## 1 Introduction  
Plants in the genus _Lavandula_ (lavenders) within the family Lamiaceae (Labiatae), (which are native to the Arabian Peninsula, the Mediterranean, Russia, Southeast India, and Africa) are noted for their important medicinal properties [1]. There are currently 39 known species of lavender that can be classed based on the following sectional classification: section _Lavandula_; _Dentatae_ Suarez-Cerv. and Seoane-Camba; section _Stoechas_ Ging; section _Peterostoechas_ Ging; section _Subnudae_ Chaytor; section Chaetostachys Benth; and unclassified taxa [2]. Among the species in section _Peterostoechas_, _Lavandula__pubscens_ is a species native to the Yemen Arab Republic, Israel, Jordan, Saudi Arabia, Egypt, and Syria, and this aromatic plant is comprised of a distinctive stem with a long white stiff indumentum over a number of short-stalked glandular hairs, two to three pinnatisect leaves, and dense unbranched flower spikes [2,3]. The bracts are oval-shaped and slightly shorter or longer than the calyx. A wide variety of lavenders have similar chemical compositions and ethnobotanical properties [4]. The phenotype of _L.__pubscens_ grown in a greenhouse is shown in Figure 1. Moreover, they are characterized by the production of essential oils containing monoterpenes, dirpenes, sesquiterpenes, and aromachemicals, which are utilized in veterinary medicine, aromatherapy products, cosmetics, fragrances, and food processing [1,3]. Numerous lavender varieties have been developed and cultivated for a diverse range of medicinal, ornamental, and commercial purposes [1,3]. Terpenes are both the largest class of natural plant products and the most structurally diverse group in nature, the biosynthesis of which is initiated by the formation of two types of isoprene unit, isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP), derived from the mevalonate (MVA) or 1-deoxy-d-xylulose5-phosphate (MEP) pathway (Figure 2). In the subsequent metabolic step, geranyl diphosphate synthase (GPPS) catalyzes the formation of geranyl diphosphate (GPP) (a C\({}_{10}\) precursor for monoterpene biosynthesis) via the condensation between IPP and DMAPP. The condensation between GPP and IPP is further catalyzed by farnesyl pyrophosphate synthetase (FPPS) to generate famesyl pyrophosphate (FPP), known to be a C\({}_{15}\) precursor of sesquiterpenes. The formation of geranyl geranyl diphosphate, also known as the C\({}_{20}\) precursor of diterpenes, is catalyzed by geranyl geranyl diphosphate synthetase (GGPPS), which binds the resulting FPP with a further IPP. In the biosynthesis of triterpenes, there is a head-to-head dimerization between two FPP molecules, followed by an NADPH-requiring reductive rearrangement to form a squalene containing a C1-C1\({}^{\prime}\) linkage between the two molecules, which is catalyzed by squalene synthase (SQS). Thereafter, squalene epoxidase (SQE) catalyzes the oxygenation of squalene to produce 2, 3-oxidosqualene, a common C\({}_{30}\) precursor of triterpenes. Biosynthesis of oleanolic and ursolic acids involves the cyclization of 2,3-oxidosqualene to generate \(\alpha\)-amyrin and \(\beta\)-amyrin, catalyzed by \(\alpha\)-amyrin (aAS)  
Figure 1: The phenotype of _L.__pubscens_ grown in a greenhouse. and \(\beta\)-amyrin (bAS) synthases, respectively, which are successively 28-oxidized to unsolic acid and oleanolic acid via the activity of cytochrome P450 family members [5; 6]. Advances in next-generation sequencing (NGS) technologies now enable cost-effective whole-transcriptome sequencing rather than whole-genome sequencing, with RNA-seq techniques providing functional information from genome transcripts [7]. Among the well-established NGS technologies, Illumina Solexa GA, one of the \"Short-read\" (36-72 bp) sequencing technologies, has been successfully applied in the whole-transcriptome analysis and de novo assembly of laverders, including _L. angustifolia_ 'Lady', _L. lufifolia_, and their natural breed _L. \(\times\) intermedia_ '[PERSON]' [8], and _L.