Figure 2.

Photosynthesis is the source of carbon for CW synthesis. Fixed CO₂ in leaves by photosynthesis in the chloroplasts is transported through the phloem and unloaded into the cytosol as triose phosphates. In growing sink organs such as meristems, leaves, roots, tubers, and seeds, most of the primary CW synthesis takes place using sucrose as the source of C and energy. Sucrose synthase (SuSy) and invertases are key enzymes controlling sugar precursors for CW synthesis [41,42,43,44]. Sucrose synthase catalyzes the reversible transformation of sucrose into fructose and UDP-Glc. Invertases catalyze the irreversible breakdown of sucrose into glucose and fructose. For a comprehensive view of starch synthesis see [45,46,47,48].