Understanding the Composition, Biosynthesis, Accumulation and Transport of Flavonoids in Crops for the Promotion of Crops as Healthy Sources of Flavonoids for Human Consumption

Abstract

Flavonoids are a class of polyphenolic compounds that naturally occur in plants. Sub-groups of flavonoids include flavone, flavonol, flavanone, flavanonol, anthocyanidin, flavanol and isoflavone. The various modifications on flavonoid molecules further increase the diversity of flavonoids. Certain crops are famous for being enriched in specific flavonoids. For example, anthocyanins, which give rise to a purplish color, are the characteristic compounds in berries; flavanols are enriched in teas; and isoflavones are uniquely found in several legumes. It is widely accepted that the antioxidative properties of flavonoids are beneficial for human health. In this review, we summarize the classification of the different sub-groups of flavonoids based on their molecular structures. The health benefits of flavonoids are addressed from the perspective of their molecular structures. The flavonoid biosynthesis pathways are compared among different crops to highlight the mechanisms that lead to the differential accumulation of different sub-groups of flavonoids. In addition, the mechanisms and genes involved in the transport and accumulation of flavonoids in crops are discussed. We hope the understanding of flavonoid accumulation in crops will guide the proper balance in their consumption to improve human health.

Keywords: ABC transporters; MATE transporters; biosynthesis pathway; crops; flavonoids; health; nutrition; phenolic compounds.

Figure 1

The molecular structure of genistein in (A) aglycone form (genistein), (B) glycoside form (genistin) and (C) conjugated beta-glycoside form (6”-O-malonyl genistin).

Figure 2

Simplified flavonoid biosynthetic pathway. All classes of flavonoids can be further modified to yield different derivatives with a variety of functions, such as storage and transportation. Malonyl-CoA results from the carboxylation of acetyl-CoA, which is a product of glycolysis and fatty acid β-oxidation. PAL: phenylalanine ammonia-lyase; C4H: cinnamate 4-hydroxylase; 4CL: 4-coumarate-CoA ligase; CHS: chalcone synthase; CHR: chalcone reductase; CHI: chalcone isomerase; IFS: isoflavone synthase; FNS: flavone synthase; FLS: flavonol synthase; F3H: flavanone 3-hydroxylase; F3’H: flavanone 3’-hydroxylase; DFR: dihydroflavonol 4-reductase; LDOX: leucoanthocyanidin dioxygenase; UFGT: UDP-glucose: flavonoid-3-O-glycosyltransferase; LAR: leucoanthocyanidin reductase.

Figure 3

Phylogenetic analysis of ATP-binding cassette (ABC) transporters from G. max, S. lycopersicum and A. thaliana. The amino acid sequences of the ABC transporters identified from G. max [115], S. lycopersicum [116] and A. thaliana [119] were aligned using Clustal Omega [126] with default parameters. The phylogenetic tree was constructed using RAxML [127] with 1000 times rapid bootstrapping. The protein model was selected automatically by the maximum likelihood criterion.

Figure 4

Phylogenetic analysis of multi-antimicrobial extrusion protein (MATE) transporters from G. max, Z. mays, S. lycopersicum and A. thaliana. The amino acid sequences of the MATE transporters identified from G. max, Z. mays, S. lycopersicum and A. thaliana [129,130,131,134] were aligned using Clustal Omega [126] with default parameters. The phylogenetic tree was constructed using RAxML [127] with 1000 times rapid bootstrapping. The protein model was selected automatically by the maximum likelihood criterion.