Genetic Analysis Indicates Maize Pullulanase- and Isoamylase-type Starch Debranching Enzymes Have Partially Overlapping Functions in Starch Metabolism

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The functions of the two known plant classes of α(1→6) glucan hydrolase were investigated by genetic analysis in maize. A null mutation in the zpul gene, coding for the pullulanase-type starch debranching enzyme (DBE), was coupled with three different alleles of the sul gene, coding for the isoamylase-type DBE . In two instances the starch biosynthesis abnormality conditioned by the isoamylase-type DBE mutation, namely a decrease in starch content and an increase in phytoglycogen content, was exacerbated by the additional loss of pullulanase-type DBE activity. These data confirm previous findings that pullulanase-type DBE contributes to starch biosynthesis in developing endosperm. An appreciable level of starch was still formed, however, in mutant endosperm bearing null mutations for both classes of DBE, and in this instance the pullulanase-type DBE had no effect on the amount of starch produced. The data suggest that a critical level of isoamylase-type DBE protein or activity may be required in order for pullulanase-type DBE to be able to function productively in starch biosynthesis. Furthermore, the data show clearly that a significant degree ofstarch biosynthesis, approximately 40% of normal, occurs in the absence of any detectable DBE activity in the endosperm of maize kernels.
日本応用糖質科学会の論文