## 4 Jasmonates: Biosynthesis, Signaling, and Functions
### MeJA as an Elicitor of Secondary Metabolism  
Among the various jasmonate derivatives, MeJA stands out for its remarkable ability to induce secondary metabolism [36]. When applied to either foliage or roots, MeJA simulates the biochemical signals associated with wounding or herbivore attack, priming the plant to redirect resources away from primary growth and toward the production of protective or specialized metabolites [37] (Figure 2). Well-documented examples include increases in nicotine content in _Nicotiana tabacum_ L., where MeJA causes transcriptional activation of core nicotine biosynthetic genes (e.g.,  
Figure 2: Methyl jasmonate (MeJA) and Jasmonic acid (JA) as elicitors of secondary metabolism: activation of defense responses and gene expression, leading to enhanced secondary metabolite production. Figure created using Biorender.com. PMT and QPT), effectively doubling alkaloid levels [38]. Another noteworthy illustration is the several-fold rise in paclitaxel (Taxol) production in Taxus cell suspension cultures exposed to MeJA [39]. Salvia species respond to MeJA by enhancing key enzymes in the phenylpropanol pathway, resulting in higher levels of rosmarinic acid and related antioxidant compounds [36]. In the context of cannabis, recent studies indicate that concentrations of MeJA ranging from 1 mM to 15 mM can notably elevate THC and CBD in floral tissues [40]. This upregulation aligns with the broader pattern of jasmonate-driven enhancement of specialized metabolites, and appears to be attributable to several factors. First, MeJA prompts the initiation of additional glandular trichomes, effectively increasing the number of sites capable of synthesizing cannabinoids and terpenes [19]. By generating extra \"biochemical factories,\" cannabis plants may produce more secondary metabolites without necessarily overexpressing the final enzymes, such as THCA synthase or CBDA synthase. Second, MeJA upregulates upstream components of the cannabinoid pathway, notably genes involved in fatty acid metabolism or polyketide synthesis, which leads to olivetolic acid production [24; 41]. In particular, an increase in hexanoyl-CoA availability is crucial for supporting higher rates of CBGA biosynthesis, which subsequently contributes to the formation of THC and CBD. Third, MeJA often redirects carbon skeletons away from vegetative growth and towards defense-oriented metabolism, a phenomenon that clarifies why plants occasionally exhibit stunted growth, yet display more abundant resin production [31; 42]. Similarly, MeJA application was found to increase artemisinin and related secondary metabolites in _Artemisia annua_ L., highlighting its broad influence in triggering the plant's secondary metabolism [43]. In controlled-environment agriculture facilities, where potency is occasionally favored over biomass, this trade-off may be advantageous for producers. Interactions between MeJA and other hormones such as ethylene, salicylic acid, or gibberellins can produce synergistic or antagonistic effects on plant physiology. MeJA often collaborates with ethylene to bolster defenses against heterotrophic pathogens [44], yet it can clash with salicylic acid, which governs many responses to biotrophic pathogens [45]. Jasmonate signaling also commonly inhibits gibberellin-related growth processes, thereby shifting resource allocation toward defense and secondary metabolism [46]. Taken as a whole, these interactions mold the final phenotype of MeJA-treated cannabis, reconciling height reduction with improved cannabinoid yield [40]. From a cultivation standpoint, exogenous MeJA application may be a reliable method to boost trichome density and enhance the phytochemical profile of cannabis, particularly under controlled conditions such as greenhouses or vertical farms [20]. Applicators must, however, calibrate dosing and treatment schedules carefully to avoid excessive biomass loss.