A critical role for disproportionating enzyme in starch breakdown is revealed by a knock-out mutation in Arabidopsis.
Disproportionating enzyme (D-enzyme) is a plastidial alpha-1,4-glucanotransferase but its role in starch metabolism is unclear. Using a reverse genetics approach we have isolated a mutant of Arabidopsis thaliana in which the gene encoding this enzyme (DPE1) is disrupted by a T-DNA insertion. While D-enzyme activity is eliminated in the homozygous dpe1-1 mutant, changes in activities of other enzymes of starch metabolism are relatively small. During the diurnal cycle, the amount of leaf starch is higher in dpe1-1 than in wild type and the amylose to amylopectin ratio is increased, but amylopectin structure is unaltered. The amounts of starch synthesised and degraded are lower in dpe1-1 than in wild type. However, the lower amount of starch synthesised and the higher proportion of amylose are both eliminated when plants are completely de-starched by a period of prolonged darkness prior to the light period. During starch degradation, a large accumulation of malto-oligosaccharides occurs in dpe1-1 but not in wild type. These data show that D-enzyme is required for malto-oligosaccharide metabolism during starch degradation. The slower rate of starch degradation in dpe1-1 suggests that malto-oligosaccharides affect an enzyme that attacks the starch granule, or that D-enzyme itself can act directly on starch. The effects on starch synthesis and composition in dpe1-1 under normal diurnal conditions are probably a consequence of metabolism at the start of the light period, of the high levels of malto-oligosaccharides generated during the dark period. We conclude that the primary function of D-enzyme is in starch degradation.
Critchley
J H
JH
Institute of Cell and Molecular Biology, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JH, UK.
Zeeman
S C
SC
Takaha
T
T
Smith
A M
AM
Smith
S M
SM
D000687:Amylopectin / Q000378:metabolism; D000688:Amylose / Q000378:metabolism; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D004269:DNA, Bacterial / Q000235:genetics; D006004:Glycogen Debranching Enzyme System / Q000235:genetics / Q000378:metabolism*; D015151:Immunoblotting; D008320:Maltose / Q000378:metabolism; D016254:Mutagenesis, Insertional; D009844:Oligosaccharides / Q000378:metabolism; D017440:Photoperiod; D018515:Plant Leaves / Q000235:genetics / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D013213:Starch / Q000096:biosynthesis / Q000378:metabolism*
D004269:DNA, Bacterial; D006004:Glycogen Debranching Enzyme System; D009844:Oligosaccharides; D010940:Plant Proteins; C036483:T-DNA; D008320:Maltose; D013213:Starch; D000688:Amylose; D000687:Amylopectin; C020569:4 alpha-glucanotransferase