## 2 Types and Biosynthesis of Plant Secondary Metabolites
Phenolic compounds can be further divided into four classes: flavonoids,  
Figure 1: The biosynthetic pathway of terpenoids (**A**), alkaloids (**B**), and phenylpropanoids (**C**) in plants. The orange shaded area represents endogenous metabolic pathways present in microorganisms. The catalytic enzymes are shown in blue, intermediates in black. Solid arrow represents one step reaction, and dashed arrow represents multiple steps. The structures of typical natural products with underlines are displayed. Green circles indicate compounds for which the synthesis pathways have not been fully elucidated. Abbreviations: DXS,1-deoxy-D-xylulose 5-phosphate synthase; DXR, Ideoxy-D-xylulose 5-phosphate reductio isomerase; MCT, 2-C-methyl-D-eryththitol 4-phosphate cytidylltransferase; CMK,4-(cytidine 5’diphospho)-2-Cmethyl-D-eryththitol kinase; MECPS:ME-CDP synthase; HDS, 4-hydroxy-3-methyl-2-enyl-diphosphate synthase; IDS: isoprenyl diphosphate synthase; IDI, isoptenenyl diphosphate isomerase; GPP, geranyl diphosphate; GPPS, GPP synthase; GGPP, geranyldigphosphate; GGPPS, GGPP synthase; TPS: terpene synthase; AACT, acetyl-CoA acetyltransferase; HMGS, hydroxymethylglutural-CoA synthase; HMGR, 3-hydroxy-3-methylglutural-CoA reductase; MVK, mevalonate kinase; PMK, phosphomechanate kinase; MPDC: mevalonate diphosphate decarboxylase; FPP, farnesyl diphosphate; FPPS, FPP synthase; STR: strictosi-dine synthase; PAL: phenylalanine ammonia lyase; C4H, cinnamate 4-hydroxylase; 4 CL, 4-couramate: CoA ligase; CHS, chalcone synthesethe; CHI, chalcone isomerase; F3H, flavanone 3-hydroxyylase; F3’H, flavonoidoid-3’-hydroxyylase; F5’H, flavonoidoid-5’-hydroxyylase; DFR, dihydroflavonol reductase; ANS, Anthocyanidin synthesethe; 3 GT: flavonoid 3-O-glucosyltransferasetransferase; LAR: Leucanthocyanidin reductase; ANR: anthocyanin reductase. phenolic acids that derived from hydroxybenzoic acids or hydroxycinnamic acids, stilbenes, and lignans [20]. Flavonoids originate from the phenylpropanol metabolic pathway, with their basic structure composed of a C6-C3-C6 unit formed by connecting two phenolic ring moieties (A and B rings) with three carbon atoms. The C3 part can be a linear chain or form a five-membered or six-membered oxygen heterocycle with the C6 portion. Depending on differences in whether the C3 portion is cyclized, the degree of oxidation, and the substitution patterns, flavonoids can be further classified into various subclasses, including chalcones, flavones, anthocyanin, flavanones, flavonols, and isoflaves [21]. Phenolic compounds in plants have various biosynthetic pathways, with the biosynthesis of flavonoids being one of the more extensively studied and understood processes. First, phenylalanine ammonia lyase, as the first crucial enzyme in plant phenolic compound production, catalyzes the deamination of phenylalanine to form trans-cinnamic acid. Subsequently, under the action of cinnamate 4-hydroxylase, cinnamic acid is hydroxylated, forming p-comamic acid. Then, p-coamuric acid, in conjunction with coenzyme A, is converted to p-coumaroyl-CoA by the action of 4-coumaroyl:CoA ligase. This p-coumaroyl-CoA is further catalyzed to form chalcone when combined with malonyl-CoA, and this product serves as a precursor for various flavonoid compounds (Figure 1C). Many natural phenolic compounds are beneficial for human health. Some extensively researched examples include resveratrol, a stilbene found in grapes and red wine; epigallocatechin-3-gallate, a flavanol found in tea; and quercetin and rutin, flavonols present in various plants such as tea, onions, and Ginkgo biloba [21].