Identification of Major Genes Affecting Nutritional Element Concentrations in Rice Grains.

Biofortification is the process by which the nutritional quality of food crops is improved through conventional plant breeding and/or use of biotechnology.  Biofortification differs from conventional fortification in that biofortification aims to increase nutrient levels in crops during plant growth rather than through manual means during processing of the crops.  The first step towards targeted breeding is the identification of genes responsible for orchestrating desirable concentrations of various elements in the grain. 

With the aim of identifying germplasm and genes useful for breeding biofortified rice varieties, 1763 rice accessions collected from 114 countries around the world were grown side by side in a replicated field study, so that rice lines expressing exceptionally high or low concentrations of one or more of 16 elements could be identified.  The selected rice lines were crossed to produce several segregating progeny populations in which to further characterize and identify the biofortification genes contained in these rice lines.  Mendelian segregation patterns (i.e. 1:2:1 or 3:1) among F2 progeny, indicative of single major-gene control of the exceptional grain concentrations (biofortification), were seen for six elements: arsenic, cobalt, magnesium, manganese, molybdenum, and zinc.  Linkage was identified in three separate progeny populations between grain molybdenum concentration and markers on chromosome 8, which is known to contain the molybdenum transporter 1 (MOT1) gene.   Molecular characterization continues for the other F2 populations found to segregate in a Mendelian fashion for grain biofortification.   Interestingly, although increased grain arsenic was associated with later maturity among the 1763 divergent rice accessions, in contrast, all three F2 populations found to segregate for a single major gene affecting grain arsenic show early maturity to be associated with increased grain arsenic.  It was also interesting to note that, although grain concentrations of magnesium and potassium were the most strongly correlated among the 16 elements within the 1763 diverse rice accessions, the single F2 population found to segregate 1:3 for grain-magnesium segregated quantitatively for grain-potassium, and showed independence (no correlation) for grain concentration of these two elements.