## 3 Factors Influencing PSMs Production in Plants
### Effects of Ozone on PSMs
Nevertheless, some plants (such as tobacco plants) do not require SA accumulation to form PAL transcripts [246]. These examples suggest that O\({}_{3}\) induces at least two signaling pathways, the SA-dependent pathway associated with pathogen defense response and the SA-independent pathway in the protective response to O\({}_{3}\). Isoprene in tree foliage is known to protect foliage from oxidative stress. For instance, when [PERSON] et al. [204] applied isoprene (2-3 ppm) exogenously to tobacco and birch leaves fumigated with O\({}_{3}\) (300 ppb), photosynthesis was consistent throughout the treatment period with the less accumulation of ROS compared to their fosmidomycin-treated control (showed more ROS accumulation and decreased rate of photosynthesis). Moreover, after three days of O\({}_{3}\) treatment, they observed that areas of leaves treated with isoprene were intact, suggesting that isoprene protects photosynthetic tissues and stabilizes the thylakoid membrane. Isoprene protects photosynthesis in those plants exposed to acute thermal and O\({}_{3}\) stress through antioxidative action (quench H\({}_{2}\)O\({}_{2}\)) and preventing membrane lipid peroxidation. For instance, leaves of _Phragmites australis_ for which their endogenous isoprene production was inhibited by applying fosmidomycin become more sensitive to O\({}_{3}\) stress than isoprene-producing leaves [204]. Figure 6: General phenylpropanolid pathway and flavonoid biosynthesis (adapted from [247; 248]. Solid arrows represent single enzymatic reaction; dashed arrows represent multiple sequential reactions. Enzymes involved: PAL—phenylalanine ammonia lyase; CHS—chalcone synthase; STS—stilbene synthase; CHR—chalcone reductase. Exposure to high O\({}_{3}\) concentration causes VOC emission, but at chronic O\({}_{3}\) level, it modifies compositions of BVOCs, consequently affecting tri-trophic interactions and weakening plants' response to arthropod attack [245; 249]. During such situations, isoprenoids (mainly herimetepenes, monoterpenes, and sesquiterpennes) are synthesized by plants to tolerate O\({}_{3}\)-induced damages. Hemiterpene is an example of an isoprenoid released in the leaves, as it can protect photosynthetic apparatus and scavenge O\({}_{3}\) by-products and ROS due to its antioxidative activity [204]. The effect of O\({}_{3}\) on alkaloid biosynthesis remains less elucidated, but polyamines in plants, which is an important alkaloid precursor, are correlated to O\({}_{3}\) tolerance [234]. Polyamines in plants possess a wide array of physiological functions [240] in addition their involvement in response to both abiotic and biotic stresses [250]. 4 Reported Pharmacological Properties of PSMs Present in Plants Affected by Ex Situ Abiotic Stresses  
Plant protective secondary metabolites are diverse in structure and biological properties, and they have been continuously exploited for pharmaceutical, nutraceutical, and cosmetic uses [251] (Figure 7). Flavonoids and other phenolic compounds are predominant among secondary metabolites produced in response to climatic/or abiotic stress (Table 1). Flavonoids confer protection against inflammation, allergy, and bacterial infections [252]. Flavonols (or 3-hydroxy flavones), one of the main subclass of flavonoids, are apparent antioxidants in stressed plants, and they are known to prevent nuclear DNA damage by free radicals like H\({}_{2}\)O\({}_{2}\)[253]. Flavonols are polyaromatic secondary metabolites with three rings, and many of them are bioactive. Many flavonoids possess antiviral properties. For instance, the hydroxy (OH) group in the ring-C of flavonoids makes them more effective against herpes simplex virus type I than flavones [254]. Fisetin is another example of an active flavonoid produced by plants under oxidative stress, preventing membrane lipid peroxidation, DNA damage, and protein carbonylation [247].