The Genetic Basis and Nutritional Benefits of Pigmented Rice Grain

Abstract

Improving the nutritional quality of rice grains through modulation of bioactive compounds and micronutrients represents an efficient means of addressing nutritional security in societies which depend heavily on rice as a staple food. White rice makes a major contribution to the calorific intake of Asian and African populations, but its nutritional quality is poor compared to that of pigmented (black, purple, red orange, or brown) variants. The compounds responsible for these color variations are the flavonoids anthocyanin and proanthocyanidin, which are known to have nutritional value. The rapid progress made in the technologies underlying genome sequencing, the analysis of gene expression and the acquisition of global 'omics data, genetics of grain pigmentation has created novel opportunities for applying molecular breeding to improve the nutritional value and productivity of pigmented rice. This review provides an update on the nutritional value and health benefits of pigmented rice grain, taking advantage of both indigenous and modern knowledge, while also describing the current approaches taken to deciphering the genetic basis of pigmentation.

Keywords: flavonoids; genetics; metabolites; nutrition; pigmented rice grain.

FIGURE 1

Genetic variation for grain pigmentation in rice. Grains featuring (a,b) white, (c,d) brown, (e–h) purple, (i,j) dark purple or black, (k–n) red, and (o,p) mixed colored pericarp. The application of various genomic approaches to understand the genetic pathway of grain pigmentation is outlined.

FIGURE 2

Secondary metabolism in rice. (A) A schematic representation of the shikimic acid pathway. DAHP, 3-deoxy-D-arabino-heptulosonic acid 7-phosphate; DAHPS, 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase; DHQ/SDH, 3-dehydroquinate dehydratase/shikimate 5 dehydrogenase; DHQS, 3-dehydroquinate synthetase; DHS, 3-dehydroshikimic acid; SDH, shikimate dehydrogenase; SK, shikimate kinase; S3P, shikimic acid 3-phosphate; EPSPS, 5-enolpyruvylshikimate 3-phosphate synthase; EPSP, 5-enolpyruvylshikimate 3-phosphate; CS, chorismate synthase; CM, chorismate mutase; PAT, prephenate aminotransferase; ADT, arogenate dehydratase; PDT, prephenate dehydratase; PPY-AT, phenylpyruvate aminotransferase (Tzin and Galili, 2010; Widhalm and Dudareva, 2015; Santos-Sánchez et al., 2019). (B) Possible routes to the production of benzoic acid, benzoic acid-derived compounds and lignin. CNL, cinnamate-CoA ligase; CHD, cinnamoyl-CoA-dehydrogenase/hydratase; KAT1, 3-ketoacyl-CoA thiolase; TE, CoA thioesterase; BA2H, benzoic acid 2-hydroxylase; BALDH, benzaldehyde dehydrogenase; AO, aldehyde oxidase; C4H, cinnamate 4-hydroxylase; 4CL, 4-coumarate:CoA ligase; ICS, isochorismate synthase; CCR, cinnamoyl-CoA reductase; CCoAOMT, caffeoyl-CoA O-methyltransferase; F5H, ferulate 5-hydroxylase; CSE, caffeoyl shikimate esterase; COMT, caffeic acid O-methyltransferase; CAD, cinnamyl alcohol dehydrogenase; LAC, laccase; POD, peroxidase; p-HBD, p-hydroxybenzaldehyde; HBDS, 4-hydroxybenzaldehyde synthase; HCHL, 4-hydroxycinnamoyl-CoA hydratase/lyase; HBD, 4-hydroxybenzaldehyde dehydrogenase (Qualley et al., 2012; Gallage and Møller, 2015; Widhalm and Dudareva, 2015; Liu et al., 2018). (C) Flavonoid metabolism. PAL, phenylalanine ammonia lyase; C4H, cinnamate 4-hydroxylase; CHS, chalcone synthetase; CHI, chalcone isomerase; F3′H, flavone 3-hydroxylase; DFR, dihydroflavonol 4-reductase; ANS, anthocyanin synthase; ANR, anthocyanin reductase; GT, glucosyltransferase; LAR, leucoanthocyanidin reductase; MT, O-methyltransferase; F2H, flavanone 2-hydroxylase (Chen et al., 2013; Galland et al., 2014). The square dot arrows indicates steps which have not yet been fully elucidated, while the black arrows indicate steps supported by genetic evidence.