Farhad Nazarian Firouzabadi: "In planta modification of potato starch granule biogenesis by different granule-bound fusion proteins"

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8 Jan 2007 13:30
Unit: Wageningen University
Location: Aula, building 362, Gen. Foulkesweg 1, Wageningen
Organisation: Wageningen University
Promotor: prof.dr. R.G.F. Visser (Plant Breeding (Genetical Variation and Reproduction))
Co Promotor: Dr.ir. J-P. Vincken

 Starch is composed of amylose and amylopectin and it is deposited in amyloplasts/choloroplasts as semi-crystalline granules. Many biosynthetic enzymes are involved in starch degradation and biosynthesis. Some microbial starch degrading enzymes have a Starch Binding Domain (SBD) which has affinity for the starch granules on its own. In our laboratory, expression of SBD alone or fused to other effector proteins has been demonstrated. In industry, starch is modified after harvesting by chemical, enzymatic or physical treatments. In this thesis, the in planta modification of starch composition, structure and consequently its properties are demonstrated. Expression of multiple repeat SBDs (SBD2-SBD5) resulted in starch granule cluster formation. However despite the high intrinsic affinity for starch granules, they could not displace GBSSI from the starch granules. Acetylation of starch at low degree of substitution was obtained when E. coli maltose acetyltransferase (MAT) was expressed in potato amyloplasts. Expression of truncated form of a mutansucrase from Streptococcus downei fused to SBD led to mutan production in transgenic potato plants. Moreover starches with irregular surfaces and porous appearance were obtained in amylose-containing (Kardal) and amylose-free (amf) potato genetic background, respectively. Punctured and irregular starch granules were obtained when fusion proteins of SBD fused to an E. coli branching enzyme were expressed in potato amyloplasts. All these different approaches led readily to altered phenotypes of starches but without large consequences for their physical properties.
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