Breeding rice varieties with improved grain and nutritional quality

Abstract

Worldwide, 3 billion people depend on rice for more than 20% of their daily calorie intake. Like yield, rice grain quality has now become a primary consideration for producers and consumers. However, preference for grain quality is highly variable in different regions, countries and among different consumers. The grain consists of ≈80% starch and its quality is dependent on a combination of several traits, some of which are influenced by environmental factors. The major physical traits include grain shape, size, chalkiness, degree of milling, and head rice recovery, and chemical characteristics-amylose content, gelatinization temperature and gel consistency. A number of Quantitative Trait Loci (QTLs) have been identified and molecular markers for some of the traits are available. Starch biosynthesis, however, is a complex system involving as many as 18 genes including starch synthases, branching and debranching enzymes, which act in concert to affect starch structure and function at least at six organizational levels. Aroma carries a special place in rice and is controlled by 1 or 2 major genes, and molecular markers for three alleles are available. To improve the nutritional quality, major focus is on developing rice varieties with Pro-vitamin A, high iron and zinc content in the polished grain. Pro-vitamin A enriched rice, Golden Rice, has been used to develop elite indica breeding lines, through marker assisted selection (MAS). Screenhouse and confined field tests have shown that these transgenic Golden Rice lines are similar in agronomic performance to IR64, PSBRc82 and BR29. Iron content is being increased using both conventional and transgenic approaches. Through conventional breeding, enhanced levels of zinc have been produced and QTLs are being mapped. We plan to pyramid genes for Pro-vitamin A, high iron and zinc content to develop micronutrient enriched rice. Some of the priority areas for the genetic enhancement of grain quality include; (1) development of precise quantitative assays for different quality traits, (2) dissect complexity of the genetic control of eating and cooking quality characteristics, (3) discovery of genes known to enhance milling yield, head rice recovery and bran oil quality, (4) better understanding of the chemistry and genetics of cooking and eating quality traits of traditional varieties and of leading varieties like IR64, Basmati 370, Koshihikari, Khao Dawk Mali, (5) formulate selection indices to breed for improved grain quality, (6) develop varieties possessing slowly digestible starch (low glycemic index) that could assist in managing the global pandemic of diabetes. With the recent advances in analytical tools, molecular markers, applied genomics, proteomics and metabolomics, the scope for improving grain and nutritional quality in rice, and combining that with high yield, seems more promising than before.