Tuber physiology and properties of starch from tubers of transgenic potatoplants with altered plastidic adenylate transporter activity

We showed recently that antisense plants with decreased activity of the pla stidic ATP/ADP-transporter protein exhibit drastically reduced levels of st arch and a decreased amylose/amylopectin ratio, whereas sense plants with i ncreased activity of the transporter possessed more starch than wild-type p lants and an increased amylose/amylopectin ratio. In this paper we investig ate the effect of altered plastidic ATP/ADP-transyorter protein expression on primary metabolism and granule morphology in more detail. Tuber tissues from antisense and sense plants exhibited substantially increased respirato ry activity compared with the wild type. Tubers from antisense plants conta ined markedly increased levels of free sugars, UDP-Glc, and hexose phosphat es, whereas phosphoenolpyruvate, isocitrate, ATP, ADP, AMP, UTP, UDP, and i norganic pyrophosphate levels were slightly decreased. In contrast, tubers from sense plants revealed a slight increase in adenine and uridine nucleot ides and in the levels of inorganic pyrophosphate, whereas no significant c hanges in the levels of soluble sugars and metabolites were observed. Antis ense tubers contained 50% reduced levels of ADP-Glc, whereas sense tubers c ontained up to 2-fold increased levels of this sole precursor for starch bi osynthesis. Microscopic examination of starch grain morphology revealed tha t the size of starch grains from antisense tubers was substantially smaller (50%) compared with the wild type. The large starch grains from sense tube rs appeared of a more angular morphology, which differed to the more ellips oid shape of wild type grains. The results suggest a close interaction betw een plastidial adenylate transport and starch biosynthesis, indicating that ADP-Glc pyrophosphorylase is ATP-limited in vivo and that changes in ADP-G Lc concentration determine starch yield, as well as granule morphology. Pos sible factors linking starch synthesis and respiration are discussed.