Flavonoids in major cereal grasses: distribution, functions, biosynthesis, and applications

Cereal food crops like maize ( Zea mays ), rice ( Oryza sativa ), sorghum ( Sorghum bicolor ) and wheat ( Triticum aestivum ) are members of the grass family (Poaceae). Together with bioenergy grasses, they play essential roles in agriculture and biorefinery, comprising an important component of bio-economy. Grasses accumulate a unique range of flavonoids (a class of plant-specific specialized metabolites) that contribute to defense responses, fertility, pigmentation and cell wall lignification. This review summarizes the distribution, functions, biosynthesis and regulation of flavonoids, including flavanones, tricin and flavone O -conjugates, flavone C -glycosides, 3-deoxyanthocyanidins, phlobaphenes, anthocyanins, proanthocyanidins (condensed tannins) and flavonols, in grasses focusing on model cereal grasses. In particular, the constitutive, predominant and widespread tricin-lignin, flavone O -conjugates and flavone C -glycosides as well as the inducible and/or lineage-specific sakuranetin (a flavanone), maysin (a flavone C -glycoside), 3-deoxyanthocyanidins and phlobaphenes, are highlighted. Finally, the impacts and importance of cereal grass flavonoids on scientific research, agricultural production, food and health, and biorefinery are discussed.