Content area
Full Text
Starch is the major storage carbohydrate in higher plants and of considerable importance for the human diet and for numerous technical applications. In addition, starch can be accumulated transiently in chloroplasts as a temporary deposit of carbohydrates during ongoing photosynthesis. This transitory starch has to be mobilized during the subsequent dark period. Mutants defective 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 the Arabidopsis sex1 mutant that has been proposed to be defective in the export of glucose resulting from hydrolytic starch breakdown. The mutated gene in sexI was cloned using a map-based cloning approach. By complementation of the mutant, immunological analysis, and analysis of starch phosphorylation, we show that sexl is defective in the Arabidopsis homolog of the RI protein and not in the hexose transporter. We propose that the SEX1 protein (Ri) functions as an overall regulator of starch mobilization by controlling the phosphate content of starch.
INTRODUCTION
During photosynthesis, a proportion of the photoassimilates are exported from the chloroplast stroma in the form of triose phosphates that serve as precursors for sucrose biosynthesis in the cytosol. A proportion also is retained within the chloroplast and stored transiently as starch. The accumulation of transitory starch in Arabidopsis is relatively constant throughout the light period and occurs concurrently with sucrose synthesis (Zeeman et al., 1998a). In the subsequent dark period, starch is mobilized and thereby provides a steady supply of carbon for export to sink organs and for energy metabolism (Zeeman and ap Rees, 1999). The alteration of net biosynthesis and net degradation of starch is reflected by the diurnal variation of leaf starch contents. Both starch synthesis and degradation are tightly controlled to adapt plant metabolism to changing environmental conditions. Starch biosynthesis is controlled by the 3-phosphoglycerate (3-PGA)/Pi ratio, which increases when sucrose biosynthesis becomes limited. Because of the allosteric activation of ADP-glucose pyrophosphorylase (AGPase) by 3-PGA and its inhibition by Pi, starch biosynthesis approaches maximum rates when stromal 3-PGA/Pi ratios are high (Smith et al., 1997). This view has been reinforced by analysis of mutants defective in AGPase (Neuhaus and Stitt, 1990). On the contrary, our understanding of...