Scientists discover molecular mechanism of RS biosynthesis in rice

The sedentary lifestyle and refined food consumption significantly lead to obesity, type 2 diabetes, and related complications, which have become one of the major threats to global health. These diseases could be effectively guarded against by daily foods rich in resistant starch (RS). Rice is a traditional staple in China. However, common rice varieties are deficient in RS, so to breed high-RS rice varieties remains a challenge.

LI Jiayang from the CAS Institute of Genetics and Developmental Biology and WU Dianxing & SHU Xiaoli from the Zhejiang University Institute of Nuclear Agriculture Sciences made new breakthroughs in the molecular mechanism of RS biosynthesis in rice recently. Their findings were published in an article titled “Loss of function of SSIIIa and SSIIIb coordinately confers high RS content in cooked rice” in the journal PNAS.

Fig.1. Characterization of the high-RS mutant rs4 and cloning of SSIIIb.

The researchersreported a high-RS mutant rs4 with an RS content of ~10.8% in cooked rice. Genetic studies revealed that the loss-of-function SSIIIb and SSIIIa together with a strong Wx allele in the background contributed to the high-RS phenotype of the rs4 mutant. The increased RS content in ssIIIa and ssIIIassIIIb mutants was associated with the increased amylose and lipid content. SSIIIb and SSIIIa proteins were functionally redundant, whereas SSIIIb mainly functioned in leaves and SSIIIa largely in the endosperm owing to their divergent tissue-specific expression patterns. Furthermore, they found that SSIII experienced duplication in different cereals, of which one SSIII paralog was mainly expressed in leaves and another in the endosperm. SSII but not SSIV showed a similar evolutionary pattern to SSIII. The copies of endosperm-expressed SSIII and SSII were associated with high total starch content and low RS levels in the seeds of tested cereals, compared with low starch content and high RS levels in tested dicots.

This study provides critical genetic resources for breeding high-RS rice varieties, and the evolutionary features of these genes may facilitate to develop high-RS varieties in different cereals.