The first step on the pathway of starch degradation in Arabidopsis (Arabidopsis thaliana) leaves at night is the phosphorylation of starch polymers, catalyzed by glucan, water dikinase (GWD). It has been suggested that GWD is important for the control of starch degradation, because its transcript levels undergo strong diel fluctuations, its activity is subject to redox regulation in vitro, and starch degradation is strongly decreased in gwd mutant plants. To test this suggestion, we analyzed changes in GWD protein abundance in relation to starch levels in wild-type plants, in transgenic plants in which GWD transcripts were strongly reduced by induction of RNA interference, and in transgenic plants overexpressing GWD. We found that GWD protein levels do not vary over the diel cycle and that the protein has a half-life of 2 d. Overexpression of GWD does not accelerate starch degradation in leaves, and starch degradation is not inhibited until GWD levels are reduced by 70%. Surprisingly, this degree of reduction also inhibits starch synthesis in the light. To discover the importance of redox regulation, we generated transgenic plants expressing constitutively active GWD. These plants retained normal control of degradation. We conclude that GWD exerts only a low level of control over starch degradation in Arabidopsis leaves.