Changes in structure of starch and enzyme activities affected by sugary mutations in developing rice endosperm. Possible role of starch debranching enzyme (R-enzyme) in amylopectin biosynthesis

In maturing endosperms of a variety of sugary mutants of rice, phytoglycogen-like polysaccharides with highly branched a -glucans were accumulated instead of amylopectin. while the amylose content greatly decreased. Measurement of activities per endosperm of the 10 major enzymes involved in starch and sucrose metabolism revealed that the activity of starch debranching enzyme (R-enzyme) was specifically reduced in the sugary mutants. The activity of starch branching enzyme I (Q-enzyme I) was also significantly decreased, but less so than the R-enzyme, in the mutants, suggesting some coordination of the expression of the genes coding for R-enzyme and Q-enzyme I. Western blot analysis showed that the sugary mutations of rice resulted in a decrease in the amount of R-enzyme protein, but not in major modification of the enzyme. These findings strongly suggest that R-enzyme plays a critical role in determining the amylopectin fine structure, since at the extremely low level of R-enzyme activity as compared with Q-enzyme activity, as found in sugary mutants, the rice endosperm produced phytoglycogen. We hypothesize that balance of activities or interaction between Q-enzyme and R-enzyme may be responsible for the fine structure of a -polyglucans in plant tissues.