Poster: Photosynthesis
Abs #
324: Maltose metabolism in the cytosol
Transitory starch is stored during the day inside chloroplasts and then broken down at night for export. Recent data indicates that maltose is the predominant form of carbon exported from the chloroplast at night but its fate in the cytosol is unknown. An amylomaltase gene (malQ) cloned from Escherichia coli and other bacteria is necessary for maltose metabolism in E. coli. However, whether there is an amylomaltase in the cytosol of plant leaves, and what the role of this enzyme is, are unclear. Using a reverse genetics approach two mutants of Arabidopsis thaliana were isolated in which the gene encoding a putative amylomaltase enzyme (cytosolic glucanotransferase, CGT1) is disrupted by a T-DNA insertion. Both cgt1-1 and cgt1-2 plants show retarded growth, and accumulated a large amount of maltose. In addition, cgt1 plants accumulated starch and a water-soluble, KCl-ethanol precipitable polyglucan while having less sucrose. One mutant of Arabidopsis in which cytosolic starch phosphorylase (Pho2) was disrupted by T-DNA was isolated. In the homozygous state this mutation was embryo-lethal. This gene is similar to the malP gene of E. coli, which works in concert with malQ. These results show that Arabidopsis has an amylomaltase and maltodextrin phosphorylase that is involved in the conversion of maltose to sucrose in the cytosol. We propose that a similar maltose metabolism is present in the cytosol of Arabidopsis leaves as in the cytoplasm of E. coli. These enzymes have been considered to be starch metabolism enzymes, and their presence in the cytosol has been puzzling. Our results explain their presence in the cytosol as enzymes needed for maltose metabolism.
