# New Insights into the Mechanism of Spatiotemporal Scent Accumulation in Orchid Flowers
## 1 Introduction  
Flower scents can help plants to attract pollinators [1] and are an important commodity for ornamental flowers [2; 3; 4]. Thus, the biochemical mechanism of flower scent formation is an attractive topic to scientists and breeders and has been widely described for many plants [5; 6; 7; 8]. Orchids are one of the most famous plants worldwide due to their beautiful, colorful, and fragrant flowers, with three sepals and three petals, of which one is differentiated into the lip (labellum) that is unique [9]. There is limited understanding of the composition and intensity of scent volatiles among different flower parts, although the underlying biochemistry of volatile synthesis has been investigated in some _Phalaenopsis_ and _Dendrobium_ orchids [10; 11; 12; 13; 14]. Terpenoids, one of the main components of floral scents, are classified into several types and consist mainly of sesquiterpenes, monoterpenes, diterpenes, and volatile carotenoid derivatives. These groups are classified according to the number of five-carbon (C\({}_{5}\)) units and their biosynthesis via either the cytosolic mevalonic acid (MVA) or the plastitial methylerythitol phosphate (MEP) pathways [15]. Monoterpenes have been found to be especially important floral volatile organic compounds (VOCs) in plants [3; 16; 17; 18]. They also act as the main compounds of floral scents in orchid species. Linalool and geraniol and their derivatives, _beta_-Ocimate and myrcene, determine the floral scent of _Phalaenopsis bellina_, whereas these monoterpenes are not found in the scentless species _P. eugstris_ and _P. aplrodite_[10; 13]. The floral scent of the fragrant _Phalaenopsis_ orchid _P._ I-Hsin Venus KHM221 is dominated by three monoterpenes, including linalool, geraniol,and eucalyptpt [12]. Additionally, linalool and geraniol have also been indicated to be the main VOCs that determine the floral scent of _Dendrobium officinal_[19]. Based on these previous studies, linalool and geraniol are common VOCs emitted by flowers in _Plalenaopsis_ and _Dendrobium_ species, and there are differences in the types of other monoterepenes in different orchid species. Monoterepenes are products of the MEP pathway, which has been clearly described and mainly consists of ten enzymes: 1-deoxy-D-xylulose 5-phosphate synthase (DXS), 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR), 2-C-methyl-D-erythitol 4-phosphate cytidylyltransferase (MCT), 4-(cytidine 5'-diphospho)-2-C-methyl-D-erythitol kinase (CMK), 2-C-methyl-D-erythitol 2,4-cyclodiphosphate synthase (MDS), 4-hydroxy-3-methyl-2-enyl diphosphate synthase (HDS), 1-hydroxy-2-methyl-2-_trans_-butenyl 4-diphosphate reductase (HDR), isopentenyl diphosphate isomerase (IDI), geranyl diphosphate synthase (GDPS), and terpene synthase (TPS) [15; 20]. In the MEP pathway, monoterepne synthases (MTPSs) belonging to the TPS family are involved in the final step of catalyzing geranyl pyrophosphate (GPP) to produce monoterepenes [21]. MTPSs are regarded as among the important enzymes determining the types and contents of monoterepenes in flowers [15; 22]. In _Freesia \(\times\) hybrida_, the spatiotemporal expression of _FhTPS1_ plays an important role in floral scent and is responsible for linalool biosynthesis [17]. _Linalool synthase_, an _MTPS_ from _Clarkia breweri_, was found to be overexpressed in carnations (_Dianthus carophyllus_ 'Eilat'), which resulted in the emission of linalool and its derivatives from the leaves and flowers of transgenic carnation lines [23]. Additionally, the transient overexpression of _PbTPS1_ and _PbTPS5_ identified from _Phalanopsis belina_ in \(P\). I-Hsin flowers increased the emission of linalool [24]. _Rhyncholalieocattleya_ Beauty Girl 'KOVA' (KOVA), with strongly scented flowers, is an ornamental orchid cultivar cultivated in South America and is a cross of _Rhyncholenia_ species and _Cattleya_ species.