SS III (SSIII) has been reported to play a regulatory role in the synthesis of transient starch. SSIII from Arabidopsis thaliana contains 1025 amino acid residues and has an N‐terminal transit peptide for chloroplast localization followed by three in tandem starch‐binding domains (SBDs D1, D2, and D3, residues 22‐591). Its C‐terminal catalytic domain (residues 592–1025) is similar to bacterial glycogen synthase. Binding studies to raw starch and its individual components, AM or AP show that the SBD region binds preferentially to AM, and that the D1 domain is mainly responsible for this selective binding. The D2 domain contains two binding sites which include amino acid residues Y394 (binding site 1) and W366 (binding site 2) acting cooperatively with the D1 domain in the binding process while G335 and W340 have a minor role. In addition, mutations in these residues also affect the kinetic parameters for the polysaccharide substrate of SSIII.

Starch binding domains of starch synthase III from Arabidopsis thaliana (SBD123) binds preferentially to cell wall polysaccharides rather than to starch in vitro. Transgenic plants overexpressing SBD123 in the cell wall are larger than wild type. Cell wall components are altered in transgenic plants. Transgenic plants are more susceptible to digestion than wild type and present higher released glucose content. Our results suggest that the transgenic plants have an advantage for the production of bioethanol in terms of saccharification of essential substrates. The plant cell wall, which represents a major source of biomass for biofuel production, is composed of cellulose, hemicelluloses, pectins and lignin. A potential biotechnological target for improving the production of biofuels is the modification of plant cell walls. This modification is achieved via several strategies, including, among others, altering biosynthetic pathways and modifying the associations and structures of various cell wall components. In this study, we modified the cell wall of A. thaliana by targeting the starch-binding domains of A. thaliana starch synthase III to this structure. The resulting transgenic plants (E8-SDB123) showed an increased biomass, higher levels of both fermentable sugars and hydrolyzed cellulose and altered cell wall properties such as higher laxity and degradability, which are valuable characteristics for the second-generation biofuels industry. The increased biomass and degradability phenotype of E8-SBD123 plants could be explained by the putative cell-wall loosening effect of the in tandem starch binding domains. Based on these results, our approach represents a promising biotechnological tool for reducing of biomass recalcitrance and therefore, the need for pretreatments.