ABSTRACT
Terpenoids are an important class of structurally diverse plant metabolites synthesized from C-5 precursor units (isopentyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMPP)). These precursors are produced by two alternative pathways, i.e., cytosolic mevalonate pathway (MVA) and plastidial methylerythritol phosphate pathway (MEP). Based on the number of carbon atoms, terpenoids can be classified into hemiterpenoids, monoterpenoids , sesquiterpenoids, diterpenoids, triterpenoids, tetraterpenoids, and polyterpenoids. The majority of the terpenoids produced are common to all plants, while some of them are species-specific. Terpenoids are known for their role as accessory pigments, phytoalexins, allelopathic agents, pheromones , repellents, and attractants in plant-insect or plant-animal interaction. Their accumulation in plants is known to be affected upon encountering various biotic and abiotic stresses. Terpenoids such as zealexins are elicited 104in response to fungal infection, whereas citral induces senescence. Moreover, the induction of terpenoid production is one of the primary defense mechanisms in plants responding to herbivory attacks. This chapter provides a comprehensive account of the critical roles of terpenoids in plants in response to various environmental stresses.
