Flavonoids: a metabolic network mediating plants adaptation to their real estate

Abstract

From an evolutionary perspective, the emergence of the sophisticated chemical scaffolds of flavonoid molecules represents a key step in the colonization of Earth's terrestrial environment by vascular plants nearly 500 million years ago. The subsequent evolution of flavonoids through recruitment and modification of ancestors involved in primary metabolism has allowed vascular plants to cope with pathogen invasion and damaging UV light. The functional properties of flavonoids as a unique combination of different classes of compounds vary significantly depending on the demands of their local real estate. Apart from geographical location, the composition of flavonoids is largely dependent on the plant species, their developmental stage, tissue type, subcellular localization, and key ecological influences of both biotic and abiotic origin. Molecular and metabolic cross-talk between flavonoid and other pathways as a result of the re-direction of intermediate molecules have been well investigated. This metabolic plasticity is a key factor in plant adaptive strength and is of paramount importance for early land plants adaptation to their local ecosystems. In human and animal health the biological and pharmacological activities of flavonoids have been investigated in great depth and have shown a wide range of anti-inflammatory, anti-oxidant, anti-microbial, and anti-cancer properties. In this paper we review the application of advanced gene technologies for targeted reprogramming of the flavonoid pathway in plants to understand its molecular functions and explore opportunities for major improvements in forage plants enhancing animal health and production.

Keywords: anthocyanins; evolution; flavonoids; metabolism; proanthocyanidins; transgenic.

FIGURE 1

Schematic representation of flavonoid pathway in plants. ANR, anthocyanidin reductase; ANS, anthocyanidin synthase; DFR, dihydroflavonol 4-reductase; F3H, flavonoid-3′ hydroxylase; FLS, flavonole synthase; FNS, flavone synthase; GT, glucosyltransferase; LAR, leucoanthocyanidin reductase.

FIGURE 2

Accumulation of proanthocyanidins in Lotus corniculatus and Trifolium repens organs. Abeynayake et al. (2011,2012). (A,B) L. corniculatus leaf stained with DMACA; (C) mature T. repens inflorescence; (D) longitudinal section through mature T. repens inflorescence stained with DMACA; (E) longitudinal section through a trichome; (F) transverse section through an immature petal in which proanthocyanidins accumulated only on the abaxial side. (G) transverse section through a mature standard petal showing the accumulation of proanthocyanidins on both the abaxial and adaxial sides; (H) transverse section through anther filaments and carpel; (I) longitudinal section through a developing seed in a mature flower. Bars = 200 μm (A,B); 1 mm (C,D); 5 μm (E); 50 μm (F,G), 100 μm (G), and 10 μm (I).

FIGURE 3

Developmentally regulated biosynthesis of proanthocyanidins and anthocyanins in white clover flowers (Abeynayake et al., 2012). Upper figure shows six stages of flower development in white clove. Middle figure shows flavonoid gene families and gene members related to the biosynthesis of proanthocyanidins and anthocyanins, differentially expressed at stages 1–3 and 4–6, respectively.

FIGURE 4

Phenotypes (A) and biochemical profiles (B) of TrANRhp transgenic white clover lines with down-regulated TrANR gene (Abeynayake et al., 2012). (A): (a) wild-type white clover inflorescence; (b) wild-type flowers at different developmental stages; (c) TrANRhp inflorescence; (d) TrANRhp flowers at different developmental stages; (e) wild-type mature flower; (f) wild-type carpel; (g) cross-section of a carpel stained with DMACA; (h) epidermal cells of anther filaments stained with DMACA; (i) TrANRhp mature flower; (j) TrANRhp carpel; (k) cross-section of TrANRhp carpel; (l) epidermal cells of TrANRhp anther filaments. (B) level and composition of flavan 3-ols; (C) level and composition of anthocyanins. Bars = 2 mm (a–d); 1 mm (e,f,i,j); 500 μm (g,k) and 75 μm (h,l). GC, gallocatechin; EGC, epigallocatechin; A1, delphinidin 3-sambubioside; A2, cyanidin 3-sambubioside.

FIGURE 5

Cross-talk between monolignol and flavonoid pathways ( Tu et al., 2010). The yellow route toward the production of monolignols is conserved in angiosperms. The orange route is found in perennial ryegrass. The blue route is found in some species. CAD, cannery alcohol dehydrogenase; 4CL, 4-coumarate:CoA ligase; C3H, p-coumarate 3-hydroxylase; C4H, cinnamate 4-hydroxylase; CCoAOMT, caffeoyl-CoA O-methyltransferase; HCT, p-hydroxycinnamoyl-CoA:quinate shikimate p-hydroxycinnamoyltransferase; F5H, ferulate 5-hydroxylase; PAL, phenylalanine ammonia-lyase; SAD, sinapyl alcohol dehydrogenase. Compounds marked in red were downregulated in hpCCR1-1 lines. Those marked in blue were upregulated in hpCCR1-1 lines.

FIGURE 6

Inflorescence color changes with the production of delphinidin-based anthocyanins (Brugliera et al., 2013). (1; A–H) The host is on the left and the transgenic on the right. The percentage of delphinidin (of total anthocyanidins) detected in hydrolyzed petal extracts is given under the transgenic inflorescence. The chrysanthemum cultivars: IR, Improved Reagan; DSR, Dark Splendid Reagan; Sei; Sei Titan; Sei Spire. The transgenic line number is given next to the cultivar/construct. (2) Schematic representation of the T-DNA components of selected binary plasmid vectors used for plant transformation.