Health Benefits of Flavonoids: Comparison
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Flavonoids are health supplements commonly known in their tablet forms. Crops are rich in various sub-classes of flavonoids that could be used for human consumption. The biosynthesis and transport of flavonoids are major factors contributing to the accumulation of flavonoids in crops. On the other hand, the bioavailability of flavonoids to the human body governs the beneficial effects of the flavonoids on human health.

  • crops
  • phenolic compounds
  • flavonoids
  • biosynthesis pathway
  • ABC transporters
  • MATE transporters
  • nutrition
  • health

Background

With the improved awareness of nutrition and health worldwide, the consumption of health

 supplements has been ever increasing. Crops, including a variety of fruits, vegetables and legumes,

 are rich in flavonoids, which are known to exhibit antioxidative and antimicrobial activities.

The Bioavailability of Flavonoids

Various studies have suggested the potential nutritional and health-promoting effects of flavonoids. The bioavailability of different classes of flavonoids is varied. In general, the most poorly absorbed flavonoids upon ingestion are proanthocyanidins and anthocyanins, while the most readily absorbed flavonoids are isoflavones [1][2]. Ingested flavonoids experience a series of modifications, such as deglycosylation, methylation, glucuronidation and sulphation, along the gastrointestinal tract and circulatory system to yield different conjugates of flavonoids [3][4]. These modifications affect the bioavailability of the flavonoids by affecting their stereochemical configurations and enzyme specificities [2][4][5].

Regulation of Flavonoid Biosynthesis

The flavonoid contents in plants are tightly regulated by a complex network of regulators.

 Transcription factors (TFs), including members of the MYB, bHLH, MADS, WRKY and WD40 families,

 play important roles in regulating flavonoid biosynthesis in plants. These transcription factors

 may form regulatory complexes. For example, complexes of MYB-bHLH-WD40 (MBW) have been

 suggested in plants such as A. thaliana, Camellia sinensis, Fragaria × ananassa and Vitis vinifera in order to

 fine-tune flavonoid levels [6][7][8][9][10]. The various combinations of the components in the MBW complex

 lead to yet another level of regulation in the flavonoid biosynthesis pathways. In general, plant

 species share common regulators involved in the early steps of the flavonoid biosynthesis pathway.

 However, different regulators are involved in the regulation of biogenesis genes in the later part of the

 biosynthesis pathways to determine the specific flavonoids produced and their levels in different plant

 species [6][11][12][13][14].

Transport of Flavonoids

Flavonoids are found both intracellularly and extracellularly in plants. Inside plant cells,

 flavonoids are distributed in various compartments, including nuclei, ERs, vacuoles, vesicles and

 chloroplasts. In soybean, it has been reported that isoflavones are stored in vacuoles in the glycosylated

 or malonylated form [15]. On the other hand, in pea, it was found that isoflavone synthase, CYP93C18,

 is localized in the ER [16]. These patterns of intracellular storage and biosynthesis locations highlight

 the significance of flavonoid transport in influencing the storage and the bioavailability of flavonoids.

The transportation of flavonoids can be facilitated by vesicles or transporter proteins. One of

 the transporter proteins is glutathione-S-transferase (GST). GST acts as a carrier protein by binding

 to flavonoids. In soybean, the binding of a putative lambda class GST, GmGSTL1, to flavonoids

 was reported [17]. ATP-binding cassette (ABC) transporters and multidrug and toxic compound

 extrusion (MATE) transporters have been characterized in various plant species as the transporters of

 flavonoids [18]. In general, transporter proteins are integral membrane proteins which facilitate the

 movement of molecules across membranes [19]. The specific protein domains of ABC transporters

 and MATE transporters which confer their functions will be discussed below. These two types of

 transporters mediate the secretion or the accumulation of flavonoids in plants. Since the mechanisms of

 secretion and uptake of flavonoids by plant cells have been discussed in previous reviews [18][20][21],

 this review will discuss the transportation of flavonoids from the perspective of the nutritional contents

 of plants for human consumption; in other words, the types of transportation that can facilitate the

 accumulation of flavonoids in the edible portion of crop plants.

Conclusion

Common crops for human consumption, including fruits, vegetables and legumes, are rich in

 different forms of flavonoids. Tea, which is an important plant for making beverages in a lot of Asian

 countries, is abundant in catechin, a flavanol shown to have health benefits. Flavonoid molecules

 have similar yet different structures. The subtle molecular mechanisms in the regulation of their

 biosynthesis bring forth a vast variety of flavonoids and their derivatives. The different molecular

 structures of flavonoids give rise to the different functions of these molecules. Equally important is the

 transport of flavonoids in the regulation of their contents in plant cells and thus their availability for

 human consumption. The understanding of the biological functions of flavonoids and the molecular

 mechanisms regulating their abundances in food crops will facilitate the smart use of crops in our diet

 and enable breeding programs to produce crops with desirable contents of flavonoids.

 

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