# Biosynthetic Pathways of Hormones in Plants
## 4 Salicylic Acid  
Salicylic acid (SA) participates in plant growth and development. In heat-generating plants, it initiates the process of heat production by triggering an alternative respiration pathway, leading to the release of foul-smelling compounds that lure pollinators. In terms  
Figure 3: The biosynthetic pathways of ethylene (phytohormone is green). of plant defense, SA acts as a messenger, managing the expression of plant pathogenesis-related genes and fostering disease resistance. Additionally, it helps to orchestrate plant reactions to diverse types of non-biological stress factors, like extreme temperatures, saline conditions, and oxidative stress. SA also has a role in controlling a range of other activities in plants, including photosynthesis, respiration, growth of vegetation, seed germination, flowering, and aging [42; 43; 44]. In plants, SA occurs via two pathways: (i) the isochorismate synthase (ICS) (major fraction) and (ii) the phenylalanine ammonia-lyase (PAL) pathways (Figure 4). Both biosynthetic pathways start in chorismate plastids and vary between plant species [42]. Although plants employ both pathways simultaneously, the ICS pathway is the primary contributor, accounting for approximately 95% of SA synthesis. Chorismate is a primary metabolic precursor of both pathways [45]. This compound is the end product of the shikimate pathway, which is initiated by erythrose-4-phosphate and phosphoenolpyruvate [46; 47]. The ICS pathway was first discovered in bacteria. Later, its presence was confirmed in vascular plants [48; 49]. In the ICS pathway, the first identified step is the conversion of chorismate into its isomer, which is known as isochorismate, via the reaction catalyzed by ICS or its homologous. These enzymes are common to both bacteria and plants. _A. thaliana_ and soybean (_Glycine max_) possess two genes that encode ICS, while rice (_Oryza sativa_) has only one gene that encodes this enzyme. Although the activities of ICS enzymes may vary between plant species, their primary structures are almost conserved [47]. Isochorismate is, indeed, produced in plastids and subsequently transported to the cytosol. The transportation of isochorismate from plastids to the cytosol is facilitated by the Enhanced Disease Susceptibility 5 (EDS5) protein, which functions as a MATE transporter [48]. Once it is present in the cytosol, isochorismate undergoes conjugation with L-glutamate and is converted into isochorismate-9-glutamate (ICS-Glu) through the action of a cytosolic amidotransferase. In the last step, the spontaneous decomposition of ICS-Glu leads to accumulation of the following products: SA and 2-hydroxy-acryloyl-N-glutamate. The activity of the cytosolic amidotransferase, which converts isochorismate into isochorismate-9-glutamate (ICS-Glu), is, indeed, inhibited by salicylic acid (SA), which acts as a negative feedback regulation mechanism. This feedback inhibition by SA helps to regulate the biosynthesis of SA and control its levels within the plant [42]. In _A. thaliana_, the ICS pathway plays a significant role in the accumulation of SA in response to pathogen infection. The ICS pathway is known to be a major contributor to SA biosynthesis, particularly during pathogen-induced defense responses. Upon pathogen recognition, the activation of defense signaling pathways leads to an increase in ICS enzyme activity, resulting in the production of isochorismate and its subsequent conversion into SA. This pathogen-induced SA accumulation plays a crucial role in triggering and amplifying plant defense responses against pathogens [47]. Figure 4: The biosynthetic pathways of salicylic acid (phytohormone is green). The second route of SA biosynthesis, which is known as the PAL pathway, is responsible for the synthesis of only a small percentage of SA in plants such as _A. thaliana_ and _Nicotiana benthamiana_, which were infected by pathogens or treated with UV during the reviewed studies [50]. This pathway occurs entirely within the cytosol.