Title : Characterization of a new isoform of starch synthase in Solanum tuberosum
Potato (Solanum tuberosum) is a staple crop that is cultivated for its starch rich tubers. Potato starch has many industrial uses, for example as thickener in food industry or as adhesives in cardboard production. Starch is one of the major storage polysaccharides in plants. It accumulates in plastids as insoluble semi-crystalline granules. It is composed of two polymers: amylose and amylopectin. Amylopectin represents 75% of starch dry weight. Both amylose and amylopectin are glucose polymers linked by ?-1,4 O-glycosidic linkages and branched by ?-1,6 bounds. Amylopectin displays complex and very organized secondary and ternary structures. The latter are tightly regulated and essential for granule formation. Starch biosynthesis requires the concerted activities of distinct enzyme families including Starch Synthases (SSs), Branching Enzymes (BEs) and DeBranching Enzymes (DBEs). Moreover, each family of enzymes comprises several isoenzymes regulating amylopectin chain length distribution. Starch synthases catalyze the formation of ?-1,4 bounds using ADP-glucose as a donor substrate and transferring the glycosyl moiety onto an elongating glucan chain. Five starch synthases are already well characterized. Among them, starch synthases 1, 2 and 3 are respectively involved in elongation of short, medium and long chains of amylopectin whereas Starch synthase 4 is involved in the initiation process. GBSS (Granule bound starch synthase), which is present within the granule matrix, is responsible for the synthesis of amylose. In the lab, with identified 2 other SS isoforms, SS5 and SS6, by proteomics of potato starch. Phylogenetic analysis of SSs sequences including SS6 have shown that the latter is most closely related to SS4. Furthermore, sequence alignment highlighted the presence of two highly conserved motifs responsible for SS catalytic activity in SS6, suggesting that this isoform plays a yet uncharacterized function in starch metabolism. My thesis work is to decipher SS6 activities and also understand its function in planta. For this purpose, Solanum tuberosum is a good subject of study, due to the relative ease of production of genetically transformed plants, the existence of genome sequence but also because of the economic importance of the potato starch production. This presentation will report the advances in characterization of this new isoform of starch synthase in Solanum tuberosum.