# Biosynthetic Pathways of Hormones in Plants
## 12 Jasmonates  
Jasmonates are plant hormones that originate from oxylipins involved in plant development and stress responses [198]. Their major representatives are the isomers of jasmonic acid (JA): (+)-7-iso-JA and (-)-JA. JA can be metabolized into methyl jasmonate (MeJA), which is a volatile phytohormone. Jasmonates have previously been defined  
Figure 13: The biosynthetic pathways of different types of strigolactones (phytohormones are green, blue, and red). as JA and its diverse metabolites derived from various reactions created via esterification, methylation, sulfation, glycosylation, conjugation, de-carboxylation, hydroxylation, and carboxylation [199]. Recently, an alternative and probably more ancient pathway was proposed. In this route, JA originates from minor-OPDA (2,3-dinor-12-oxo-10,15(Z)-phytodienoic acid [dn-OPDA]) [200]. Thus, 12-oxo-10,15(Z)-phytodienoic acid (OPDA), dn-OPDA, and their derivatives are members of JAs family. JA and MeJA are the best characterized group of jasmonates in plants, and they are regarded as one of the major phytohormones that regulate both defense responses and the development process [201]. Jasmonate biosynthesis pathways have been extensively investigated in dicotyledonous plants, such as _A. thaliana_, tobacco, and tomato (Figure 14). In monocotyledonous species, only a few biosynthetic enzymes of JA have been identified and characterized [202; 203; 204]. JA biosynthesis is initiated via the release of the polyunsaturated fatty acid \(\alpha\)-linolenic acid 18:3 (\(\alpha\)-LeA) from galactolipids, which are present in chloroplast membranes. This reaction of hydrolysis is catalyzed by phospholipase A\({}_{1}\) (PLA1), which is characterized by specificity of sn-1 in the structure of fatty acids. Thus, \(\alpha\)-LeA oxygenation is the first step in JA biosynthesis. The flower-specific protein DAD1, which is a PLA1, is required for the formation of JA [205; 206]. The formation of JA occurs via one of the seven different branches of lipoxygenase (LOX) pathway [199]. Enzymes LOXs (lipoxygenases) are non-heme-iron-containing dioxygenases that produce hydroperoxides of fatty acid from polyunsaturated substrates. These enzymes can be sub-divided into 9-LOX and 13-LOX types, depending on the carbon atom  
Figure 14: The biosynthetic pathways of jasmonic acid (phytohormones are green). to which molecular oxygen is added. In _A. thaliana_, the 13-LOX enzymes, such as AtLOX2, 3, 4, and 6, are pre-sent to the chloroplast. On the other hand, 9-LOX members (e.g., AtLOX1 and AtLOX5) are probably localized in the cytosol, although their functions are poorly understood [207,208]. The distribution and transport of \(\alpha\)-LeA between the organelles, such as chloroplast and the cytosol, regulate the metabolic flow between 13-LOX- and 9-LOX-derived oxylipins [202]. In the chloroplast, (135)-hydroperoxy octadecatrienoic acid (13-HPOT) is formed after oxygenation of \(\alpha\)-LeA by the enzyme 13-LOX (Figure 14). This compound can play a role as a substrate of other enzymes, such as cytochrome P450 enzymes that belong to the CYP74 family, hydroperoxide lyase, allene oxide synthase (AOS), divinyl ether synthase, and epoxyalcohol synthase [209]. The next enzyme involved in the octadecanoid pathway is responsible for the biosynthesis of JA is AOS, which prefers to use 9- or 13-hydroperoxide derivatives as substrates. Thus, like LOX, this enzyme is characterized by 9- and 13-positional specificity. AOS is responsible for the formation of unstable allylic epoxides that can be spontaneously hydrolyzed in the presence of molecules of water in 13-hydroxy-12-oxo-octadecanionic (\(\alpha\)-ketols) and 9-hydroxy-12-oxo-octadecanionic acids (\(\gamma\)-ketols). Moreover, the cyclization to a racemic mixture of _cis_-(+) and _cis_-(\(-\)) enantiomers is observed [210]. The _cis_-(+) isomer of oxylipin OPDA is produced in this reaction. This reaction requires the neighborhood of AOC.