With the improvement of living standards and refined diet, obesity, diabetes and other metabolic chronic diseases caused by reduced exercise and overeating are becoming increasingly serious. Dietary intervention and reasonable diet are one of the effective strategies for chronic diseases prevention and control. Resistant starch (RS) is a general term for starch and degradation products that are difficult to digest and absorb in the small intestine of healthy individuals. As a new type of dietary fiber, resistant starch has important physiological functions in preventing and controlling diabetes, reducing blood lipids, controlling weight and maintaining intestinal health. Rice is the main grain crop in China, but the content of resistant starch in ordinary rice varieties is very low. At present, little is known about the functional genes involved in the synthesis of resistant starch, and it is difficult to improve and cultivate high resistant starch rice varieties without theoretical support.
The research team of Li Jiayang from the Institute of Genetics and developmental biology of the Chinese Academy of Sciences and the research team of Wu Dianxing from the Institute of Nuclear Agricultural Sciences of Zhejiang University have made new progress in the research on the synthesis mechanism of rice resistant starch. Previously, a combination of two research projects found that mutations in SSIIIa (soluble starch synthase) increased the resistant starch content of indica rice from less than 2% to 6%. In the latest study, the collaborating team re sequenced and gene cloned a rice mutant rs4 with a 10% resistant starch content obtained through physical mutagenesis, and found that the high resistant starch phenotype of the mutant was caused by a co mutation of the SSIIIa and SSIIIb genes. There was no significant change in the resistant starch content of the ssIIIb single mutant, but the resistant starch content of the ssIIIa ssIIIb double mutant was significantly increased compared to the ssIIIa single mutant. With the increase of resistant starch content, the content of amylose and starch lipid complexes in the ssIIIa ssIIIb double mutant also significantly increased.
Subsequently, the study further elucidated that both SSIIIa and SSIIIb belong to the homologous genes of SSIII, but SSIIIb is mainly highly expressed in leaves, while SSIIIa is specifically expressed in seeds. Introducing the strong promoter driven SSIIIb gene into the ssIIIa mutant background can restore the resistant starch content to the wild-type level, indicating that the protein functions of SSIIIa and SSIIIb are the same, while the functional differences between SSIIIa and SSIIIb genes are mainly caused by the different promoters and expression patterns of the two. Meanwhile, the resistant starch content of ssIIIa ssIIIb mutant in japonica rice background was significantly lower than that of ssIIIa ssIIIb mutant in indica rice background, indicating that the synthesis of resistant starch mediated by SSIIIa and SSIIIb depends on the Wx alleles differentially expressed in indica and japonica rice.
Analysis of the phylogenetic tree and expression profile of the water-soluble starch synthase SS family revealed that gene replication occurred between the SSII family and the SSIII family in cereal plants, and the genes before and after replication had specific expression patterns. One type was specifically expressed in seeds, while the other type was mainly highly expressed in leaves. Meanwhile, cereal plants that replicate the SSII and SSIII genes have increased starch content in their seeds while having lower resistant starch content (less than 2%), while dicotyledonous plants with only one homologous gene between SSII and SSIII have lower starch content and generally higher resistant starch content (3-10%). It is speculated that the differentiation of gene replication and expression patterns between SSII and SSIII may be related to a decrease in resistant starch content in grain seeds.
In summary, this study identified and cloned a new gene SSIIIb that controls the synthesis of resistant starch, and analyzed the molecular mechanism and evolutionary significance of SSIIIb and SSIIIa jointly regulating the synthesis of resistant starch. This provides important genetic resources for improving the nutritional quality of rice and cultivating high resistant starch nutritional functional rice varieties.
The research team of Li Jiayang from the Institute of Genetics and developmental biology of the Chinese Academy of Sciences and the research team of Wu Dianxing from the Institute of Nuclear Agricultural Sciences of Zhejiang University have made new progress in the research on the synthesis mechanism of rice resistant starch. Previously, a combination of two research projects found that mutations in SSIIIa (soluble starch synthase) increased the resistant starch content of indica rice from less than 2% to 6%. In the latest study, the collaborating team re sequenced and gene cloned a rice mutant rs4 with a 10% resistant starch content obtained through physical mutagenesis, and found that the high resistant starch phenotype of the mutant was caused by a co mutation of the SSIIIa and SSIIIb genes. There was no significant change in the resistant starch content of the ssIIIb single mutant, but the resistant starch content of the ssIIIa ssIIIb double mutant was significantly increased compared to the ssIIIa single mutant. With the increase of resistant starch content, the content of amylose and starch lipid complexes in the ssIIIa ssIIIb double mutant also significantly increased.
Subsequently, the study further elucidated that both SSIIIa and SSIIIb belong to the homologous genes of SSIII, but SSIIIb is mainly highly expressed in leaves, while SSIIIa is specifically expressed in seeds. Introducing the strong promoter driven SSIIIb gene into the ssIIIa mutant background can restore the resistant starch content to the wild-type level, indicating that the protein functions of SSIIIa and SSIIIb are the same, while the functional differences between SSIIIa and SSIIIb genes are mainly caused by the different promoters and expression patterns of the two. Meanwhile, the resistant starch content of ssIIIa ssIIIb mutant in japonica rice background was significantly lower than that of ssIIIa ssIIIb mutant in indica rice background, indicating that the synthesis of resistant starch mediated by SSIIIa and SSIIIb depends on the Wx alleles differentially expressed in indica and japonica rice.
Analysis of the phylogenetic tree and expression profile of the water-soluble starch synthase SS family revealed that gene replication occurred between the SSII family and the SSIII family in cereal plants, and the genes before and after replication had specific expression patterns. One type was specifically expressed in seeds, while the other type was mainly highly expressed in leaves. Meanwhile, cereal plants that replicate the SSII and SSIII genes have increased starch content in their seeds while having lower resistant starch content (less than 2%), while dicotyledonous plants with only one homologous gene between SSII and SSIII have lower starch content and generally higher resistant starch content (3-10%). It is speculated that the differentiation of gene replication and expression patterns between SSII and SSIII may be related to a decrease in resistant starch content in grain seeds.
In summary, this study identified and cloned a new gene SSIIIb that controls the synthesis of resistant starch, and analyzed the molecular mechanism and evolutionary significance of SSIIIb and SSIIIa jointly regulating the synthesis of resistant starch. This provides important genetic resources for improving the nutritional quality of rice and cultivating high resistant starch nutritional functional rice varieties.
