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.