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, phytogl ycogen-like polysaccharides with highly branched alpha-glucans were ac cumulated instead of amy amylopectin, while the amylose content greatl y decreased. Measurement of activities per endosperm of the 10 major e nzymes involved in starch and sucrose metabolism revealed that the act ivity of starch debranching enzyme (R-enzyme) was specifically reduced in the sugary mutants. The activity of starch branching enzyme I (Q-e nzyme I) was also significantly decreased, but less so than the R-enzy me, in the mutants, suggesting some coordination of the expression of the genes coding for R-enzyme and Q-enzyme I. Western blot analysis sh owed that the sugary mutations of rice resulted in a decrease in the a mount of R-enzyme protein, but not in major modification of the enzyme . These findings strongly suggest that K-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-enzym e and R-enzyme map be responsible for the fine structure of alpha-poly glucans in plant tissues.