Biochemical and genetic analysis of the effects of amylose-extender mutation in rice endosperm

Biochemical analysis of amylose-extender (ae) mutant of rice (Oryza saliva) revealed that the mutation in the gene for starch-branching enzyme IIb (BE IIb) specifically altered the structure of amylopectin in the endosperm by reducing short chains with degree of polymerization of 17 or less, with the greatest decrease in chains with degree of polymerization of 8 to 12. The extent of such change was correlated with the gelatinization properties of the starch granules, as determined in terms of solubility in urea solution. The ae mutation caused a dramatic reduction in the activity of BEIIb. The activity of soluble starch synthase I (SSI) in the ae mutant was significan tly lower than in the wild type, suggesting that the mutation had a pleiotr opic effect on the SSI activity. In contrast, the activities of BEI, BEIIa, ADP-Glc pyrophosphorylase, isoamylase, isoamylase, pullulanase, and Suc sy nthase were not affected by the mutation. Therefore, it is stressed that th e function of BEIIb cannot be complemented by Ella and BEL These results st rongly suggest that BEIIb plays a specific role in the transfer of short ch ains, which might then be extended by SS to form the A and B-1 chains of am ylopectin cluster in rice endosperm.