i>p</i>≤0.01 as determined by Student’s <i>t</i>-test.</p