# Biochemistry of Terpenes and Recent Advances in Plant Protection
## 1 Introduction  
Plants and a multitude of pathogenic microbes are in a constant battle for supremacy. While pathogens adopt novel means to maintain their nutrition and shelter sources in plants, the former undergoes immunity evolution to keep off the latter by eliciting defense molecules. Despite the absence of motile defensive cells and adaptive immunity, plants possess a robust immune system with a mirage of defense molecules to enhance their physical and chemical immunity against environmental stresses [1]. Plants produce two types of metabolites; primary metabolites are involved in cellular survival and propagation, and secondary metabolites play a crucial role in defense against pathogens and pests. Plants synthesize over 300,000 secondary metabolites, and muting or silencing their synthesis severely impairs their effectiveness to withstand biotic stresses [2].  
PSMs are categorized as phenolics, terpenoids, and alkaloids, among others. They play several roles in plant defense against fungi, viruses, bacteria, and feeding herbivores [3]. Some PSMs perform allebotraltic activities, while others serve as signal transducers for chemical communication with symbiotic insects, such as pollinators and seed dispersers [4]. The utilization of PSMs for pharmacological purposes spanned several thousands of years. However, analysis of PSMs for specific functional annotation came into lamplight only a couple of hundred years ago, with morphine from _Papaver somniferum_ (opium poppy) among the earliest studied. Several PSMs have recently been investigated for pesticides/insecticidal and herbicidal activities. Approximately 40% of commercial medicines are synthesized using PSMs as the active ingredients [5; 6]. For example, Taxol\({}^{\text{\textregistered}}\), an anticancer drug, and artemisinin, an antimalarial drug, are synthesized from terpenes [7]. Diterpenenes and phenylpropanoids secretion, for instance, confer phytoplankton properties against pathogenic microbes [8]. Antifeedant metabolite synthesis also enhances plant resistance to herbivores. For example, alkaloids deter _Emposca fabue_ and _Leptinotarsa declineata_[9], and phenolic compounds repel _Rhopalsiphum padi_ and _Galerucella lineola_ from feeding on wheat plants and also delay maturation and lower fecundity in _Aphis gossypii_ on cotton plants [10].  
Terpenes are the largest and most diverse PSMs in nature [11; 12] and are the informative and defensive vehicles used by plants for antagonistic and mutualistic interactions. The production of terpenes by plants to counter biotic (pathogenic microbes, herbivore pests, and weeds) and abiotic (water, temperature, light, and salt) stresses is widely studied [8; 13; 14; 15]. While phytoanticipins terpen are constitutively secreted in the absence of pathogen effectors, phytoalexins are elicited in response to inducible pathogenic microbes or feeding herbivores [7]. Terpene research as an active ingredient in pesticide development has seen a global resurgence due to its low risk to the environment and human health. For example, a consistent but deliberate policy in the European Union reduced the number of permissible synthetic compounds from 1000 in 1993 to 250 in 2011. The shift from risk to a hazard-based assessment of the effect of synthetic pesticides on humans and the environment accounted for this reduction. Costs for outdooring a new synthetic active product hiked amidst overwhelming cases of pesticide resistance, as [PERSON] and [PERSON], 2015 reported that over 580 species of arthropods globally developed resistance to synthetic pesticides [16; 17].  
The crucial role of PSMs and terpenes makes them a critical topic of concern to plant protectionists. This write-up reviews established knowledge of terpenes, types, the chemistry of their biosynthesis, and function in protecting plants from pests, diseases, and weeds. These highlights will reinvigorate the debate on using terpenes as active ingredients in developing biopesticides.