GBSSI loss-of-function in potato affects dynamics in starch biosynthesis and breakdown in leaves and alters leaf starch multi-level structure

Starch biosynthesis, breakdown, and multi-scale leaf starch granule structure in the potato variety Wotan with knock-out of the Granular Bound Starch Synthase (GBSS) I gene were assessed. In wild-type, starch in the leaves had compared to the tuber significantly lower amylose content, smaller amylopectin molecules, more amylopectin side chains with a degree of polymerization (DP) of 6–12, fewer amylopectin sidechains (DP 12–24), higher degree of branching, thicker amorphous lamellae, higher crystallinity, higher molecular order, and much smaller granules. Knock-out of GBSSI resulted in reduced leaf starch amylose content and larger amylopectin and amylose molecules, more amylopectin sidechains (DP 6–8 and 12–16), fewer internal chains (DP 18–24), increased branching, thickening of the crystalline lamellae, lowered crystallinity and increased molecular order. Leaf starch content assayed over 24 h with a diurnal rhythm of 16/8 h (light/dark) revealed that the gbssI mutant accumulated leaf starch at a faster rate, while degrading leaf starch at a much slower rate compared to wild-type. This study provides a systematic analysis of the multi-scale structure of leaf starch in potato and is the first study to demonstrate that the loss of GBSSI causes significant structural differences in leaf starch while also affecting leaf starch synthesis and degradation.