# Biochemistry of Terpenes and Recent Advances in Plant Protection
## 3 Role of terpenes in Plant Protection
### Antimicrobial (Phytoalexin) Activities of Terpenes  
There is a substantial amount of literature on phytoplankton terpene research. Phytoalexin camalexin synthesis in _Arabidopsis_ defends against _Pletosphaerella cucumerina_, _Botrytis cinerea_, and _Alternaria brasscicola_[89; 90]. Figure 5: Terpenes’ role in plant protection. Four critical functions of terpenes in plant protection: (**a**) insecticidal role of terpenes, (**b**) antimicrobial activities of terpenes against rice blast disease and _Fusarium_ head blight, and (**c**) alleopathy of root exudates of plants. Momilactone B inhibits the growth and development of _Monochloria vaginalis_. (**d**) Terpenes indirectly attract natural enemies of pests, e.g., (E)-\(\beta\)-caryophyllene attracts entomopathogenic nematodes on corn root borer. (Structures were drawn and analyzed with ChemDraw software, version 20.0.0.41, and uploaded on BioRender.com to create the illustration). Terpenes are a vital member of the VOCs group of weapons often unleashed by plants to defend against various fungi, bacteria, and viruses. (_E_)-\(\beta\)-caryophyllene is a widespread VOC in the plant kingdom and is produced from the floral whorls. This terpene critically inhibited a rod-shaped Gram-negative bacterium, _Pseudomonas syringae_, on Solanaceae and _Arabidopsis thaliana_ plants. In the flowers of _Arabidopsis thaliana_, bacterial growth exponentially increased when the production of (_E_)-\(\beta\)-caryophyllene was muted and critically inhibited and triggered defense signaling pathways when it was restored [91]. Capsidiol is another sesquiterpene consisting of toxic compounds with a significant phytoalexin property. Capsidiol inhibited _Phytophthora capsaicin_ and _Botrytis cinerea_ virulence in _Nicotiana plumbaginifolia_. Recent molecular evidence in NbEAS or NbEAH-silenced _N. benthamiana_ suggests a compromised defense function against _P. infestans_[92] and Potato Virus X (PVX) [13]. [PERSON] et al. also revealed that capsidiol inhibited _A. alternata_ under in vitro conditions. They argued that the ERF2-like transcriptional factor is responsible for the biosynthesis of capsidiol, and its role as a phytoalexin is independent of ethylene and JA signaling pathways [93]. Diterpenes also play a role in the plant's immune response against microbial pathogens, particularly _M. oryzae_. For example, the rice plant produces four major labdane-related diterpenoids that function as phytoalexins. They are momilactone A and B, phytocassanes A-F, oryzaexin A-F, and oryzaexin S [94,95]. The identification of these diterpenes was based on their defense activities against rice blast fungi, _M. oryzae_, leaf blight pathogen, and _Xanthomonas oryzae_ pv _oryze_ (Xoo) [96]. Diterpene (11E,13E)-labda-11,13-diene-8 alpha,15-diol, designated WAF-1, was implicated in immune defense signaling against tobacco mosaic virus (TMV) in _Nicotiana tabacum_. WAF-1 induced salicylic acid protein kinase by activating pathogenesis and wound healing-related genes. The endogenous level of WAF-1 in TMV tobacco leaves increased due to the hypersensitive response (HR) [91,92]. Oryzalexin diterpenoids isolated from rice leaves induced strong resistance to bacterial leaf spot disease. Notable among them are oryzaelide-related diterpenes: oryzalide, acid A, oryzalide, ent-15,16-epoxy-3\(\beta\)-palmitoyloxy-kauran-2-one, ent-15,16-epoxy-3\(\alpha\)-palmitoyloxy-kauran-2-one, ent-15,16-epoxy-3\(\beta\)-hydroxy-kauran-2-one, ent-15,16-epoxy-2,3-dihydroxy-kaurane, and oryzadione ent-15,16-epoxy-kauran-3-one [24]. Following _M. oryzae_ infection of rice plants, 9\(\beta\)-primara-7,15-diene-3\(\beta\),6\(\beta\),19-triol, phytocassane F, and stemar-13-en-2\(\alpha\)-ol accumulation increased in the leaves. Among these three compounds, phytocassane F showed stronger inhibition of the fungal mycelial growth [97]. Triterpenes are weak phytoalexin compounds. However, quinoa, a rice husk triterpene, exhibited effective molluscicidal and antimicrobial activities [98]. Phytocassanes A-D were also reported to inhibit _M.