Starch branching enzyme: structure, key functional sites, and regulation of starch characteristics and rice qualities

Starch branching enzymes (BEs) play crucial roles in determining amylopectin structure, starch components, and ultimately, starch properties and crop quality. This review begins by outlining starch biosynthesis and its relationship with starch properties and rice qualities. We summarized the current understanding of the tertiary structure, catalytic mechanism, and key functional sites of BEs, encompassing catalytic residues, substrate-binding sites, and phosphorylation sites. The regulating BEIIb for modulating amylopectin chain length distribution, starch crystalline lamellar structure, starch granule morphology, starch gelatinization resistance, and starch resistance to enzymatic hydrolysis is reviewed. The regulatory impacts of BEIIb deficiency (via frameshift mutation or expression downregulation) and amino acid substitution on rice qualities are critically discussed. Finally, we proposed some future research directions, including: high-throughput screening and identification methods for BEIIb allelic mutants exhibiting a transparency endosperm, utilizing base editing technology to create novel elite BEIIb alleles, pyramiding multiple genes to develop novel rice germplasm rich in resistant starch while maintaining elite grain quality, especially for appearance quality and eating and cooking quality, and integrating AI and machine learning to predict regulation effects. This review not only enriches the theoretical framework concerning BEIIb-mediated regulation of starch components and rice qualities but also provides some specific molecular targets within BEIIb for targeted quality improvement strategies.