Carbon fixation, sucrose synthesis and its transport to storage tissues

Abstract

All oxygenic organisms including land plants assimilate atmospheric CO₂ to sugar phosphates by reductive pentose phosphate (RPP) pathway located in chloroplasts. This is the primary carboxylating mechanism in plants and is comprised of thirteen reactions catalyzed by eleven enzymes distributed in three distinct phases namely, carboxylation, reduction and the regeneration. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyzes the primary carbon fixation, in which ribulose-1,5-bisphosphate (RuBP) and CO₂ are converted to two molecules of 3-carbon compounds, 3-phosphoglycerate (3-PGA). In subsequent reactions, 3-PGA is phosphorylated and reduced by the products of light reactions of photosynthesis (ATP and NADPH) to produce triose phosphates, which ultimately serve as the source of carbon for sucrose synthesis in cytosol and starch synthesis in chloroplast. The pathway of CO₂ assimilation is under strict regulatory control and the key enzymes of the pathway, besides being regulated by metabolites are also subjected to regulation by light. Carbon in the form of dihydroxy acetone phosphate (DHAP) leaves chloroplasts through phosphate translocator and is converted to sucrose in the cytosol. The major regulatory enzymes in sucrose biosynthetic pathway include cytosolic fructose-1,6-bisphosphatase (FBPase) and sucrose phosphate synthase (SPS), both of which catalyze irreversible reactions and have tight allosteric regulation by cytosolic metabolites and intermediates in vivo. The rate of sucrose synthesis is also controlled via molecular regulation by changes in the amount of these regulatory proteins and/or post translational modifications of the pre-existing enzyme. Sucrose synthesized in the mesophyll cells is then translocated to various sink tissues through phloem. Further transport of sucrose from phloem parenchyma cells to the sieve tubes can occur symplastically via plasmodesmata without involving translocators or apoplastically. From apoplast to the companion cells, sucrose transport proceeds via proton symport driven by proton gradient.