Identification of Structure-Controlling Rice Biosynthesis Enzymes

Area of Science:

• Plant Biochemistry
• Molecular Genetics
• Food Science

Background:

• Starch chain-length distributions (CLDs) are crucial for rice quality and are controlled by starch synthases (SSs), starch branching enzymes (SBEs), and debranching enzymes (DBEs).
• Existing models for CLDs use enzyme activity ratios but do not specify individual isozymes involved.
• Identifying specific isozymes controlling CLDs is essential for targeted breeding of improved rice varieties.

Purpose of the Study:

• To explore and specify the roles of individual starch-modifying enzyme isoforms in controlling CLDs for amylose and amylopectin synthesis in rice endosperm.
• To link specific isozymes to the degree of polymerization (DP) ranges they dominate.
• To provide a basis for breeders to develop rice with desired starch properties.

Main Methods:

• Analysis of molecular weight distributions of starch chains from 87 rice varieties using size-exclusion chromatography after enzymatic debranching.
• Fitting the obtained CLD data using biosynthesis-based models.
• Identification of enzyme isoform mutants through amino acid mutations in coding sequences of extracted whole gene sequences.

Main Results:

• Significant differences in CLDs were observed between mutant groups of different isoforms, enabling the identification of key enzymes.
• Isoforms SSI, SSII-3, SSIII-1, SSIII-2, SBEI, and GBSSI were identified as controlling amylose biosynthesis.
• GBSSI was also found to be involved in controlling amylopectin chain synthesis.

Conclusions:

• This study successfully specifies the roles of individual isozymes and the DP ranges they control across the entire DP spectrum.
• The findings enable precise targeting of enzyme isoforms for manipulating starch biosynthesis.
• This knowledge can significantly aid rice breeders in developing varieties with enhanced functional properties.