Asia-Pacific Biotech News

Developing High-Resistant Starch for a Healthier Diet

Using biotechnology, scientists were able to develop healthier rice that could be essential in the diet of Type 2 diabetics.

Rice is a major staple food for half of the world’s population, providing up to half of one’s daily energy intake. However, the rapid digestion and absorption of starch from rice consumption may lead to blood glucose abnormalities and serious health issues, such as Type 2 diabetes. Due to the prevalence of Type 2 diabetes worldwide and its status as a public health concern, there is a growing demand and greater push towards the production of Resistant Starch (RS), a special type of starch that cannot be digested in the small intestine and instead passed to the large intestine for slow fermentation. In other words, RS lowers the possibility of acquiring Type 2 diabetes as it becomes more difficult for the body to break down starch into glucose.

Current rice varieties in the market have a low RS value of less than 2%. Recently however, scientists from Prof. Li Jiayang’s team at the Institute of Genetics and Developmental Biology (IGDB) of the Chinese Academy of Sciences and Prof. Wu Dianxing’s team at Zhejiang University have managed to increase the low RS value of current rice varieties to the recommended RS value (> 10 %) of a healthy diet. Their study was published in PNAS.

The key to their findings is the manipulation of rice content using a combination of mutagenesis, selective breeding, and gene editing methods. Rice is a starch made up of the carbohydrate macromolecules, amylopectin and amylose, both of which are synthesised by a group of proteins called soluble starch synthases (SSs). The rice genome is known to express nine SSs that can be grouped into five classes, from SSI to SSV. Tweaking these genes to enhance amylose biosynthesis and inhibit amylopectin has been reported to increase the RS content of rice.

Previously, the researchers have identified that having a defective SSIIIa gene increases the RS content in cooked indica rice to ~5.5%. Further investigation into the SSIII gene reveals that creating a double mutant SSIIIa SSIIIb significantly increases the RS content of cooked rice to 10.8%. In addition, they found that the SSIIIa SSIIIb mutant leads to the formation of more amylose-lipid complexes, which further increases RS content.

Aside from developing high-RS content in rice, the researchers were also able to study the evolution of the SS genes. Among the five SS classes, they found that SSII and SSIII underwent a duplication event before the divergence of rice from other grasses. As a result, these genes developed new functions, with SSIIa and SSIIIa genes being preferentially expressed in rice grains while SSIIb and SSIIIb genes are preferentially expressed in leaves. The authors suggest that this duplication event may be the cause of the low-RS content and densely packed starch granules in rice.

Overall, the researchers report that the loss-of-function mutations in SSIIIa and SSIIIb genes synergistically lead to a high-RS content in rice. However, they noted that such mutations tend to form irregular starch shapes and reduce grain yield. They stress the importance of using genome editing technologies to systematically study different combinations of amylopectin biosynthetic genes to further increase RS content in rice without compromising grain quality and yield. Further advancement in high-RS rice will be critical in the fight against the diabetes epidemic. [APBN]

Source: Wang, A., Jing, Y., Cheng, Q., Zhou, H., Wang, L., Gong, W., … Li, J. (2023). Loss of function of ssiiia and ssiiib coordinately confers high RS content in cooked rice. Proceedings of the National Academy of Sciences, 120(19). doi:10.1073/pnas.2220622120