# Biosynthetic Pathways of Hormones in Plants
## 1 Introduction  
Phytohormones, which are also known as plant hormones, are small, naturally occurring organic compounds that significantly influence the growth, development, defense, productivity, and physiological mechanisms of plants. They also orchestrate various cellular activities within the plant. Even at minimal concentrations, they are operative in plant cells, tissues, and organs. They are found in all vascular plants and a substantial number of non-vascular species (Table 1). From the initial discovery of auxin to the most recent unearthing of strigolactones (SLs), 12 groups of phytohormones--auxins, cytokins (CKs), gibberellins (GAs), abscisic acid (ABA), ethylene, brassinosteroids (BRs), salicylic acid (SA), jasmonates, polyamines (PAs), melatonin, SLs, and peptide hormones--have been identified in numerous plant species. The various chemical structures of phytohormones are pivotal for their diverse biological functions and biosynthesis (Figure 1):  
* auxins and melatonin are indole derivatives;
* ABA is a sesquiterpene;
* ethylene is the simplest alkene;
* CKs are adenine analogues;
* GAs are tetracyclic diterpenoid acids;
* BRs are polyhydroxystroids;
* jasmonates are derived from fatty acids;
* PAs are aliphatic nitrogenous bases;
* SA is a phenolic organic acid;
* SLs are terpenoid lactones [1; 2]. In addition, peptide hormones regulate many developmental and defense processes, such as meristem maintenance, xylem and phloem differentiation, stomata patterning, pollination, embryo and endosperm development, cell division, nodulation, and systematic response [3]. They are divided into secreted and non-secreted types. Secreted peptidehormones are further divided into post-translationally modified peptides and cysteine-rich peptides. Peptide hormones are synthesized as larger precursor molecules, which are then cleaved to produce active peptides, e.g., CLAVATA3 (CLV3)/Embryo Surrounding Region-Related (CLE), Phytosulfokine (PSK), Plant Peptide-Containing Sulfated Tyrosine (PSY) peptides belong to post-translationally modified peptides; Rapid Alkalinization Factor (RALF) peptides belong to cysteine-rich peptides; Plant Elicitor Peptides (PEP)--to non-secreted peptides. Peptide hormones are also generated by plant pathogens, symbionts, and microbes that interact with plants. They are crucial in establishing a molecular interface that allows them to co-exist with the host plant. These organisms produce effectors that mimic peptide phytohormones and other effectors of pathogens and symbionts. Plant receptors recognize these effectors, which primarily regulate growth rather than defense responses. However, the origin of non-plant peptide phytohormones is still a subject of controversy [3; 4; 5; 6]. Although peptide hormones are considered a legitimate group of plant hormones, their biosynthesis is not covered by the scope of this article because their biosynthesis shares all of the typical characteristics of peptide biosynthesis. As such, plants host diverse phytohormone pathways. Significant advancements have been made in the study of phytohormone biology and synthesis over the past decade. An