Mechanistic insights into the role of starch multi-level structures in functional properties of high-amylose rice cultivars

The multi-level structures of starch (including molecule, crystalline and granular morphology) and functional properties (pasting behavior, gelatinization and in vitro digestion) of starch granules from different rice cultivars were investigated and their relationships were analyzed. At the molecular level, the transgenic rice cultivar RS10 modified by inhibition of starch branching enzyme IIb (SBE IIb) presented significantly higher amylose content (46.7%), longer amylose chains, smaller whole molecular size and higher amount of amylopectin long B₃ branch-chains compared to its wild-type line RS10W and the other two regular cultivars. All the rice cultivars showed A-type crystalline structure but the relative crystallinity varied significantly with the lowest in RS10 and the highest in the low-amylose cultivar Chang5. Pearson correlation analysis indicated that the gelatinization temperature and RS content showed significantly positive correlation with amylose content and average amylopectin chain length, but were negatively correlated with short branch-chains in amylopectin (A chains). The gelatinization enthalpy, relative crystallinity, pasting viscosity, and digestion rate and extent were observed showing significantly negative correlation with amylose content and B₃ chains. The structure-functionality relationships illustrated the variations in the functional properties among the rice cultivars could be largely attributed to the different starch molecular and crystalline structures. Nonetheless, the highly aggregated compound voluminous granules could also be responsible for the remarkable lower digestion rate and improved RS content in the native starch as observed in the transgenic high-amylose RS10. This work is meaningful for plant breeders to develop rice cultivars with tailored starch functionality and enriched RS content.