The Arabidopsis sex1 mutant is defective in the R1 protein, a general regulator of starch degradation in plants, and not in the chloroplast hexose transporter

Starch is the major storage carbohydrate in higher plants and of considerab le importance for the human diet and for numerous technical applications. I n addition, starch can be accumulated transiently in chloroplasts as a temp orary deposit of carbohydrates during ongoing photosynthesis. This transito ry starch has to be mobilized during the subsequent dark period. Mutants de fective in starch mobilization are characterized by high starch contents in leaves after prolonged periods of darkness and therefore are termed starch excess (sex) mutants. Here we describe the molecular characterization of t he Arabidopsis sex1 mutant that has been proposed to be defective in the ex port of glucose resulting from hydrolytic starch breakdown. The mutated gen e in sex1 was cloned using a map-based cloning approach. By complementation of the mutant, immunological analysis, and analysis of starch phosphorylat ion, we show that sex1 is defective in the Arabidopsis homolog of the R1 pr otein and not in the hexose transporter. We propose that the SEX1 protein ( R1) functions as an overall regulator of starch mobilization by controlling the phosphate content of starch.