STAT6 review namely, chalcones, flavanones, dihydroflavonols, and leucoanthocyanidins (Figure 2). 2.2. Chalcone: The initial Key Intermediate Metabolite in Flavonoid Biosynthesis The entry of p-coumaroyl-CoA into the flavonoid biosynthesis pathway represents the start out of the synthesis of specific flavonoids, which starts with chalcone formation [2]. 1 molecule of p-coumaroyl-CoA and 3 molecules of malonyl-COA, derived from acetyl-CoA through the activity of acetyl-CoA carboxylase (ACCase), produce naringenin chalcone (four,2 ,4 ,6 -tetrahydroxychalcone [THC] [chalcone]) by way of the action of chalcone synthase (CHS) [25]. CHS, a polyketide synthase, will be the key and very first rate-limiting enzyme within the flavonoid biosynthetic pathway [26,27]. In tomato (Solanum lycopersicum), RNA interference (RNAi)-mediated suppression of CHS results in a reduction in total flavonoid levels [28]. Chalcone reductase (CHR), an aldo-keto reductase superfamily member, acts on an intermediate in the CHS reaction, catalyzing its C-6 dehydroxylation, yielding isoliquiritigenin (four,2 ,4 -trihydroxychalcone [deoxychalcone]) [29]. Overexpressing the CHR1 gene from Lotus japonicus in petunia results in the formation of isoliquiritigenin as well as a reduce in anthocyanin content material [30]. Since THC is readily converted to a colorless naringenin below the action of chalcone isomerase (CHI) or by way of spontaneous isomerization, it isInt. J. Mol. Sci. 2021, 22,5 offrequently converted towards the additional stable THC 2 -glucoside (isosalipurposide [ISP]) beneath the action of chalcone 2 -glucosyltransferase (CH2 GT) in plant vacuoles [31,32]. Differences in CH2 GT gene expression or enzymatic activity may possibly account for the difference in ISP content material within the petals of distinct varieties of yellow carnation [33]. Chalcone is definitely the very first crucial intermediate item inside the flavonoid metabolic pathway, giving a fundamental skeleton for downstream flavonoid synthesis. Chalcone (THC, isoliquiritigenin, and ISP, amongst others) can also be a vital yellow pigment in plants [31]. 2.3. Stilbene Biosynthesis: The initial Branch of the Flavonoid Biosynthesis Pathway Stilbene synthase (STS) also makes use of p-coumaroyl-CoA and malonyl-CoA as substrates and catalyzes the formation of the stilbene backbone, for example resveratrol [34,35]. The stilbene pathway may be the first branch of the flavonoid biosynthesis pathway and exists only within a few plants, like grapevine, pine, sorghum, and peanut [36,37]. STS, a member in the type III polyketide synthase loved ones, is definitely the initially and essential enzyme in stilbene biosynthesis, and is closely connected to, and evolved from, CHS [34]. Nonetheless, STS generates a compound having a unique C14 backbone (C6-C2-C6) together with the release of four carbon dioxide (CO2 ) molecules, even though CHS catalyzes the formation of C15 skeletons (C6-C3-C6), with only three molecules of CO2 getting released [38]. In Vitis amurensis calli, the overexpression of Picea jezoensis PjSTS1a, PjSTS2, and PjSTS3 considerably increases the total stilbene content material [39]. Most plant stilbenes are derivatives from the simple unit transresveratrol (3,5,four -trihydroxy-trans-stilbene) which has undergone numerous modifications, for instance isomerization, glycosylation, methylation, oligomerization, and prenylation [36]. Trans-resveratrol is usually converted to polydatin, pterostilbene, and 5-HT4 Receptor Modulator site piceatannol by glycosylation, methylation, and hydroxylation, respectively [35]. In peanuts, the major prenylated stilbene compounds are trans-3 -(3-methyl-2-butenyl)-resveratrol and transarachidin-1/