## 3 Discussion  
Terpenoids are important components of citrus fruit volatiles. Finger citron has unique aroma, but the molecular mechanism of terpenoid biosynthesis remains unclear. In order to identify the characteristic compounds in this cultivar, we profiled the volatiles in 10 tissues from different finger citron organs. In total, 62 volatile chemicals were characterized, most of which were terpenoids. Consistent with previous studies in finger citron [38; 39], the most abundant volatile is limonene followed by \(\gamma\)-terpinene. Variations in volatile compositions were also observed in different organs of _Citrus limon_ and other _Citrus_ species [52; 53; 54]. In the clustering heat map (Figure 2), we observed that most components are differentially expressed in different organs. The main component limonene, decreased when fruit matured and a similar pattern in finger citron was also observed by [PERSON] et al. [40]. A decrease in limonene during fruit ripening was also detected in _Satsuma mandarin_[55; 56]. We identified six characteristic components in finger citron (Figure 3). Apart from \(\beta\)-bisabolene, the other five volatiles are cyclic monoterpenoids. They are enriched in fruit and accumulated during fruit ripening. These components are suitable biomarkers that could be used to distinguish finger citron organs and finger citron extracts from those of other _Citrus_ species. We identified 58 members of the finger citron TPS family. This family size is smaller than in grapevine (152) and sweet orange (95), but similar to rice (57) and _Sorghum bicolor_ (48) [12]. The largest clade of _TPS_ genes found in finger citron, with 20 members, is the TPS-a clade. In sweet orange, TPS-a is also the largest clade [15]. In other plants where the TPS family has been analyzed at genome wide level, TPS-a clade is also the largest clade, including sweet orange [15], tomato [8], apple [11], and grape [10]. It has been suggested that more recent _TPS_ gene duplications are indicated by an additional TPS-a clade [11]. Sesquiterpenes are major products produced by TPS-a enzymes, and the high number of TPS-a clade was suggested to be related to their possible role in plant developmental responses, such as defense [15]. RNA-Seq has the ability to simultaneously detect whole gene expression levels, and the availability of such a complete transcriptome profile has been a powerful tool to obtain insights into the molecular mechanisms underlying different conditions. In the present study, transcript levels of finger citron _TPSs_ were widely detected in different finger citron organs during development. Similar observations were also made for apple and tomato TPS family in developing organs [9; 11], suggesting important biological roles of TPS family members in plants. To understand the regulation of finger citron terpenoid biosynthesis, expressed genes detected by RNA-Seq were connected by weighted gene co-expression network analysis (WGCNA). This provided a network made of nodes (genes) and edges (connections). Connections in the network were obtained based on gene co-expression data. This strategy has been applied in mining potential targets genes and transcription factors in plants [45; 46; 47]. In the present study, the top three modules enriched for _TPS_ genes were selected for further analysis. Briefly, thirteen _TPS_ genes were screened based on WGCNA, including four from Module Red (Cm269930, Cm207000, Cm105420, and Cm186730), six from Module Green (Cm028750, Cm116770, Cm257260, Cm074160, Cm252580, and Cm310520) and three from Module Blue (Cm206970, Cm107390, and Cm279170). Moreover, a total of 15 TFs were identified, including five from Module Red (Cm127980, Cm258540, Cm279030, Cm303080, and Cm308600) and 10 from Module Green (Cm022190, Cm027020, Cm028420, Cm061930, Cm086640, Cm110180, Cm111830, Cm175400, Cm215520, and Cm227880). These TFs consisted of 12 families, including ARR-B, bHLH, MYB, ARF, MYB, MIKC-MADS, HD-ZIP, AP2/ERF, G2-like, C2H2, NAC, and CPP.