## 3 Discussion  
In this study, the DEGs associated with terpenoid biosynthetic processes during _L. cubeba fruit development were examined using DGE profiling technology. A total of 8459 DEGs were identified across four respective comparisons of developmental stages. Among the identified DEGs, 26 DEGs were associated with terpenoid biosynthesis pathway as identified using pathway analysis. The average percentage of reads mapped to the reference transcriptome was 63.85%. The lack genome sequence date and incomplete transcriptome reads of _L. cubeba_ explain the occurrence of the unmapped tags. Among those differentially expressed genes, more DEGs were down-regulated than up-regulated, and a similar phenomenon was observed in the DEGs involved in terpenoid backbone biosynthesis pathways (24 vs. 2). The fact suggested that these genes may be the negatively controlled genes in terpenoid biosynthesis. However, it didn't mean lower expression of these genes leaded to lower volatile constituent contents. In other words, the expression levels of terpenoid biosynthetic genes did not correspond with the storage of volatile constituents, since the terpenoids biosynthetic pathway is a complex pathway. Most of the key enzymes involved in this pathway were encoded by multiple genes with different expression patterns and subcellular localizations [21]. Another reason might be plants often transport natural products from a synthesis site to an accumulation site [22]. This phenomenon was verified in _Arabidopsis thaliana_, _Astragalus membranaacus_, and _Tropaeolum majus_[23; 24; 25]. The biosynthesis pathways of all terpenoids include three main processes, the synthesis of the C5 precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), the synthesis of the immediate diphosphate precursors, and the formation of the diverse terpenoids. A large group of enzymes play a key role in volatile tempere synthesis [4; 5]. In this study, we investigated the genes with key roles in the terpenoid biosynthetic process. The most predominant chemical constituent in _L. cubebe_ essential oil was citral (60-80%), which is an isomeric mixture of gerani and neral [26]. Gerani is produced by geraniol dehydrogenase (GEDH), which catalyses the oxidation of the geraniol [27], and can be transformed into neral by keto-enol tautomerization [28; 29]. GEDH was cloned and characterized in a few plant species. [PERSON] identified two cDNAs encoding NADP+dependent dehydrogenase that can use geraniol to form citral in _Ocimtum basilicum_ cv. Sweet Dani, and found that GEDH activity levels were the highest in young leaves which have more glands per unit area [30]. The authors also isolated and characterized a cDNA encoding GEDH in _Zingiber officinal_. The expression levels of ZoGeDH in various ginger plant tissues were in accordance with the accumulation of geraniol [27]. In addition, [PERSON] cloned and characterized GEDH in _Perilla_[31]. In our research, although it did not reach the selection criteria for the DEGs (\(\,\mathrm{l}\mathrm{log}_{2}\) (\(\,\mathrm{fold-changed}\,\)) \(\,\mathrm{|<1}\)), the RPKMs of g36695, which KEGG annotation identified as GEDH, were increased from 60 DAF to 90 DAF, and then decreased until 135 DAF. This trend was inconsistent with the accumulation of citral. It is believed that GEDH expression is decreased during the maturation of fruit and the citral is synthesized during the young stage of fruit development and stored during maturation in _L. cubeba_. This inconsistent phenomenon was also found in _Zingiber officinal_ and _Citrus unshiu_. In old rhizomes of _Zingiber officinal_, the loss of ZoGeDH expression was concomitant with the accumulation of abundant gerani [27].