Mutan produced in potato amyloplasts adheres to starch granules

Abstract

Production of water-insoluble mutan polymers in Kardal potato tubers was investigated after expression of a full-length (Gtfl) and a truncated mutansucrase gene referred to as GtflCAT(Gtfl without glucan-binding domain) from Streptococcus downei. Subsequent effects on starch biosynthesis at the molecular and biochemical levels were studied. Expression of the GfflCAT gene resulted in the adhesion of mutan material on starch granules, which stained red with erythrosine, and which was hydrolysed by exo-mutanase. In addition, GtflCAT-expressing plants exhibited a severely altered tuber phenotype and starch granule morphology in comparison to those expressing the full-length Gffl gene. In spite of that, no structural changes at the starch level were observed. Expression levels of the sucrose-regulated, AGPase and GBSSI genes were down-regulated in only the GTFLCAT transformants, showing that GtflCAT expression interfered with the starch biosynthetic pathway. In accordance with the down-regulated AGPase gene, a lower starch content was observed in the GTFLCAT transformants. Finally, the rheological properties of the GTFLCAT starches were modified; they showed a higher retrogradation during cooling of the starch paste. © 2005 Blackwell Publishing Ltd.