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

Abstract

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.