SummaryThe biochemical lesion of the sugary-1 mutation was examined in five different mutants of rice with varying phenotypes but with mutations at the same locus. The cells in the inner part of the endosperm of all mutants tested contained phytoglycogen instead of starch, while the cells located in the outer part of the endosperm tissue from some mutants were filled with numerous starch granules. The molecular size of phytoglycogen was markedly smaller than that of amylopectin as measured by Sephacryl S-1O00 chromatography. Analysis of the distribution of a-1,4 chain lengths revealed that in phytoglycogen the number of A-chains dramatically increased, while long B chains with DP I> 37 remarkably decreased or were almost absent, which resulted in the disappearance of the cluster structure. The results suggest that changes in the balance of enzymic activities induced by the mutations brought about a drastic alteration in polyglucan structure and the shape of the polyglucan granule. The greater the extent of phytogiycogen regions in sul endosperm tissues became, the greater was the phytoglycogen content, and the greater the reduction in the activity of starch debranching enzyme, a type of enzyme referred to as R-enzyme (RE}, limit dextrinase or puUulanase. Immunoblot analysis showed that the reduction in RE activity was due to a decrease in the amount of RE protein, and that the reduction in RE was specific since proteins of starchbranching enzymes I and Ila and ADP-glucose pyrophosphorylase were not markedly affected by sul mutations.The proportion of starch region to the whole endosperm Received

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