Despite diabetes being an age-old disease of humanity, it still leans in healthcare management, relying on evidence-based interventions termed the lean management system rather than cure. Obesity is the risk factor for the progression of diabetes and the greater risk exerted by diabetes for cardiovascular disease due to the impact of this metabolic disease on the blood vessels and the nerves that control the heart. Diabetes is one of the prominent causes of death and disability.
The keystone of diabetes is regulating blood sugar or blood glucose level, which can be measured to improve its level through physical activity, lifestyle modifications, or medication adherence. For managing diabetes or preventing the ailment, attention is paid to the role of dietary components of the food eaten, as the sugar comes from it. Disturbances in the blood glucose homeostasis mediated by the peptide hormone insulin produced by the beta cells in the pancreas result in diabetes. Type 1 diabetes is a chronic autoimmune condition (life-long) in which the immune system mistakenly permanently damages the healthy body cells, in this case, the b-cells of the pancreas, limiting insulin, a peptide hormone availability. Type 2 diabetes is caused either by insufficient insulin, a peptide hormone that regulates the metabolism of carbohydrates, fats, and protein or by insulin resistance, in which the body cells do not respond to the hormone for transporting blood glucose to the cells. Both genetic and lifestyle conditions can influence insulin resistance. The third is gestational diabetes, caused by interference with the insulin function (a form of insulin resistance) by other hormones arising during pregnancy.
The amount of carbohydrates consumed decides the glucose level in the blood and how much insulin one needs to keep the optimum blood sugar level. The consumption of starchy food and sugar intake need to be regulated regardless of the type of diabetes to maintain the proper blood sugar level. For this purpose, carbohydrate-based foods are classified by glycemic (glycaemic) index (GI), which measures the impact of carbohydrates on blood glucose levels and how quickly and much the blood sugar increases on a scale of one to 100 in response to consumption. It is categorised from low to high: low – equals to or less than 55, medium – 56-69, high – equals to or higher than 70.
Rice is the principal source of staples and energy for half the global population, providing satiety and satisfaction with a complete meal. The diabetes-affected population that depends on rice as the staple is predominantly spread in South Asia and Central Africa, and its proportion is alarmingly increasing. Protein deficiency is also familiar to many of these people. Rice is generally considered an energy source with a moderate GI, varying between 60-70. The literature has a more comprehensive range of variants up to GI 90. GEB24 (syn: Kichilli samba), a spontaneous mutant of landrace Konamani, selected by the Government Economic Botanist, Dr. K. Ramiah, was the first released cultivar in 1921 by the Paddy Breeding Station, Coimbatore, Tamil Nadu, India. Incidentally, this rice has a low GI of 50 (and 5.6% resistant starch—discussed below) with a unique citrusy flavour (Selvakumar et al. 2014). This variety has played a significant role in the development of rice globally by its usage in many national and international breeding programmes. Characteristically, the quality of milled rice consumed has an elevated level of easily digestible carbohydrates. However, despite being an excellent source of protein with an adequate balance of amino acids except for the essential amino acid lysine, the nutritional value of rice is incomplete and places rice eaters at higher risk for diabetes and malnutrition.
Moreover, the type and amount of processing used to convert rice increase its GI. All white rice starts as brown rice. The average GI for brown rice is low, at 55, while the GI for white rice is higher, at 64. Wild rice has a lower GI value than white and brown rice. Long-grain rice with higher amylose content, like Basmati and Jasmine rice, tends to have lower GI values of 50 and 45-50, respectively. Short-grain waxy (glutinous) rice with lower amylose content has higher GI values. A recent attempt to unveil the genic regulation of aroma in rice, predominantly caused by 2-acetyl-1-pyroline, showed that it is boosted by the availability of the amino acid ornithine together with the elevation of amylose content in rice (Li et al. 2024). However, the basis of this influence is not known. The compartmental separation of two biosynthetic pathways in plant cells is not absolute. Unearthing the intricate mechanisms of this phenomenon surrounding amylose biosynthesis might be of interest in associating it with low GI values. Earlier, IRRI developed two salt-tolerant rice lines, IRRI147 and IRRI125, with low glycemic indexes (GIs) of 55 and 51, respectively, and released them for cultivation in the Philippines.
Genetically managing the GI level has been a longstanding quest. This includes genomically targeting starch synthesis and structure and amylopectin in rice science, which has gradually spread to other food crops yielding carbohydrate-rich food materials. However, evaluating diverse germplasm collections to isolate low GI genotypes is laborious, expensive, and impracticable.
During the past one-and-a-half decades, the biomolecular and genomic information gathered led to new insights, especially mobilisation patterns of total starch in the germinating seeds, which is similar to its digestion in the human gastrointestinal tract (de Guzman et al. 2017), the influence of amylose on starch physicochemical behaviour, and its contribution to resistant starch (Butardo et al. 2011, 2012; Seung 2020) paved the way for new directions in developing suitable rice for people with diabetes. Starch or amylum contains a mixture of two glucose polymer molecules: amylose (10-30%) and amylopectin (70-90%). Amylose, a polysaccharide composed of long unbranched chains of glucose units, is embedded in amylopectin, a highly branched polymer of glucose units in starch granules. Starch branching enzymes (SBEs) are essential for starch biosynthesis. SBEs control the synthesis of the branched amylopectin in plants, and silencing of STARCH BRANCHING ENZYME 2B (SBEIIb) increases the amylose content (Shimada et al. 2006). The linear structure of amylose contributes to its ability to form helical structures and its resistance to digestion, making it a critical factor in determining the glycemic index of starchy foods. Therefore, the proportion of amylose and amylopectin influences the GI and how easily the starch is digested. A higher proportion of amylose in starch makes it resistant and unamenable for digestion until it reaches the colon, where it is broken down by gut microbes and converted into short-chain fatty acids. Thus, resistant starch helps digest lesser amounts of carbs to be absorbed in the blood, reducing GI values.
In a prelude study (Guo et al. 2020), CRISPR-mutated loss-of-function recessive OssbeIIb mutants were characterised by high levels of amylose, reflecting a doubling effect. They also had an enhanced proportion of resistant starch (RS, 6%) and low glutelin, the primary rice seed storage protein. This suggests a negative relationship between amylose and grain protein content.
Recently, in a remarkable breakthrough study (Badoni et al. 2024), a team of international scientists led by Dr Nese Sreenivasulu at the International Rice Research Institute, the Philippines (IRRI) pinned down the culpable gene, OsSBEIIb maintaining high GI-low amylose values in rice grains using a multi-omics approach. Consumption of food with a high level of GI raises the level of blood sugar, causing diabetes syndrome, a chronic disease associated with the consumption of carbohydrate-rich food for people who are either resistant to or deficient in insulin.
Ultralow GI (below 45), low GI (below 50), medium GI (65), and a range of apparent amylose (18 to 37%) and grain protein (6-16%) contents were recorded in the amylose bulk pool analyses. Using quantitative trait loci (QTL) sequencing with the high amylose and high protein (HAHP) and low amylose and low protein (LALP) bulk segregant lines arising from the parental lines, IR36 amylose extender (IR36ae) mutant line (high amylose (35%) and resistant starch and also low GI=54) and Samba Mahsuri (good grain quality with medium, slender grains, amylose (25%, GI -not available), the IRRI team identified the genes governing the desired trait of HAHP in the advanced breeding lines. This has ensured the feasibility of separating the undesired association between high amylose and low protein traits to facilitate the pyramiding of genes governing HAHP traits in single breeding lines.
Even though rice is a grain and is most well-known for carbohydrates, the protein in rice adds nutritional benefits. White rice is considered a refined grain, and it has the same amount of protein as other types of rice, such as brown rice and wild rice. This has prompted conventional rice breeding programmes to pursue the development of high-protein rice, resulting in an increase from the average protein content between 7% and 9% in the past. Rice with 10% protein is also available. Foods with low GI contain higher amounts of protein, fat, or fibre, ideal for maintaining good health. True to this relationship, the dual presence of high amylose and protein content is possible in the presence of the recessive allele sbeIIb. Combinations of the novel-created alleles generated ultra-low to low GI-high amylose and notable differences in amylopectin structure and ultra-high protein phenotypes. Importantly, the high amylose and high protein recombinant inbred line (HAHP_101) rates a GI of 55 and is enriched with 16% (500% increase over conventional milled rice) protein comprising the essential amino acids, including lysine, as revealed by metabolomic analysis.
Genetic techniques and classification models (statistical techniques involving machine learning algorithms) were used in unison with the recombinant breeding strategy undertaken in this study. This has helped dissociate the grain hardness of cooked grain related to high grain protein, affecting grain quality (Fukai and Mitchell 2024). This is similar to breaking the relationship between high amylose and low protein traits, as the HAHP_101 line maintains good grain quality. As this inbred line is a result of crossing between two closely related rice variants, this can reach farmlands suitable for growing without any difficulty.
The association of OsSBEIIb with GI is confirmed in a uniquely obtained CRISPR mutant of the high-yielding rice line IRRI154 in this study. The recessive gene, OssbeIIb parallels fgr and sd1 genes, contributing to grain fragrance and semi-dwarf stature traits (Bradbury et al. 2005), respectively, and rod1 (RESISTANCE OF RICE TO DISEASES1) conferring multi-pathogen (including fungal and bacterial pathogens Magnaporthe oryzae causing blast, Rhizoctonia solani causing sheath blight and Xanthomonas oryzae pv. oryzae causing bacterial blight of rice) resistance in rice (Wu et al. 2024, see the blog “Balancing trade-off between disease resistance and plant growth” in this collection). These genes have arisen from the loss-of-function of their respective dominant alleles without affecting the basic survival ability of the plants and, therefore, do not warrant any trade-off with desirable agronomic traits.
In addition to generating low-GI and high-protein inbred lines, the role of the OsSBEIIb gene in lowering the glycemic index (GI) was validated using CRISPR-mediated targeted mutagenesis on this gene with the high-yielding rice line IRRI154. This exercise also aimed to minimise the mutational effects on grain chalkiness and size. The GI of the transgenic rice lines obtained in this study varied between 50 and 53. The resulting marker-free lines will enter multi-location trials to evaluate their geographical suitability and undergo biosafety assessments.
Monitoring disease incidence, including bacterial blight, blast, and other minor panicle diseases, is crucial because the source-sink relationships are active during the heading stage. This activity facilitates the transfer of assimilates (organic carbon and nitrogen) to the grains, enabling the assembly and synthesis of complex molecules like starch and protein. This process leads to the efflux of nutrients from host cells, essential for pathogen multiplication and development, ultimately resulting in diseases. Pathogen susceptibility mediated by transporters is common; sugar transporters (SWEET genes, Chen et al. 2010) and amino acid transporters (UmamiT genes, Prior et al. 2024) serve as mechanisms for pathogen virulence.
The recombinant inbred rice lines generated by IRRI, which offer the dual benefit of low GI and high protein content, pave the way for incorporating rice into healthcare strategies to combat diabetes. Apart from making lines such as HAHP_101 available to farmers for cultivation, envisioning methods to manipulate the production of low-GI rice with higher protein content using the genetic resources generated in this study is bright.
References
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Suggested citation:
Sridhar, R. (2024, November 22) Nailing down the culprit gene for beating diabetes—a case of duelling effect on rice grain protein. https://sridharr.com/nailing-down-the-culprit-gene- for-beating-diabetes-a-case-of-duelling-effect-on-rice-grain-protein
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