Understanding the genetics and chemistry of important quality traits in rice (Oryza sativa L.)

Abstract

Rice grain quality is a determinant of rice breeding success. Although several studies have independently looked at the different traits of quality to infer the phenotype of the grain, few studies have identified the various metabolites that are produced in high quality rice. In recent years, genetic markers have proven to be effective in increasing the efficiency of breeding line selection and therefore shortening the breeding cycle. In complex traits such as rice grain quality, the availability of more specific phenotypes will increase the value of these genetic markers. Many traits of quality in rice are associated with starch; however, lipids interact with starch and have been shown to influence many of the traits of eating quality and aroma. To identify these new phenotypes and understand their genetic basis, using relevant tools that can discriminate between the phenotypes of varieties at the metabolite and genetic levels is necessary. To identify these new phenotypes, in Chapter 3, a set of 40 Cambodian rice varieties were screened for physical and texture-associated traits including apparent amylose content (AAC), gelatinisation temperature (GT), gel consistency (GelCon) and pasting properties and then a subset from this set was screened for volatile compounds using a two-dimensional gas chromatography-time-of-flightmass spectrometer (GC×GC-TOF-MS) and fatty acids (FA) using gas chromatography-mass spectrometry (GC-MS). The FA analysis showed that unsaturated FA (UFA) oleic (C18:1n-9) and linoleic acid (C18:2n-6) were the most abundant in milled rice grains, followed by palmitic acid (C16:0). These results infer about the lipid origin of odour-active volatile compounds, which were found to be characterising the group of known fragrant, low amylose and soft-textured rice varieties, including the Jasmine-style indica variety Phka Rumduol (PRD). To identify the genetic determinants of these important traits of quality, in Chapters 4 and 5, quantitative trait loci (QTL) mapping was carried out for the texture and aroma traits in PRD using a population of about 300 F6 recombinant-inbred lines (RIL) derived from an intraspecific cross between PRD and Thmar Krem (TMK), both of which were identified as the most discriminating varieties in Chapter 3. This RIL population was genotyped for single nucleotide polymorphisms (SNPs). QTL analysis revealed large-effect QTLs for AAC, GT and all of the pasting properties. Candidate genes for these QTLs relate back to the starch biosynthesis pathways. Major QTLs were colocalised to the Waxy (Wx) gene and starch synthase IIa (SSIIa) on chromosome 6. Minor QTLs on chromosomes 3 and 8 were identified for AAC, SB, PV and HPC, and at chromosomes 4 and 7 for GT and pasting temperature (PTemp). This chapter has shown that the rice texture is regulated by multiple genes and that although correlations do exist among the routinely measured textural traits, these correlations may not necessarily translate into the already existing molecular markers. Publications included in this thesis...