Flavonoids Metabolomics: Structure and Functions

Flavonoids, owing to their versatile functionalities, have emerged as prominent molecules in the realm of plant material research. In this article, our focus lies on elucidating the classification, synthesis pathways, and functional aspects pertinent to flavonoids.

I. Classification and Structural Characteristics of Flavonoids

Flavonoids represent the most extensive category of polyphenolic compounds known to date, estimated to encompass over 8000 distinct flavonoid metabolites¹. These compounds share a common structural framework characterized by a diphenylpropane A (C6-C3-C6) skeleton, wherein two aromatic rings are interconnected by a three-carbon chain (Figure 1). The A-ring is typically biosynthesized through the acetate pathway, stemming from either resorcinol or pyrogallol molecules and exhibiting distinct hydroxylation patterns at the C5 and C7 positions. In contrast, the B-ring originates from the more comprehensive cinnamic acid pathway and frequently bears hydroxylation at positions 3', 4' and 5'.

Flavonoids are mainly classified into chalcones, flavones, flavonols, flavanones, isoflavones, dihydroflavonols, flavanols, anthocyanins, flavonoid glycosides, and proanthocyanidins.

II. Biosynthesis Pathways of Flavonoids: An In-Depth Exploration

The biosynthetic journey of flavonoid compounds commences with phenylalanine, whereby phenylalanine ammonia-lyase (PAL), cinnamic acid 4-hydroxylase (C4H), and 4-coumaroyl CoA ligase (4CL), enzymes encoded by core phenylalanine-related biosynthetic genes, orchestrate the conversion of phenylalanine. This process serves as a pivotal precursor for the biosynthesis of major secondary metabolites within higher plants. Despite their remarkable diversity, the core biosynthetic pathway for flavonoids remains evolutionarily conserved.

Table 1. Structural genes in the flavonoid synthesis pathway

Flavonoid compounds exhibit extensive variability across diverse taxa, exemplified by the synthesis of isoflavones in soybeans², anthocyanins in Arabidopsis and alfalfa³, flavone-C-glycoside synthesis in kudzu⁴, and flavonoid biosynthesis in monocotyledonous maize⁵. Intriguingly, distinct accumulation patterns are discernible among different species and even within specific genotypes, tissues, and developmental stages.

III. Multifaceted Functions of Flavonoids in Plants

Flavonoids play multifaceted roles within extant plant systems.

1. They actively participate in pivotal growth and developmental processes. Notably, in Arabidopsis flavonoids exert influence over cell wall synthesis. The abrogation of flavonol synthesis ensures the unimpeded transport of quercetin to the cell wall⁶. Furthermore, flavonols intricately regulate root phototropism and root growth through intricate interactions with phytohormones such as auxins, cytokinins, and reactive oxygen species (ROS)⁷. Gibberellins, on the other hand, promote root growth by directly curtailing flavonol biosynthesis⁸. Additionally, in cabbage, the downregulation of flavonoid synthesis within the stigma promotes self-pollination⁹.

2. Flavonoids are pivotal in pigment formation, exemplified by the striking coloration of grape skins attributable to the accumulation of anthocyanins¹⁰.

3. Flavonoids contribute significantly to abiotic stress responses. For instance, the synthesis of quercetin glucosides heightens cold resistance in Arabidopsis¹¹. In rice, the extent of flavonoid accumulation correlates directly with ultraviolet (UV) resistance¹².

4. Flavonoids are integral components of biotic stress resistance mechanisms. Elevated levels of flavonoids in rice enhance resistance against brown planthoppers¹³, with hesperetin demonstrating enhanced effectiveness against bacterial pathogens and cyanidin exhibiting superior antifungal properties¹⁴. An augmentation in isoflavone content bolsters soybean leaf resistance to soybean mosaic virus¹⁵.

In summary, flavonoids occupy a prominent position within the sphere of plant research. At Metware lab in Boston, we offer Flavonoids Metabolomics service which can detect more than 3700 flavonoids in plant samples, contact us to speed up your research!

Reference

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