## 1 Introduction  
More than 1700 volatile organic compounds (VOCs) have been identified from 90 different plant families belonging to both angiosperms and gymnosperms [1, 2]. In nature, VOCs play important roles in plants' defenses against abiotic/biotic stresses, herbivores, and pathogens, as well as in pollinator attraction, and in plant-plant communication [3, 4, 5]. VOCs also possess immense economic importance as they are extensively used in cosmetics, food, and pharmaceutical industries. Based on their biosynthetic origins, plant VOCs can be divided into several classes: terpenoids, benzenoids/phenylpropanoids, fatty acid derivatives, and amino acid derivatives [2]. Terpenoids represent the largest and the most diverse class of plant metabolites. The majority of terpenoids are volatile and contribute significantly to the aroma of fruits, flowers, and essential oils [6].  
In plants, terpenoids are produced by a highly diversified and large class of proteins called terpene synthases (TPSs) [7, 8]. Biosynthesis of terpenoids are regulated by abiotic stress factors, such as temperature, light, drought, and salt, as well as by herbivores and microbes [9, 10, 11, 12]. For instance, emission of \(\alpha\)-pinene, sabinene, and thujene were affected by temperature as well as light intensity in beech (_Fagus sylvarica_ L.) [9]. Similarly, egg deposition by the pine sawfly (_Diprion pini_) induced the emission of mono- and sesquiterpenes from Scots pine (_Pinus sylvestris_) needles [11]. In addition, the production and emission of terpenoids are also regulated in a tissue- and/or organ-specific manner. Genes involved in the biosynthesis of terpenoids are expressed at particular stages of plant development and/or in specific tissues [13; 14; 15; 16; 17].  
Stevia (_Stevia rehaudiana_), a perennial shrub belonging to Asteraceae (2n = 22), is appreciated worldwide for its sweetness. Sweetness in Stevia comes from diterpenoid steviol glycosides (SGs), which are about 300 times sweeter than sugar. Thus, they are widely used in the food industry as a sugar substitute and sweetener [18]. In addition to SGs, Stevia tissues also produce diverse terpenoids [16; 19]. Aerial parts of Stevia plants grown in Italy were shown to produce mainly mono- and sesquiterpenoids [19]. Trichomes isolated from Stevia leaves accumulated high amounts of mono-, sesqui-, and diterpenoids, along with minor quantities of fatty acid derivatives [16].  
So far, studies in Stevia have focused mainly on types and accumulation of diterpenoid SGs, their biosynthetic genes, and pathways. In contrast, research efforts related to profiling and biosynthesis of other chemicals are largely missing in Stevia. Here, we showed comprehensive profiles of terpenoids in four types of Stevia tissues--leaves, flowers, stems, and roots. From the transcriptome data of Stevia, we identified four sesqui-TPSs and one mono-TPS, which were later found to be responsible for the production of major terpenoids in Stevia tissues via biochemical assays. Furthermore, our study identified two root-specific, multifunctional sesqui-TPSs that were capable of catalyzing multiple products, \(\gamma\)-curcumene/zingiberene/\(\beta\)-sesquiphellandrene and \(\alpha\)-longipinene/\(\beta\)-himachalene/himachalol. We also found that most _SrTPSs_ are highly responsive to environmental stresses showing different inducibility in leaves and roots of Stevia. These results suggest that Stevia could use terpenoids produced by these SrTPSs in each tissue for the defense mechanism against biotic and abiotic stresses.