## 2 Types and Biosynthesis of Plant Secondary Metabolites  
Plant secondary metabolites are diverse in type and originate from various biosynthetic pathways. Based on their biosynthetic origins, plant natural products can be categorized into three main classes: terpenoids, alkaloids, and polyphenols [8]. Terpenoids are a group of compounds that include all isoprene polymers and their derivatives. They are based on the five-carbon isoprene unit and are also known as isoprenoids. Terpenoids are categorized into acyclic (linear) and cyclic terpenoids based on the way the isoprene units are connected [9]. These compounds are synthesized by the condensation of isopenetyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) (Figure 1A) [10]. IPP is converted into DMAPP by the action of IPP isomerase, and both IPP and DMAPP are further converted into geranyl pyrophosphate (GPP) by the action of isoprenyltransferases. Famesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP) serve as non-cyclic intermediates and are further transformed into various classes of terpenoid compounds through the action of terpenoid synthases [11; 12]. Terpenoids are the most diverse class of secondary metabolites in plants, with highly variable chemical structures. They play crucial roles in various aspects of plant growth and development, such as in the biosynthesis of plant hormones like gibberellins, abscisic acid, strigolactones, and cytokins. Terpenoids also contribute to the adaptation to environmental stresses, as they include compounds like allelopathic substances, defensive compounds against herbivores, and chemicals involved in allelopathy [13]. Moreover, plant terpenoids have significant medicinal and economic value, including well-known drugs like artemisinin for malaria treatment and paclitaxel as an anti-cancer agent [10]. Alkaloids are the largest class of nitrogen-containing organic compounds among secondary metabolites. They mainly include terpenoid indole alkaloids, isoquinoline alkaloids, tropane alkaloids, and pyridine alkaloids [14]. Most alkaloids are synthesized from L-amino acids (such as tryptophan, tyrosine, phenylalanine, lysine, and arginine) individually or in combination with steroids, secoiridoid (e.g., strychnoside), or other terpene ligands (Figure 1B). Terpenoid indole alkaloids consist of an indole ring and a secologanin. Among the key enzymes in the metabolic pathway of this class is the enzyme strictosidine synthase, and its product, strictosidine, is an important branching point in this pathway. It can be further transformed into various related alkaloids like ajmalicine, quinine, and strychnine [15]. In the biosynthesis pathway of tetrahydropyhenylisoquinoline alkaloids, the branching point is (S)-reticuline. Under the action of specific synthases, it can be further converted into alkaloids such as berberine, sanguinar, and morphine [16]. Precursors for alkaloids like nicotine and scopolamine are derived from ornithine or arginine. Key enzymes involved in the biosynthesis of these compounds include ornithine decarboxylase, tropinone reductase, and hoyosamine hydroxylase [17]. Since the isolation of the first alkaloid, morphine, from opium in 1816, a variety of alkaloids such as piperine, atropine, caffeine, and quinine have been sequentially isolated. Currently, there are over 27,000 reported alkaloids with diverse chemical structures. Among these, 88 of them have various pharmacological properties, including analgesic (morphine), stimulant (caffeine and ephedrine), psychoactive (mescaline and cocaine), antimicrobial (quinine), and anticancer (vinblastine and vincristine) activities [18]. The plant kingdom comprises more than 8000 to 10,000 phenolic compounds, which exhibit excellent antioxidant activity due to one or more aromatic rings with hydroxyl groups. Additionally, they serve as defense compounds in plants against bacteria, fungi, and insects [19].