|De Graaf, Albert|
Submitted to: Metabolic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2007
Publication Date: 7/10/2007
Citation: Alonso, A.P., Raymond, P., Hernould, M., Rondeau-Mouro, C., De Graaf, A., Chourey, P.S., Lahaye, M., Shachar-Hill, Y., Rolin, D., Dieuaide-Noubhani, M. 2007. A metabolic flux analysis to study the role of sucrose synthase in the regulation of the carbon partitioning in central metabolsim in maize root tips. Metabolic Engineering. 9:419-432. Interpretive Summary: Our previous collaborative studies done at the CMAVE, USDA ARS, SAA and the University of Florida, Gainesville, FL, have provided substantial evidence to show that three separate genes encode for three forms of the enzyme sucrose synthase (SuSy) which is critical in providing building blocks and precursors for starch and cellulose in developing seeds of maize. The same three genes are also critical in young roots especially in providing protection against flooded soil conditions (anoxic environment). Biochemical and metabolic basis of this protection remained unclear until now. The collaborative studies done through a group of scientists from Chemistry Unit, CMAVE, USDA ARS, SAA, the Institut National de la Recherche Agronomique, Université Bordeaux, France and the Netherlands, based on physiological, biochemical and high throughput chemical approaches present an unexpected finding that the same three genes may perform a different function in roots than in the seeds. Overall, these basic studies point to a new metabolic complexity on the role of SuSy in plants.
Technical Abstract: In order to understand the role of sucrose synthase in carbon partitioning, metabolic fluxes were analyzed in maize root tips of lineage-related genotypes of a double mutant (dm) of sucrose synthase genes, sh1 sus1 and the corresponding wild type, Sh1 Sus1, in W22 inbred background. Pulse labeling experiments were performed using [U-14C]glucose to quantify unidirectional fluxes into sucrose, starch and cell wall polysaccharides. Isotopic steady state labeling with [1 13C]glucose, [2-13C]glucose or [U 13C]glucose followed by an analysis by 1H-NMR and 13C-NMR of labeling in carbohydrates and amino acids was also used to determine fluxes through central metabolism using computer-aided modelling of labeling data. The combination of short-term and steady-state labeling experiments allowed us to determine 33 fluxes of intermediary metabolism involved in glucose partitioning. We found that the low starch level observed in the mutant maize root tips was not caused by a decrease of its synthesis, and that reduced sucrose synthase activity in the mutant leads to an increase in the wall synthesis flux, accompanied by an increase in cell wall polysaccharide content. Taken together, these observations suggest that SH1 and SUS1 are not major providers of substrates for starch and cell wall biosynthesis in maize root tips. A non catalytic role of mitochondrial SH1 and SUS1 sucrose synthase (Subbaiah et al., 2006), suppressed in the double mutant, could explain the metabolic perturbations.