## 1 Introduction  
Peppers are one of the most important vegetables and originate from the tropical and subtropical regions of Central and South America, where more than thirty closely-related species of the genus Capsicum are found. There are six economically cultivated species: _Capsicum chinense_, _C. annuum_, _C. frutscens_, _C. baccatum_, _C. assamicum_, and _C. pubescens_[1], with different fruit shapes, sizes, spiciness, aromas, and tastes. Pepper fruits are rich in capsaicin, minerals, vitamin C, etc., with properties such as nutritional supplement, antioxidant, antibacterial, anti-inflammatory, thrombus dissolution, blood pressure reduction, and cholesterol reduction. As a result, they are widely used as an application in medical, antiseptic, chemical, food, and other fields [2, 3]. Fruit aroma, determined by the composition and content of volatile flavor compounds, is a crucial indicator for evaluating fruit quality, guides consumer choices in many cases [4], and is closely related to human health and nutrition [5]. About 2000 volatile aroma compounds (VACs) have been identified in different species of plants, including esters, aldehydes, alcohols, phenols, terpenes, etc., which are of great significance for the complete aroma characteristics of fruits [6; 7]. As the most abundant secondary metabolites in plants and important contributors to the aroma of fruits, volatile terpenids (including sesquiterpenoids, hemiterpenoids, monotereponoids, and diterpenoids) are synthesized mainly through the mevalonic acid (MVA) and methylerythnitol phosphate (MEP) pathway, and terpene synthase (TPS) is the most critical enzyme for the final synthesis of terpenes [8; 9]. The second largest category of VACs in plants are volatile benzenoid compounds, such as phenylacetaldehyde, methyl salicate, benzyl benzoate, eugenol, and isoeugenol, which mainly come from aromatic amino acids (L-phenylalanine) and are then synthesized by the shikimate/phenylalanine biosynthetic pathways [10; 11]. Most of the volatile straight-chain aldehydes, alcohols, and esters in aroma components are synthesized using unsaturated fatty acids (linolenic acid and linoleic acid) as precursors through the lipoxygenase (LOX) pathway. Linoleic acid and linolenic acid are converted into the hydroperoxide intermediate catalyzed by LOX and then form corresponding straight-chain aldehydes, alcohols, and esters under the catalysis of hydroperoxide lyase (HPL), alcohol dehydrogenase (ADH), and alcohol acyltransferase (AAT) [12; 13]. In many flowers and fruits, valine, leucine, and isoeucine precursors can also produce VACs, such as branched-chain aldehydes, alcohols, and esters through the branched-chain amino acid pathway (BCAAP), in which branched-chain aminotransferase (BCAT) is the key enzyme [14; 15; 16]. In addition, many studies have shown that the same VACs can be regulated by multiple genes, and the same gene can regulate the synthesis of multiple VACs [17]. Meanwhile, the non-canonical biosynthesis pathways of VACs exist in plants [18], and many TFs are involved in regulating the formation of aroma substances [19; 20]. Therefore, the biosynthesis of VACs is a very complex process involving multiple metabolic pathways and gene expression regulation. Fruit aroma is an important quality component and processing demand of pepper. Therefore, as early as the 1960s, 60 VACs have been identified in Californian green bell peppers and 2-methoxy-3-isobutyllyrazine, methyl salicate, limonene, linalool has been identified as the main aroma component [21]. Thereafter, 102, 136, and 63 VACs were identified at the two ripening stages of _Capsicum chinese_ fruits, including Yucatan peppers, Cachucha peppers, and CNPH4080 peppers, where hexyl isovalerate, hexyl dimethyl butyrate, and hexyl valerate were the main components [22; 23]. In Zunyi chili pepper (_Capsicum frutscens_), among the 85 identified VACs, 40 components appeared at the green stage, 53 at the breaking stage, and 65 at the red color stage.