## 1 Introduction
After prokaryotic expression and purification, geraniol synthase (_CIGES_) transformed the geraniol diphosphate into a single product geraniol [29]. The genetic transcriptome related to the biosynthesis of terpenoids in _Cinnamomum camphora_ was analyzed preliminarily, and some candidate genes involved in the biosynthesis of terpenoids were screened. Three synthetic genes of monoterpene were found to be upregulated in the borneol chemotype [13]. The study of terpenoid synthases and their functions have made great progress. Determining how to use genetic engineering technology to regulate and even transform and improve the composition and yield of the essential oils of plants has become a research hotspot. However, the current research on the terpenoid biosyntheticpathway of _C. porrectum_ is still lacking, and there has been no gene study on the related biosynthesis of major terpenoids in the essential oil of _C. porrectum_, directly restricting the genetic improvement and exploitation and utilization of the quality of _C. porrectum_ essential oil. Transcriptome studies provide a useful perspective for expounding the molecular mechanisms of gene functions, cellular reactions, and different biological processes [30]. High-throughput sequencing (HTS) has recently been used to obtain transcriptome data for nonmodel species and provide valuable genomic information, especially for species without genomic sequences [31]. Second-generation sequencing (e.g., Illumina sequencing) has high-throughput capabilities, provides high-quality readings and enables very accurate research on gene expression and quantification. However, existing transcriptome analyses are based on incomplete genomic sequences and incomplete gene annotations. The fragmentation of genes assembled from second-generation sequencing data (usually 100-150 base pairs [bp] in length), which are limited by read length, results in a lack of information on full-length transcripts. With the advantage of an average read length of 10-15 kb, in combination with multifragment library screening technology, single-molecule real-time sequencing, also known as third-generation sequencing (e.g., PacBio sequencing), achieves transcript analysis without the need for splicing, overcomes the limitations of unigenes assembled from short spliced sequences and incomplete transcript structures in traditional second-generation transcriptomes, and can directly obtain all high-quality transcriptome information of single-RNA molecules from the 5\({}^{\prime}\) end to the 3\({}^{\prime}\) end. Third-generation sequencing is applicable to species without genomic sequences and has been used in extensive applications [32; 33; 34; 35; 36]. In view of this, the present study carried out third-generation de novo transcriptome sequencing of _C. porrectum_ leaf tissue for the first time and sequenced the expression profiles of leaf tissues from three different chemotypes (linalol chemotype, eucalypt chemotype and camphor chemotype) of _C. porrectum_ to reveal information on the transcriptome features of _C. porrectum_. Using kyoto encyclopedia of genes and genomes (KEGG) pathway analysis technology, we explored the genes of _C. porrectum_ involved in the metabolism pathways of terpenoids and further analyzed the differentially expressed genes (DEGs) of monoterpenoids in the three different chemotypes. We also preliminarily screened candidate genes for the biosynthesis of several important monoterpenoids. qRT-PCR verification of candidate genes was performed, and key genes in the synthesis of monoterpenoids, such as linalool, eucalypttol and camphor, were screened. This study lays a foundation for the cloning of key genes in the synthesis of important monoterpenoids of _C. porrectum_, functional research, and the genetic improvement and study of the special economic values of _C. porrectum_.