Maize Flavonoid Biosynthesis, Regulation, and Human Health Relevance: A Review

Abstract

Maize is one of the most important crops for human and animal consumption and contains a chemical arsenal essential for survival: flavonoids. Moreover, flavonoids are well known for their beneficial effects on human health. In this review, we decided to organize the information about maize flavonoids into three sections. In the first section, we include updated information about the enzymatic pathway of maize flavonoids. We describe a total of twenty-one genes for the flavonoid pathway of maize. The first three genes participate in the general phenylpropanoid pathway. Four genes are common biosynthetic early genes for flavonoids, and fourteen are specific genes for the flavonoid subgroups, the anthocyanins, and flavone C-glycosides. The second section explains the tissue accumulation and regulation of flavonoids by environmental factors affecting the expression of the MYB-bHLH-WD40 (MBW) transcriptional complex. The study of transcription factors of the MBW complex is fundamental for understanding how the flavonoid profiles generate a palette of colors in the plant tissues. Finally, we also include an update of the biological activities of C3G, the major maize anthocyanin, including anticancer, antidiabetic, and antioxidant effects, among others. This review intends to disclose and integrate the existing knowledge regarding maize flavonoid pigmentation and its relevance in the human health sector.

Keywords: Zea mays L.; anthocyanins; biosynthesis; health benefits; pigmented maize; regulation.

Figure 5

Biosynthetic genes of flavone C-glycosides. The flavanones naringenin and eriodictyol are the initial substrates for the other flavonoid subgroups. There are two possible ways to generate C-glycosyl flavones, indirectly or directly, from any flavanone. The indirect pathway begins through flavanone-2-hydroxylase (ZmF2H, fnsii1, EC 1.14.14.162) opening the C-ring, producing a 3-oxo-dihydrochalcone. Then, UDP C-glycosyl transferase (ZmCGT, cgt1, EC 2.4.1.360) generates a glycosidic bond in the A-ring. Then, there is a dehydration reaction (spontaneous or enzymatic) that produces the C6-flavone glycoside. The direct pathway firstly involves flavone synthase I (ZmFNSII-2, fnsii2, EC 1.14.20.5) and flavone synthase II (ZmFNSII-1, fnsi2, EC 1.14.19.76) producing the same reaction by the addition of a double bond between C2 and C3 in the flavanone. Then, a flavone functions as a substrate for the UDP C-glycosyl transferase (ZmCGT, cgt1, EC 2.4.1.360). The enzymatic action of UDP-rhamnosyl transferase (ZmCGT, sm2, EC 2.4.1.159) and glucose 4,6 dehydratase (sm1, EC 4.2.1.76) produces either apimaysin or maysin. References: [30,43,90].

Figure 1

Chemical structure of flavonoid subgroups and the basic C6-C3-C6 skeleton (2-phenyl-2H-chromene). A, B, and C refer to a specific ring of the flavonoid skeleton.

Figure 2

Early genes in the flavonoid pathway. The flavonoid pathway begins with the transformation of phenylalanine to coumaroyl-CoA. The last steps end with the intravacuolar accumulation of acylated anthocyanins. The genes responsible for supplying the coumaroyl-CoA into the flavonoid pathway are phenylalanine ammonium lyase (ZmPAL, EC 4.3.1.24), cinnamic acid 4-hydroxylase (ZmC4H, EC 1.14.14.91), and 4-coumarate CoA ligase (Zm4CL, bm5, EC 6.2.1.12). The flavonoid genes are divided into early biosynthetic genes (EBGs) and late biosynthetic genes (LBGs). EBGs comprise four genes: chalcone synthase (ZmCHS, c2, EC 2.3.1.74), chalcone isomerase (ZmCHI, chi1, EC 5.5.1.6), flavonoid 3-dioxygenase (ZmF3H, fht1, EC 1.14.11.9), and flavonoid 3′-monooxygenase (ZmF3′H, pr1, EC 1.14.14.82). References: [30,32].

Figure 3

Biosynthetic genes for maize anthocyanin pathway. After the formation of the dihydroflavonol, five enzymatic steps catalyze its biotransformation into acylated maize anthocyanins. Those genes are the following: dihydroflavonol 4-reductase (ZmDFR, a1, EC 1.1.1.219), anthocyanidin synthase (ZmANS, a2, EC 1.14.20.4), anthocyanidin 3-O-glucosyltransferase (ZmAGT, bz1, EC 2.4.1.115), malonyl-CoA: anthocyanin 3-O-glucoside-6′′-O-malonyltransferase (Zm3MAT, aat1, EC 2.3.1.171), and flavonoid 3′,5′-O-methyltransferase (ZmAOMT, EC 2.1.1.267). The glutathione S-transferase (ZmGST, bz2, EC 2.5.1.18) and multidrug resistance protein (ZmABCC3 and ZmABCC4, MRP3 and MRP 4, EC 7.6.2.2) are required to deliver them inside the vacuole. References: [30,32].

Figure 4

The biosynthetic genes of flavonol and phlobaphenes. The flavanones naringenin and eriodyctiol are the starting substrates for the other flavonoid subgroups. Flavonol synthesis depends on flavanone 3-dioxygenase (ZmF3H, fht1, EC 1.14.11.9) and flavonol synthase (ZmFNS1, fns1, EC 1.14.20.5). Phlobaphene synthesis begins with the action of dihydroflavonol 4-reductase (ZmDFR, a1, EC 1.1.1.219) on flavanones, generating flavan-4-ol molecules that undergo a non-enzymatic polymerization into phlobaphenes. References: [8,30,32].

Figure 6

The regulation of the MBW complex and its influence on anthocyanin biosynthesis. (A) Environmental factors such as ultraviolet light (UV) and cold temperatures and phytohormones such as abscisic acid (ABA), salicylic acid (SA), and jasmonic acid (JA) augment the expression of the MBW complex. Meanwhile, the gibberellins (GAs) downregulate the transcription of this tripartite complex. In the case of GAs and ABA, their concentrations participate in seed development. In the seed dormancy period, ABA levels increase and the aleurone starts to accumulate anthocyanins. Mutations in the vp1 gene produce embryos insensitive to ABA, suppressing the anthocyanin biosynthesis in the aleurone and resulting in a viviparous phenotype. (B) The complete MBW is necessary to activate the anthocyanin biosynthetic genes. Some gene products such as A3 and In1 compete with the bHLH member of this transcriptional complex, suppressing the anthocyanin accumulation. (C) The anthocyanin accumulation modifies the color of the plant’s tissues, turning the vegetative tissues, aleurone, and pericarp into a purple color and turning the anthers into a red color.

Figure 7

The locus p1, which regulates the biosynthesis of phlobaphenes and flavone C-glycosides, and its paramutation phenomenon. (A) UV-B produces the gene promoter demethylation of p1, with a consequent lower methylation level in the p1 promoter. (B) After demethylation, the p1 gene is expressed, and the P1 protein can function as a transcription factor. (C) P1 regulates a1 expression, leading to phlobaphene biosynthesis, and also activates essential genes for the flavone C-glucosides, such as sm1, which express the glucose 4,6-dehydratase (RHS). (D) The expression of these enzymes modifies the plant phenotype.