Health-Promoting Phytochemicals in Rice: Genotype Screening for Biofortification Breeding.

The consumption of whole grain cereal has been recommended by many governmental and health organizations because whole grains are good sources of nutrients and may reduce the risk of chronic diseases. Phytochemicals in whole grain cereal may have contributed to some of these health benefits. Most of the phytochemicals in cereal grains are present in the bran layer. In rice these include lipophilic antioxidants, e.g. tocopherols, tocotrienols and gamma-oryzanol in both non-pigmented and pigmented rice cultivars, as well as the phenolic compounds, i.e.  proanthocyanidins in pigmented rice cultivars. These phytochemicals have been shown in animal or in cell culture models to be associated with cardioprotective, anticancer, antidiabetic and anti-inflammation activities, suggesting them as health-promoting compounds to be enriched in rice.

We determined the tocopherols, tocotrienols and gamma-oryzanol concentrations in bran of 192 genotypes of diverse origin. More than two-fold differences in concentrations of total tocotrienols, tocopherols and gamma-oryzanol in bran were found. Genotypes in the indica and aus ancestral lineages had higher gamma-tocotrienol and gamma-tocopherol, while genotypes in the temperate japonica and tropical japonica subpopulations accumulated higher concentrations of alpha-tocotrienol and alpha-tocopherol. Pearson product-moment correlation analysis showed no significant correlation among total tocotrienols, total tocopherols and gamma-oryzanol suggesting each trait could be improved independently through breeding.

For the proanthocyanidins of the 32 pigmented genotypes studied, more than four-fold variation in the extractable total proanthocyanidins concentrations (TP = sum of individual proanthocyanidins of monomers to decamers and polymer fraction) in bran was found; rice extractable proanthocyanidins had high proportions of oligomers and polymers. Concentrations of individual proanthocyanidins were highly correlated with the TP; while the total proanthocyanidin concentration determined by a streamlined colorimetric method, DMAC (dimethylaminocinnamaldehyde), was highly correlated with TP suggesting that the DMAC method would be adequate for screening large numbers of breeding selections. The redness (a*) of the pigmented whole rice kernels as measured by colorimeter using the CIE L*a*b* color scale was positively correlated with the cell-wall bound proanthocyanidins and negatively correlated with TP. Thus, the redness (a*) or the degree of red bran pigmentation would not necessarily reflect the level of proanthocyanidins. Using diverse global rice varieties, genotypes high in phytochemicals were identified, which will allow future breeding and genetics studies to be conducted.