# Biosynthetic Pathways of Hormones in Plants
## 2 Polyamines  
Polyamines (PAs) play a crucial role in plant growth, metabolism, and development. Structurally, these compounds are organic polycationic alkylamines that contain two or more amino groups. PAs such as putrescine (Put), spermidine (Spd), and spermine (Spm) are the most well-studied and recognized PAs found in plants. These three PAs regulate various physiological activities, such as photosynthesis, flower generation, embryogenesis, and organogenesis. They are also accountable for maintaining the stability of nucleic acids, various protein molecules, and the membrane structure. Additionally, they play a substantial role in enhancing the tolerance of many plants to the presence of a variety of abiotic and biotic stress factors, thereby impacting crop yield. Putrescine is a key compound involved in the biosynthesis of PAs. It serves as a common intermediate in the creation of Spd, Spm, and thermospermine. The biosynthesis pathways of Put have been identified in many plants (Figure 2) [9; 10; 11; 12]. Figure 2: The biosynthetic pathways of polyamines (phytohormones are green). In the first pathway, arginine (Arg) is decarboxylated through the action of enzyme argi-nine decarboxylase (ADC). This enzyme acts on the carbon at position 8. In this reaction, agmatine (Agm) and CO\({}_{2}\) (the ADC pathway) are produced. Subsequently, Agm is used to synthesize \(N\)-carbamoyl putrescine (NCPA) and ammonia (NH\({}_{3}\)), which occurs following the removal of nitrogen. In the succeeding reaction, NCPA amidohydrolase hydrolyzes NCPA, removing its carbamoyl group to produce NH\({}_{3}\), CO\({}_{2}\), and the first PA--Put. This process is the most prevalent pathway for Put synthesis in plants [13; 14]. In the second pathway of Put biosynthesis, ornithine (Orn) is formed from Arg in the reaction catalyzed by arginase [15]. Next, the enzyme ornithine decarboxylase (ODC) removes the carboxyl group bonded to C-1 of Orn to form products such as Put and CO\({}_{2}\)[13]. On the other hand, the ODC gene that encodes this enzyme has not been identified in _Arabidopsis thaliana_ and other plants from the Brassicaceae family. Therefore, there is a high possibility that the ornithine pathway, which is known also as ODC pathway, is not important for normal plant growth and development in relation to the ADC pathway [16; 17]. However, other authors speculate that the ODC pathway is mainly involved in regulation of plant growth, development, differentiation of organs, and reproduction, while the ADC pathway is activated in plants that grow under unfavorable environmental conditions. These two pathways are commonly found in plants [10; 16; 17; 18; 19; 20]. The third pathway is the least known route of Put formation in plants. Other PAs, such as Spd and Spm, are biosynthesized from Put. Decarboxylated \(S\)-adenosyl-L-methionine (dcSAM) is a source of aminopropyl residues, which are the indispensable intermediates involved in the polyamine biosynthetic pathway [10; 21]. For dcSAM production, S-adenosyl-L-methionine (SAM) is decarboxylated through the activity of the enzyme S-adenosylyl-L-methionine decarboxylase (SAMDC) [22]. Aminopropyl groups are added to the Put molecule in a reaction catalyzed by spermidine synthase (SPDS), and Spd is produced. In _A. thaliana_, two genes, i.e., _SPDS1 (At1g23820)_ and _SPDS2 (At1g70310)_ that encode SPDS, as well as four _SAMDC1-4_ genes, i.e., _At3g02470, At3g25570, At5g15959_, and _At5g18930_, that encode SAMDC, which are essential for plant embryogenesis, have been identified [23]. The last step of the biosynthesis of PAs, i.e., the transfer of aminopropyl groups to propylamine acceptor Spd, leads to the formation of Spm. In this reaction, this Spm is formed via the action of spermine synthase (SPMS) [18; 24; 25; 26]. The biosynthesis of PAs in plants is closely connected to the biosynthetic pathways of other plant hormones.