Fig. 1

Proposed pathway of starch metabolism in leaves and tubers of potato. a In source leaves, photoassimilates are generated in the Calvin-Benson-Cycle. In form of F6P, these can be converted to G1P in two subsequent steps catalyzed by PGI and PGM. G1P serves as substrate for AGPase for starch biosynthesis. TPs from the Calvin-Benson-Cycle can also be transported to the cytosol via the TPT in exchange for Pi where they can be metabolized e.g. to sucrose. b In the tuber, sucrose is cleaved by SuSy into UDP-glucose and fructose. UDP-glucose is converted to G1P by UGPase. G1P subsequently is transferred to G6P by cytosolic PGM which can be imported into the amyloplast by GPT. In the amyloplast G6P is then reconverted into G1P by plastidial PGM and as such can serve as a substrate for starch biosynthesis. For detailed reviews on starch metabolism see [3, 4, 6] and references therein. F6P, Fructose-6-phosphate; TP, triose-phosphate; TPT, triose-phosphate/phosphate translocator; Pi, inorganic phosphate; PGI, phosphoglucoisomerase; G6P, glucose 6-phosphate; G1P, glucose 1-phosphate; PGM, phosphoglucomutase; SuSy, sucrose synthase; UGPase, UDP-glucose pyrophosphorylase; PPi, inorganic pyrophosphate; AGPase, ADP-glucose pyrophosphorylase; PPase, inorganic pyrophosphatase; SS, starch synthase; GBSS, granule-bound starch synthase; SBE, starch branching enzyme; GWD, glucan, water dikinase; PWD, phosphoglucan, water dikinase; BAM, beta-amylase; AMY, alpha-amylase; SEX4, starch excess 4; LSF, Like starch-excess Four; DPE, disproportionating enzyme; PHO, Alpha-glucan phosphorylase; GPT, glucose 6-phosphate/phosphate translocator, NTT, nucleotide translocator; GLT, glucose transporter; VGT, vacuolar glucose transporter; MEX, maltose transporter; Fk, fructokinase