Photosynthetic carbon metabolism in leaves of transgenic tobacco (Nicotiana tabacum L.) containing decreased amounts of fructose 2,6-bisphosphate

The aim of this work was to examine the role of fructose 2,6-bisphosphate ( Fru-2,6-P-2) in photosynthetic carbon partitioning. The amount of Fru-2,6-P -2 in leaves of tobacco (Nicotiana tabacum L. cv. Samsun) was reduced by in troduction of a modified mammalian gene encoding a functional fructose-2,6- bisphosphatase (EC: 3.1.3.46). Expression of this gene in transgenic plants reduced the Fru-2,6-P-2 content of darkened leaves to between 54% and 80% of that in untransformed plants. During the first 30 min of photosynthesis sucrose accumulated more rapidly in the transgenic lines than in the untran sformed plants. whereas starch production was slower in the transgenic plan ts. On illumination, the proportion of (CO2)-C-14 converted to sucrose was greater in leaf disks of transgenic lines possessing reduced amounts of Fru -2,6-P-2 than in those of the control plants. and there was a corresponding decrease in the proportion of carbon assimilated to starch in the transgen ic lines. Furthermore, plants with smaller amounts of Fru-2,6-P-2 had lower rates of net CO2 assimilation. In illuminated leaves: decreasing the amoun t of Fru-2,6-P-2 resulted in greater amounts of hexose phosphates, but smal ler amounts of 3-phosphoglycerate and dihydroxyacetone phosphate. These dif ferences are interpreted in terms of decreased inhibition of cytosolic fruc tose-1,6-bisphosphatase resulting from the lowered Fru-2,6-P-2 content.