Discovery of novel starch biosynthetic genes enhances potential for high-resistant starch varieties in rice and other cereals

A recent study conducted by Prof. LI Jiayang’s team from the Institute of Genetics and Developmental Biology (IGDB) of the Chinese Academy of Sciences and Prof. Wu Dianxing’s team from Zhejiang University has revealed that the loss of function of two similar genes involved in starch biosynthesis contributes to an increase in resistant starch (RS) content in cooked rice. This finding not only sheds light on the development of high-RS rice varieties but also provides insights into improving the nutritional value of other cereals.

Resistant starch (RS) is a unique type of starch that cannot be digested in the small intestine but undergoes slow fermentation in the large intestine. It offers several health benefits, such as promoting intestinal health, managing weight, and potentially addressing conditions like inflammatory bowel disease, insulin resistance, and type 2 diabetes.

While rice is a rich source of starch, most varieties of cooked rice contain low levels of RS, typically less than 2%. Therefore, identifying new genes responsible for RS production in rice is of significant importance for enhancing its nutritional value.

Previous studies by Li and Wu’s teams had identified a high-RS gene called SSIIIa. The RS content in a loss-of-function mutant of SSIIIa in the indica rice background increased to around 6%. Although SSIIIa and another gene called Wx contributed to RS formation, the researchers aimed to identify additional RS genes to further enhance RS levels.

To achieve this, the scientists analyzed a high-RS mutant called rs4, which exhibited an RS content of approximately 10%. Through genetic analysis, resequencing, and cloning, they discovered a novel high-RS gene named SSIIIb, which carried a frame-shift mutation in the rs4 mutant, in addition to the deficiency of SSIIIa.

Interestingly, the single mutation of ssIIIb alone did not affect RS levels. However, when combined with ssIIIa to form a double mutant called ssIIIa ssIIIb, RS levels further increased to 10% in the indica rice background. The elevated RS levels in both ssIIIa and ssIIIa ssIIIb mutants were associated with increased amylose and lipid levels.

Moreover, the researchers found that SSIIIb and SSIIIa proteins originate from similar genes within the rice SSIII family. However, they function differently due to their distinct tissue-specific expression patterns. SSIIIb mainly functions in leaves, while SSIIIa primarily operates in the endosperm. This indicates that the evolution of these genes involves gene duplication in various cereals, where one paralog is expressed predominantly in leaves and the other in the endosperm. A similar evolutionary pattern was observed for another gene called SSII. The duplication of SSIII and SSII genes in cereals is linked to high total starch content and low RS levels in the seeds, as opposed to low starch content and high RS levels observed in dicotyledonous plants.

These findings provide valuable genetic resources for the development of high-RS rice varieties. Additionally, understanding the evolutionary characteristics of these genes may facilitate the generation of high-RS varieties in different cereals.

Source: Chinese Academy of Sciences

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