Apoplastic expression of yeast-derived invertase in potato : effects on photosynthesis, leaf solute composition, water relations, and tuber composition

In potato plants (Solanum tuberosum), a chimeric yeast-derived invertase gene fused to a 35S cauliflower mosaic virus promoter has been expressed. The protein was targeted to the cell wall by using the signal peptide of proteinase inhibitor II fused to the amino terminus of the yeast invertase. The transformed plants had crinkled leaves, showed a reduced growth rate, and produced fewer tubers. Although in the apoplast of the leaves of the transformed plants the content of glucose and fructose rose by a factor of 20, and that of sucrose declined 20-fold, 98% of the carbohydrate in the phloem sap consisted of sucrose, demonstrating the strong specificity of phloem loading. In the leaf cells of the transformed plants, glucose, fructose, and amino acids, especially proline, were accumulated. Consequently, the osmolality of the cell sap rose from 250 to 350 mosmol/kg. Our results show that the observed 75% decrease of photosynthesis is not caused by a feedback regulation of sucrose synthesis and is accompanied by an increase in the osmotic pressure in the leaf cells. In the transformed plants, not only the amino acid to sucrose ratio in the phloem sap, but also the amino acid and protein contents in the tubers were found to be elevated. In the tubers of the transformed plants, the protein to starch ratio increased.     

Subcellular Volumes and Metabolite Concentrations in Potato (Solanum tuberosum cv. Désirée) Leaves1

Cellular and subcellular volumes in mature leaves of potato (Solanum tuberosum cv. Désirée) were determined stereologically from light and electron micrographs. Leaves of ten-week-old plants with a total leaf volume of 623 μL per mg chlorophyll (Chl) were found to be composed of 12 % epidermis, 68 % mesophyll, 5 % vascular tissue, 3 % apoplast and 16 % gas space. In the epidermal cells 97 % of the volume was occupied by the vacuole. The mesophyll cells consisted (as expressed per mg Chl) of 323 μL (76 %) vacuole, 35 μL (8 %) chloroplast stroma, 22 μL (5 %) cytosol plus nucleus and peroxisomes, and 4μL, (1 %) mitochondria. A comparison of these values with subcellular volumes previously determined for spinach and barley leaves, shows that the relative sizes of the subcellular volumes are strikingly similar. Subcellular concentrations of carbohydrates, of the phosphorylated intermediates of carbohydrate metabolism, of malate, and of amino acids have been evaluated from measurements of the corresponding subcellular metabolite contents determined using the non-aqueous fractionation technique. Malate, glucose and fructose were found to accumulate in the vacuole, whereas the concentration of sucrose and amino acids in the cytosol were much higher than in the vacuole. The amino acid concentration in the chloroplast stroma is similar to that in the cytosol. Phosphorylated intermediates of carbohydrate metabolism are confined to the chloroplast stroma and the cytosol, confirming the validity of the fractionation method. Whereas triose phosphates and fructose-l,6-bisphosphate are concentrated in the stroma, the concentrations of hexose monophosphates were highest in the cytosol. Since the subcellular metabolite distribution in potato leaves reported here is very similar to that previously described for spinach and barley leaves, we conclude that it may be characteristic for mesophyll cells in general.     

Leaf-Specific Antisense Inhibition of Starch Biosynthesis in Transgenic Potato Plants Leads to an Increase in Photoassimilate Export from Source Leaves during the Light Period

In an attempt to study the importance of starch synthesis inleaves with respect to sink-source interactions, we investigateddaily turnover of carbohydrates in leaves of transgenic potatoplants inhibited for ADP-glucose pyrophosphorylase (AGPase).Down-regulation of AGPase has been performed using two differentpromoters: the near-constitutive CaMV 35S promoter, and theSTLSI promoter which is active in photosynthetic cells only.Residual AGPase activity in leaves was between 6 and 30% inindividual transformants as compared to wild-type potato plants.We found that: (i) photosynthesis is not significantly alteredrelative to wild-type plants; (ii) levels of starch are markedlyreduced in leaves of transgenic plants; (iii) levels of solublesugars and malate are largely unaffected by the inhibition ofAGPase; (iv) the reduction of starch synthesis leads to a higherportion of assimilated carbon being transported from leavesto sink tissues during the light period; (v) altered leaf exportcharacteristics do not change tuber yield under greenhouse conditions.Collectively, these data demonstrate a striking flexibilityof the potato plant with respect to day/night rhythms of carbonexport from leaves and utilization by the major storage sinks,i.e. developing tubers. (Received November 1, 1994; Accepted March 2, 1995)