Peter J. Vanderveer

Modification of a Specific Class of Plasmodesmata and Loss of Sucrose Export Ability in the sucrose export defective1 Maize Mutant.

We report on the export capability and structural and ultrastructural characteristics of leaves of the sucrose export defective1 (sed1; formerly called sut1) maize mutant. Whole-leaf autoradiography was combined with light and transmission electron microscopy to correlate leaf structure with differences in export capacity in both wild-type and sed1 plants. Tips of sed1 blades had abnormal accumulations of starch and anthocyanin and distorted vascular tissues in the minor veins, and they did not export sucrose. Bases of sed1 blades were structurally identical to those of the wild type and did export sucrose. Electron microscopy revealed that only the plasmodesmata at the bundle sheath-vascular parenchyma cell interface in sed1 minor veins were structurally modified. Aberrant plasmodesmal structure at this critical interface results in a symplastic interruption and a lack of phloem-loading capability. These results clarify the pathway followed by photosynthates, the pivotal role of the plasmodesmata at the bundle sheath-vascular parenchyma cell interface, and the role of the vascular parenchyma cells in phloem loading.

Altered photosynthesis, flowering, and fruiting in transgenic tomato plants that have an increased capacity for sucrose synthesis

Photosynthesis, leaf assimilate partitioning, flowering, and fruiting were examined in two lines of Lycopersicon esculentum Mill. transformed with a gene coding for sucrose-phosphate synthase (SPS) (EC 2.3.1.14) from Zea mays L. expressed from a tobacco ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit promoter. Plants were grown at either 35 or 65 Pa CO2 and high light (1000 μmol photons·m−2·s−1). Limiting and maximum SPS activities were significantly greater (up to 12 times) in the leaves of SPS-transformed lines for all treatments. Partitioning of carbon into sucrose increased 50% for the SPS transformants. Intact leaves of the control lines exhibited CO2-insensitivity of photosynthesis at high CO2 levels, whereas the SPS transformants did not exhibit CO2-insensitivity. The O2-sensitivity of photosynthesis was also greater for the SPS-transformed lines compared to the untransformed control when measured at 65 Pa CO2. These data indicate that the SPS transformants had a reduced limitation on photosynthesis imposed by end-product synthesis. Growth at 65 Pa CO2 resulted in reduced photosynthetic capacity for control lines but not for SPS-transformed lines. When grown at 65 Pa CO2, SPS transformed lines had a 20% greater photosynthetic rate than controls when measured at 65 Pa CO2 and a 35% greater rate when measured at 105 Pa CO2. Photosynthetic rates were not different between lines when grown at 35 Pa CO2. The time to 50% blossoming was reduced and the total number of inflorescences was significantly greater for the SPS transformants when grown at either 35 or 65 Pa CO2. At 35 Pa CO2, the total fruit number of the SPS transformants was up to 1.5 times that of the controls, the fruit matured earlier, and there was up to a 32% increase in total fruit dry weight. Fruit yield was not significantly different between the lines when grown at 65 Pa CO2. Therefore, there was not a strict relationship between yield and leaf photosynthesis rate. Flowering and fruit development of the SPS-transformed lines grown at 35 Pa CO2 showed similar trends to the controls grown at 65 Pa CO2. Incidences of blossom-end rot were also reduced in the SPS-transformed lines. These data indicate that altering starch/sucrose partitioning by increasing the capacity for sucrose synthesis can affect acclimation to elevated CO2 partial pressure and flowering and fruiting in tomato.

Biogenic isoprene emission: Model evaluation in a southeastern United States bottomland deciduous forest

Isoprene is usually the dominant natural volatile organic compound emission from forest ecosystems, especially those with a major broadleaf deciduous component. Here we report isoprene emission model performance versus leaf and canopy level isoprene emission measurements made at the Duke University Research Forest near Chapel Hill, North Carolina. Emission factors, light and temperature response, canopy environment models, foliar mass, leaf area, and canopy level isoprene emission were evaluated in the field and compared with model estimates. Model components performed reasonably well and generally yielded estimates within 20% of values measured at the site. However, measured emission factors were much higher in early summer following an unusually dry spring. These decreased later in the summer but remained higher than values currently used in emission models. There was also a pronounced decline in basal emission rates in lower portions of the canopy which could not be entirely explained by decreasing specific leaf weight. Foliar biomass estimates by genera using basal area ratios adjusted for crown form were in excellent agreement with values measured by litterfall. Overall, the stand level isoprene emissions determined by relaxed eddy accumulation techniques agreed reasonably well with those predicted by the model, although there is some evidence for underprediction at ambient temperatures approaching 30°C, and overprediction during October as the canopy foliage senesced. A Big Leaf model considers the canopy as a single multispecies layer and expresses isoprene emission as a function of leaf area rather than mass. This simple model performs nearly as well as the other biomass-based models. We speculate that seasonal water balance may impact isoprene emission. Possible improvements to the canopy environment model and other components are discussed.

Carbon metabolism enzymes and photosynthesis in transgenic tobacco (Nicotiana tabacum L.) having excess phytochrome

J.M. Keller et al. (1989, EMBO J. 8, 1005–1012) introduced a phytochrome gene controlled by a cauliflower mosaic virus 35S promoter into tobacco (Nicotiana tabacum L.) providing material to test whether several photosynthesis enzymes can be increased by one modification to the plant. We report here that this transgenic tobacco had greater amounts of all enzymes examined as well as greater amounts of total protein and chlorophyll per unit leaf area. Fructose bisphosphatase (E.C. 3.1.3.11), glyceraldehyde 3-phosphate dehydrogenase (E.C. 1.2.1.12), and sucrose-phosphate synthase (E.C. 2.4.1.14) were also higher when expressed per unit protein. However, ribulose-1,5-bisphosphate carboxylase (E.C. 4.1.1.39) amount per unit leaf protein was the same in transgenic and wild-type (WT) plants. Photosynthesis in the transgenic plants was lower than in WT at air levels of CO2, but higher than in WT above 1000 μbar CO2. The photosynthesis results indicated a high resistance to CO2 diffusion in the mesophyll of the transgenic plants. Examination of electron micrographs showed that chloroplasts in the transgenic plants were often cup-shaped, preventing close association between chloroplast and cell surface. Chloroplast cupping may have caused the increase in the mesophyll resistance to CO2 diffusion. We conclude that it is possible to affect more than one enzyme with a single modification, but unexpected physical modifications worsened the photosynthetic performance of this plant.

Promoter strength and tissue specificity effects on growth of tomato plants transformed with maize sucrose-phosphate synthase

Abstract. When sucrose-phosphate synthase (SPS; EC 2.4.1.14) is expressed in tomato (Lycopersicon esculentum Mill.) from a ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) small subunit (rbcS) promoter, yields are often unchanged but when SPS is expressed from a Cauliflower Mosaic Virus 35S promoter, yield is enhanced up to 80%. Two explanations for this phenomenon are (i) that expression of SPS in tissues other than leaves accounts for the increased yield or (ii) that the lower level of expression directed by the 35S promoter is more beneficial than the high level of expression directed by the rbcS promoter. To test the first hypothesis, we conducted a reciprocal graft experiment, which showed that root SPS activity did not substantially affect growth. To test the second hypothesis, we conducted a field trial using a backcrossed, segregating, population of SPS-transformed plants derived from 35S and rbcS lines. The optimal dose of SPS activity for growth was approximately twice that of the wild type regardless of which promoter was used. The effect of SPS on growth was the result of a shift in partitioning of carbon among starch, sucrose, and ionic compounds (primarily amino acids), rather than of an increase in net photosynthesis. Excessive SPS activity resulted in a decreased rate of amino acid synthesis, which could explain the non-linear response of plant growth to the level of SPS expression.

Disequilibrium of Chloroplastic Phosphoglucose Isomerase Inferred from Deuterium Isotope Discrimination

Intramolecular deuterium (D) distributions of plant metabolites carry information whether an enzyme reaction in vivo is in equilibrium or not. We find that cytosolic phosphoglucose isomerase (PGI) is in equilibrium, but chloroplastic PGI is not.