Summary The starting point of this thesis was a screening for proteins which preferentially bind to phosphorylated starch granules. These proteins were thought to be involved in starch-related processes downstream of starch phosphorylation by the glucan, water dikinase (GWD). Therefore, a test system has been established, which allows to compare the binding of proteins from Arabidopsis thaliana leaf extracts to phosphorylated and non-phosphorylated starch granules. A so far uncharacterised protein has been identified in this tests. This protein displays rather weak homology to GWD and preferentially binds to phosphorylated starch granules. The function of this protein has been characterised in vitro as well as in vivo. The protein has been heterologously expressed in E. coli. Activity tests with this recombinant protein revealed that it has a noval enzymatic ac- tivity of a phosphoglucan, water dikinase (PWD). PWD was able to phosphorylate starch granules in vitro. However, starch granules had to be previously phosphorylated by GWD, meaning that PWD acts down- stream of GWD. The reaction mode involves the transfer of the E-P of ATP firstly to a histidine residue of the PWD protein and subsequently to a phosphoglucan. The acceptor of the J-P of ATP is water. Analysis of the in vitro phosphorylated starch substrate revealed that PWD predominantly or exclusively phosphorylates the C-3 position of glucosyl units within starch. This is in contrast to GWD which transfers phosphate to C-6 as well as C-3 positions in a ratio of about 3:1. The results obtained with the recombinant protein were approved with PWD-protein partially purified from leaves of the GWD-deficient Arabidopsis thaliana sex1-3mutant. Subcellular localisation of PWD was confirmed by immunoblot analysis on extracts from isolated chloroplasts and protoplasts with help of a polyclonal anti-PWD antibody. The plastidic localisation of PWD was further approved by non-aqueous fractionation of Arabidopsis thaliana leaf extracts. The physiological role of PWD has been analysed in transgenic Arabidopsis thaliana plants in which PWD expression was inhibited either by a T-DNA insertion (pwd) or by means of RNA interference (RNAi). RNAi as well as pwd plants displayed clearly increased leaf starch levels compared with wild type. Regarding the Glc-6-P:Glc-3-P ratio starch of transgenic plants differed from starch of wild-type plants. The slightly elevated ratio in starch of the transgenic plants was due to elevated Glc-6-P levels whereas Glc-3-P remained unchanged. The amount of starch granule associated PWD strongly increased during net starch degradation in the dark. Presumably, PWD is involved in the phosphorylation of C-3 positions of glucosyl units at the granule surface during net starch degradation in the dark.