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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Light Induces Phosphorylation of Glucan Water Dikinase, Which Precedes Starch Degradation in Turions of the Duckweed Spirodela polyrhiza^1,2

Institute of General Botany and Plant Physiology (R.R., M.H., K.-J.A.) and Institute of Nutritional Sciences (B.M.), University of Jena, 07743 Jena, Germany

Degradation of storage starch in turions, survival organs of Spirodela polyrhiza, is induced by light. Starch granules isolated from irradiated (24 h red light) or dark-stored turions were used as an in vitro test system to study initial events of starch degradation. The starch-associated pool of glucan water dikinase (GWD) was investigated by two-dimensional gel electrophoresis and by western blotting using antibodies raised against GWD. Application of this technique allowed us to detect spots of GWD, which are light induced and absent on immunoblots prepared from dark-adapted plants. These spots, showing increased signal intensity following incubation of the starch granules with ATP, became labeled by randomized [-³³P]ATP but not by [-³³P]ATP and were removed by acid phosphatase treatment. This strongly suggests that they represent a phosphorylated form(s) of GWD. The same light signal that induces starch degradation was thus demonstrated for the first time to induce autophosphorylation of starch-associated GWD. The in vitro assay system has been used to study further effects of the light signal that induces autophosphorylation of GWD and starch degradation. In comparison with starch granules from dark-adapted plants, those from irradiated plants showed increase in (1) binding capacity of GWD by ATP treatment decreased after phosphatase treatment; (2) incorporation of the -phosphate group of ATP into starch granules; and (3) rate of degradation of isolated granules by starch-associated proteins, further enhanced by phosphorylation of starch. The presented results provide evidence that autophosphorylation of GWD precedes the initiation of starch degradation under physiological conditions.

² Dedicated to Professor Dr. Aino Henssen, Marburg, Germany. Her pioneer work about starch metabolism in turions of Spirodela polyrhiza was published 50 years ago.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.103.036236.

^* Corresponding author; e-mail klaus.appenroth{at}uni-jena.de; fax 49 3641 949232.

Received November 20, 2003; returned for revision January 27, 2004; accepted January 28, 2004.

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