Evidence that amylose synthesis occurs within the matrix of the starch granule in potato tubers

Transgenic potatoes expressing reduced levels of granule-bound starch synth ase I(GBSSI) have been used to investigate whether the synthesis of amylose occurs at the surface of the starch granule or within the matrix formed by the synthesis and organization of amylopectin. Amylose in these potatoes i s wholly or largely confined to a central region of the granule. Consequent ly this core region stains blue with iodine whereas the peripheral zone sta ins red. By making extensive measurements of the relative sizes of the gran ules and their blue-staining cores in tubers over a range of stages of deve lopment, we have established that the blue core increases in size as the gr anule grows. The extent of the increase in size of the blue core is greater in potatoes with higher levels of GBSSI. These data show that amylose synt hesis occurs within the matrix of the granule, and are consistent with the idea that the space available in the matrix may be an important determinant of the amylose content of storage starches.