Fig. 5

Starch metabolism pathway in potato source (A) and sink tissues (B). A F6P produced by photosynthesis in chloroplasts generates substrates for starch synthesis, ADP-Glu, through the enzymatic reactions of PGI, PGM and AGPase. SSs, GBSSs, SBEs, and DBEs continuously elongate into amylose and amylopectin. GWD, PWD, and SEX4 phosphorylate and dephosphorylate starch particles, increasing the water solubility and promoting starch degradation. DBEs are necessary for the degradation of branched starch. Amylose is hydrolyzed by BAMs to produce maltose, and then transported to the cytoplasm for metabolic utilization. B The substrate ADP-Glu for starch synthesis is synthesized in amyloplast. Sucrose derived from the unloading of the phloem is hydrolyzed by SUSy in cytoplasm to produce UDP-Glu and Fructose, which are then catalyzed by UGPase/PGM and FK/PGI to produce G6P, respectively. With the help of GPT, G6P enters the amyloplast to generate ADP-Glu by PGM and AGPase. Subsequent synthesis and degradation are similar to those in the above-ground parts. The yellow diamond represents sucrose, and arrows indicate the direction of sucrose transport