# Biosynthetic Pathways of Hormones in Plants
## 13 Conclusions  
In this review, the biosynthesis routes of all known plant phytohormones have been explored. Understanding these intricate biosynthetic pathways of plant hormones will provide valuable insights regarding their biological functions.  
The biosynthesis of PAs begins with putrescine that leading to the production of other PAs. The ethylene synthesis pathway is more straightforward, with methionine being a precursor. Salicylic acid biosynthesis is believed to occur via two routes: the isochorismate synthase pathway and the phenylalanine ammonia-lysse pathway. Auxin synthesis in plants occurs via two main pathways: the Trp-dependent and independent routes. Four Trp-dependent IAA routes have been proposed: the IAM, IPA, TAM, and IAOX routes. Similarly, melatonin biosynthesis starts with Trp, which is synthesized de novo via the shikimate pathway. Two potential pathways of ABA biosynthesis have been identified in plants. The first pathway is direct, and the second pathway is indirect; in these pathways, ABA is derived from C\({}_{15}\) farnesyl pyrophosphate and a C\({}_{40}\) carotenoid, respectively. Brassinosterol biosynthesis occurs through three pathways that lead to C\({}_{27}\)-, C\({}_{28}\)-, and C\({}_{29}\)-type BRs, with early steps happening via the MVA or MEP pathways. Cytokines can be synthesized de novo as low-activity nucleotide mono-, di-, and triphosphates. Alternatively, cytokinins can be released from tRNA, creating nucleotide mono-phosphates, with the interconversions of nucleotides, nucleosides, and free bases catalyzed by enzymes involved in adenine metabolism. Gibberellins are formed from GGPP through IPP, which is the C-5 building block of all terpenoid/isoprenoid compounds, including BRs, CKs, ABA, and SLs. Carlactone is the main precursor of SL production in plants. Jasmonic acid synthesis begins with the release of polyunsaturated fatty acid \(\alpha\)-linolenic acid from galactolipids present in chloroplast membranes.  
The emergence of various phytohormone groups, along with their unique biosynthetic pathways, could be linked to their distinct chemical structures, biosynthetic path-ways, physicochemical properties, plant evolutionary histories, and roles in plants' adaptation to diverse environmental conditions or developmental programs. Precursors or intermediates involved in one phytohormone pathway may also participate in the biosynthesis of another hormone, thus linking the pathways (Figure S1).  
The following supporting information can be downloaded via the following link: [[https://www.mdpi.com/article/10.3390/metabo13080884/s1](https://www.mdpi.com/article/10.3390/metabo13080884/s1)]([https://www.mdpi.com/article/10.3390/metabo13080884/s1](https://www.mdpi.com/article/10.3390/metabo13080884/s1)), Figure S1: Network of plant hormones' biosynthetic pathways (all-in-one).  
Conceptualization, A.B.; methodology, A.B.; writing--original draft preparation, A.P.-N.; writing--review and editing, A.B. and A.P.-N.; visualization, A.B.; supervision, A.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
We express our sincere gratitude to the Reviewers and Academic Editors who dedicated their time and valuable insights to the process of reviewing our manuscript. A.B. thanks [PERSON] for his consultation support.  
The authors declare no conflict of interest.