Advances Made by Scientists in Breeding Rice with High Resistance to Starch

Scientists have shown that loss of function of two balanced starch synthetic genes contributes to an increase in resistant starch (RS) content in cooked rice, providing insights into the generation of high-RS cultivars in rice and possibly other grains.

The study, conducted by the team of Professor LI Jiayang of the Institute of Genetics and Developmental Biology (IGDB) of the Chinese Academy of Sciences and the team of Professor Wu Dianxing of Zhejiang University, is published online in Proceedings of the National Academy of Sciences on May 1.

Sedentary lifestyles and overeating in the long term lead to obesity, type 2 diabetes and related complications, which are becoming a major threat to global health. This occurrence can be reduced through appropriate dietary methods by regulating glucose homeostasis.

Unlike fast-digesting starch, RS is a kind of special starch that cannot be digested in the small intestine but transported to the large intestine for slow fermentation, which is beneficial to gut health and may improve various related conditions, such as inflammatory bowel disease, insulin resistance and diabetes mellitus. Type 2 and weight management.

Rice is an excellent source of starch, but most cooked rice has low levels of RS (SSIIIa, previously reported by the Li and Wu teams, is a high RS gene. RS content in loss-of-function mutant ssIIIa in an indica rice background with Wxa the allele increased to ~6%. Although SSIIIa and Wx contribute to RS formation, more functional RS genes are still required.

In light of this, the researchers used a high RS mutant rs4 (about 10% of the RS content) resulting from somatic mutations. Through genetic analysis, re-sequencing and cloning of the isolated population, they identified a new RS high gene, SSIIIb, that harbors the frameshift mutation in the rs4 mutant as well as the SSIIIa deficient.

They found that the ssIIIb single mutant had no effect on RS levels, but when co-cultured with ssIIIa to form the ssIIIa double mutant ssIIIb, RS levels increased to 10% in an indica rice background. Increased RS levels in the ssIIIa and ssIIIa ssIIIb mutants have been associated with increased levels of amylose and lipids.

Furthermore, the authors showed that SSIIIb and SSIIIa proteins are derived from Paralogous genes of the SSIII family of rice, whereas SSIIIb functions mainly in leaves and SSIIIa mainly in endosperm due to its divergent tissue-specific expression patterns. SSIII underwent gene duplication in different beans, with one paralog of SSIII expressed primarily in leaves and the other in the endosperm. SSII also showed a similar developmental pattern to that of SSIII. The SSIII and SSII pairings were associated with higher total starch content and lower RS ​​levels in the tested cereal seeds, compared with lower starch content and higher RS ​​levels in the tested pairs.

These findings provide important genetic resources for the breeding of high-RS rice cultivars, and the evolutionary characteristics of these genes may facilitate the production of high-RS cultivars in different cereals.