## 1 Introduction  
Soybean (_Glycine max_), a herbaceous annual plant in the family Fabaceae, is among the most important crops in the world, playing irreplaceable roles in providing humans and livestock with high-quality proteins and balanced fatty acids [1; 2; 3; 4; 5]. Phytonutrients such as saponins, vitamins, and flavonoids in soybean also contribute special health nutrition and pharmacological effects to humans [6; 7; 8; 9]. The issue of how to guarantee or improve soybean yield and nutrition has attracted special attention from both botanists and breeders around the world. However, soybean diseases and pests have always been great threats to soybean production, which should never be ignored [10; 11]. As the understanding of chemical pesticides deepens, studies on the environment or human-friendly pesticides or anti-pest crops have been a hotspot in recent years. As sessile organisms, plants have to face complicated potential threats in their habitat and have evolved a robust immune system for their survival, such as trichomes defending against insects, specific metabolites conferring warning color and scent, or toxic properties to plants [12; 13; 14; 15]. Terpenes are a group of plant-specialized metabolites with highly diverse structures and versatile applications, which have been reported to help plants counter various biotic and abiotic stresses [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26]. Terpenes can further be classified as monoterpenes, sesquiterpenenes, sisterlerenes, triterpenenes, and tetraterpenes/carotenoids,among which monoterpenes and sesquiterpenes are volatile terpenes playing indispensable roles in plant-insect communications such as to attract pollinators, seed dispersers or natural enemies of herbivores, or repel pathogens or herbivores directly [18; 23; 24; 25]. Generally, the classification of terpenes is based on the numbers of fundamental repeating five-carbon isoprene units: isopentenyl diphosphate (IPP) and its double-bond isomer dimethylallyl diphosphate (DMAPP). IPP and DMAPP are synthesized through two compartmentalized pathways in plants: the plasidic methylerythnitol 4-phosphate (MEP) pathway and the cytosolic mevalonic acid (MVA) pathway by sequential catalysis of a series of enzymes [15; 27; 28; 29; 30]. Subsequently, the sequential condensation of IPP units to one molecule of DMAPP by short-chain premltransferases (PTs) results in C10 geranyl diphosphate (GPP) or nervyl diphosphate (NPP), C15 farnesyl diphosphate (FPP), C20 geranylgeranyl diphosphate (GGPP), etc., which are further catalyzed into versatile terpenes with rearrangements and cyclization reactions by a group of mechanistically related enzymes: terpene synthases (TPSs), whose copy number, subcellular localization, and catalytic property could largely determine the terpene diversity in a specific plant species. Terpene synthases are encoded by a broad mid-sized _TPS_ superfamily. Phylogenetically, land-plants-derived TPS proteins can be categorized into seven subfamilies, namely TPS-a, TPS-b, TPS-c, TPS-d, TPS-e/f, TPS-g, and TPS-h [29; 31; 32; 33; 34]. Specifically, TPS-d and TPS-h subfamilies contain TPSs exclusively found in gymnosperms and _Selaginella_ spp., respectively. In comparison, TPSs responsible for volatile terpen in angiosperms are confined into TPS-a, -b, and -g subfamilies. It has generally been observed that monoterpenes are mainly yielded by TPSs from the TPS-b clade, whereas sesquiterpenes are primarily produced by the TPS-a subfamily. The TPS-g subfamily includes both monoterpene syntheses and sesquiterpene synthases, mainly represented by lanalool synthase and nerolidol synthase, respectively. Moreover, TPSs can also be divided into two classes, Type I and Type II, based on the conserved motifs in either C or N termini, representing two diverse catalyzing mechanisms of terpene formation by TPSs [28; 31]. It is widely accepted that the olfactory cues conferred by volatile terpenes, together with visualization, play important roles in the host identification of insects.