Abstract
The methylated flavone tricin has been associated with numerous health benefits, including reductions in intestinal and colon cancers in animal models. Tricin is found in a wide range of plant species and in many different tissues. However, whole cereal grains, such as rice, barley, oats, and wheat, are the only food sources of tricin, which is located in the bran portion of the grain. Variation in tricin levels was found in bran from rice genotypes with light brown, brown, red, and purple pericarp color, with the purple pericarp genotypes having the highest levels of tricin. Here, we analyzed tricin and tricin derivative levels in developing pericarp and embryo samples of a purple pericarp genotype, IAC600, that had high tricin and tricin derivative levels in the bran, and a light brown pericarp genotype, Cocodrie, that had no detectable tricin or tricin derivatives in the bran. Tricin and tricin derivatives were detected in both the pericarp and embryo of IAC600 but only in the embryo of Cocodrie. The purple pericarp rice had higher total levels of free tricin plus tricin derivatives than the light brown pericarp rice. When expressed on a per grain basis, most of the tricin component of IAC600 was in the pericarp. In contrast, Cocodrie had no detectable tricin in the pericarp samples but did have detectable chrysoeriol, a precursor of tricin, in the pericarp samples. We also used RNA-Seq analysis of developing pericarp and embryo samples of the two cultivars to compare the expression of genes involved in the flavonoid biosynthetic pathway. The results presented here suggest that understanding the basis of tricin accumulation in rice pericarp may lead to an approach to increasing tricin levels in whole grain rice. From analysis of gene expression levels in the pericarp samples it appears that regulation of the flavone specific genes is independent of regulation of the anthocyanin biosynthetic genes. It therefore may be feasible to develop brown pericarp rice cultivars that accumulate tricin in the pericarp.
Highlights
Rice (Oryza sativa) is the world’s most important food crop and is the primary food for more than half of the world’s population (Gnanamanickam, 2009)
Levels of tricin and tricin derivatives in developing embryo and pericarp Free tricin was detected in all the field and greenhouse grown IAC600 embryo and pericarp samples (Fig. 2)
The higher levels of free tricin previously detected in the bran of IAC600 (Poulev et al, 2018) may be due to the apparent overall higher synthesis of tricin in the IAC600 pericarp such that there is free tricin remaining after a subpopulation becoming bound to lignin
Summary
Rice (Oryza sativa) is the world’s most important food crop and is the primary food for more than half of the world’s population (Gnanamanickam, 2009). Tricin levels and expression of flavonoid biosynthetic genes in developing grains of purple and brown pericarp rice. A metabolomics analysis of rice bran identified 453 phytochemicals, 65 of which have been associated with health benefits (Zarei et al, 2017). Vitamin B1 deficiency has been associated with diets in which white rice is a major component (Whitfield et al, 2017). There are efforts to incorporate nutrient rich rice bran into human diets, in developing nations (Friedman, 2013; Borresen & Ryan, 2014). The bran by-product of rice milling has been used in livestock feed for over 100 years and is used in pet foods (Ryan, 2011)
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