Sucrose Synthesis

Starch and sucrose are the major end products of photosynthetic CO2 fixation. Carbon fixed during photosynthesis is either retained in the chloroplast and converted to starch or transferred to the cytosol in the form of triose phosphate through the operation of phosphate translocator and converted to sucrose (Fig. 3). Sucrose, which is the major transport form of reduced carbon in higher plants, is then either stored in the vacuole or transported to the other parts of the plant. This transported sucrose is either retained as such in sink tissues or, further metabolized to provide energy for growth and maintenance of cellular activities or converted to storage compounds (starch/fats) in reserve tissues. The relative type and size of these carbohydrate pools vary dramatically during tissue development, between different plant species and within the same species subjected to different environmental conditions (66). This has generated considerable interest in the elucidation of regulatory mechanisms that are responsible for above differences.

The pathway of sucrose biosynthesis in higher plants is well documented and has been reviewed extensively (66-71). It is synthesized in the mesophyll cells of cytosol from triose-P exported from chloroplast during photosynthesis via the phosphate translocator (44). The enzyme TPI

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