Effects of elevated sucrose-phosphate synthase activity on photosynthesis, carbohydrate partitioning and growth rate in hybrid aspen (Populus tremula x P. tremuloides)

Abstract

Starch and sucrose are synthesised from CO2 assimilated during photosynthesis. In rapidly growing plants, sucrose is the major end product of photosynthesis and the principal form of carbohydrate translocated throughout the plant. One of the key points for the regulation of the sucrose synthesis pathway is the reaction catalysed by sucrose-phosphate synthase (SPS). This enzyme occupies a strategic site downstream of the point where the pathways for sucrose synthesis and starch mobilisation converge. In herbaceous plant leaves, an increase in SPS activity often correlates with changes in the rate of sucrose synthesis and export. The consequences of this increased SPS expression on the sugar phloem transport and the sugar flows between source and sink tissues have not been investigated because of the morphological difficulties encountered with herbaceous plants. To investigate this further, hybrid aspen were transformed with the gene encoding for the maize SPS. Preliminary results show an increase in total leaf carbohydrates and surprisingly leaf starch content. The increase observed in SPS activity is correlated with a significant increase in photosynthetic activity, but no significant changes in sugar leaf partitioning. Diurnal carbohydrate content of leaves of transgenic trees is also modified, soluble sugar (glucose, fructose and sucrose) largely increase during the first part of the light period compared to wild type. Moreover, this sugar accumulation appears to improve the growth rate of the trees. Thus, genetic modification of SPS expression in trees can be used to modify traits important for the forest and paper industries.