## 1 Introduction  
Fragrance is an important feature of ornamental plants, which can provide people with a sense of pleasure. Essential oils and spices extracted from aromatic flowers, that can be used in perfume and cosmetics, also increase the economic value of ornamental species [1]. Therefore, it is especially important to pay attention to and strengthen the research on plant flower fragrance. However, since floral fragrance is a complex mixture of small volatile molecules, it is difficult to be evaluated objectively and qualitatively [2]. The collection of volatile compounds is an important step for subsequent analysis and identification, and the commonly used methods include liquid-liquid extraction (LLE), dynamic headspace sampling (DHS), simultaneous distillation extraction (SDE), solid-phase extraction (SPE), and solid-phase microextraction (SPME) [3]. Among them, SPME is a sample pretreatment technology that has developed rapidly in recent years. It has the advantages of solvent-free extraction, fast, efficient, and high sensitivity, and it has been widely used for the analysis of volatile compounds [4]. As far as we know, the floral volatiles identified in horticultural plants are mainly composed of terpenoids, phenylpropanoids/benzenoids, and fatty acid derivatives [5; 6]. As the largest group of volatile compounds, terpen are derived from isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), which are two common and interchangeable precursors [7; 8]. Studies have shown that the two precursors are synthesized by the plastid 2-C-methyl-D-erythnitol-4-phosphate (MEP) pathway and the cytosolic mevalonate (MVA) pathway [9]. Monoterpenes, the most important volatile terpen, are synthesized mainly through the MEP pathway in the plastid. The MEP pathway in plants begins with the formation of 1-deoxy-D-xylulose 5-phosphate (DXP) through DXP synthase (DXS) from pyruvate and glyceraldehyde-3-phosphate [8]. At present, the important intermediates in the MEP pathway have been known, and the enzymes that catalyze the conversion of the intermediates have also been successfully identified (as it will argued in the Section 3). Genes involved in the MEP pathway have been the subject of a large number of studies in recent years [8]. The _Arabidopsis_ DSX gene can significantly promote the content of monoterpenes in transgenic spike laveronder oil [10]. Many terpene synthase (TPS) genes responsible for the formation of volatile terpen have been identified from many plants, such as _Antirathinum majus_[11], _Catharanthus roseus_[12], _Clarkia breweri_[13], _Cymbidium_[14], _Freesia \(\times\) hybrid_[15; 16], _Lillium_[17], _Osmanthus fragrans_[18], and _Prunus mume_[19]. Phenylpropanoids/benzenoids are the second largest class of plant volatile compounds, which originate from 3-phenyl-L-alanine (L-Phe) and are produced by the shiki-mate and arogenate pathways in plastids [20]. Phenylpropanoids/benzenoids are the main components of flower fragrance in many plant varieties and are also important factors to form a unique scent of flowers [20; 21; 22]. Phenylethyl alcohol (2-PE) is one of the styrene-phenylpropanoids/benzenoids with pleasant unique rose scent, which is widely used in the production of edible essence [23]. Its metabolic mechanism has also been reported in petunia [24; 25], tomato [26; 23], and rose [27]. There are three possible biosynthetic pathways of 2-PE [26; 27; 28], and they will be showed in the \"Discussion\" section. After L-Phe is converted into phenylacetaldehyde (PALd), PALd is finally converted to 2-PE under the action of alcohol dehydrogenase (ADH) or PALd reductase (PAR). At present, the research on the mechanism of 2-PE synthesis in plants is limited, and there are few reports on some key genes in the synthesis process. More attention will be paid to the 2-PE synthesis pathway and the regulation of 2-PE synthesis at the molecular level in the future [28].