A synthase is an enzyme that causes synthesis or formation of more complex molecules. Terpene synthases are therefore the chemicals that catalyze the creation of certain terpenes. Identifying terpene synthases and their effects in Cannabis sativa L. may help plant geneticists and breeders develop chemovars with desired terpene profiles. 
Canadian researchers grew the hemp indoors from seeds. They then extracted flowers with pentane and analyzed terpene content using gas chromatography-mass spectrometry (GC-MS). This revealed abundant (though varied) concentrations of monoterpenes α-pinene, β-pinene, myrcene, and limonene, as well as sesquiterpenes β-caryophyllene and α-humulene, among others.
Researchers also used an array of techniques on plant tissues and trichomes to isolate total RNA, or transcriptomes, and synthesize complementary DNA (cDNA). cDNA is formed and analyzed using a special type of quantitative polymerase chain reaction (qPCR) known as reverse transcription qPCR (RT-qPCR). Both methods amplify DNA fragments for analysis, but RT-qPCR includes the creation and amplification of cDNA from the RNA starting material. [1,2] RNA plays a fundamental role in gene expression—how genes actually manifest physically.
In this case, the researchers declared that the nine identified CsTPS genes “account for most of the terpenes found in ‘Finola’ resin.”  Not surprisingly, these genes were expressed predominantly in trichomes. The synthases fell into two subfamilies: TPS-b, associated with monoterpenes, and TPS-a, associated with sesquiterpenes. They are summarized alongside major products below as determined after incubation/in vitro biosynthesis.
- CsTPS1FN: (-)-limonene
- CsTPS2FN: (+)-α-pinene
- CsTPS3FN: β-myrcene
- CsTPS5FN: β-myrcene and (-)-α-pinene
- CsTPS6FN: β-ocimene
- CsTPS4FN: alloaromadendrene
- CsTPS7FN: δ-selinene
- CsTPS8FN: γ-eudesmol and valencene
- CsTPS9FN: β-caryophyllene and α-humulene
Although the list makes it seem simple, the authors note “biosynthesis of the different classes of terpenoid metabolites are independently regulated,” and “genes may have evolved different functions in different [cannabis varieties] …”
One of their Finola plants did not produce any monoterpenes—only cannabinoids and sesquiterpenes. Terpinolene was a common monoterpene in several samples but was not matched to any CsTPS. The correlation between terpene composition and CsTPS gene levels only reached significance for limonene and CsTPS1FN; in the case of β-caryophyllene and α-humulene, it was not even in the ballpark of significance. When cross-referencing against intoxicating cannabis varieties (e.g., Purple Kush), distinct terpene synthases sometimes produced the same terpene, but other times, the same terpene synthase produced different terpenes. CsTPS5FN, a member of the TPS-b subfamily, also catalyzed sesquiterpene farnesene in small amounts. 
Ultimately, cannabis is complicated. Our understanding of terpene synthases and how they inform the chemovar and consumer experience is young. Still, the knowledge is advancing: the same researchers (and additional colleagues) recently identified 19 CsTPS gene models in the Purple Kush variety.  This could ultimately help equip breeders eager to deliver plants that maximize certain terpenes.
- Booth JK, et al. Terpene synthases from Cannabis sativa. PLoS ONE. 2017;12(3):e0173911. [Impact factor: 2.740; Times cited: 52]
- Adams G. A beginner’s guide to RT-PCR, qPCR and RT-qPCR. Biochem (Lond). 2020;42(3):48–53. doi:https://doi.org/10.1042/BIO20200034 [Impact factor: n/a; Times cited: n/a]
- Booth JK et al. Terpene synthases and terpene variation in Cannabis sativa. Plant Physiology. 2020;183(4). doi:https://doi.org/10.1104/pp.20.00593 [Impact factor: 5.949; Times cited: n/a]