Terpenes are diverse: more than 80,000 are known, and many remain unknown. Zhou & Pichersky  call terpenes the “largest, most chemically, structurally and functionally diverse class of chemicals in living organisms.” These researchers recently highlighted new developments in this area, emphasizing that, although basic biosynthetic pathways are shared by all plants, there are many unique ways in how plants synthesize certain terpenes.
Initially, all terpenes come from isopentenyl diphosphate (IPP) and dimethylally diphosphate (DMAPP). Plants generate these 5-carbon (C5) building blocks via two pathways: the methylerythritol 4-phosphate (MEP) pathway and the mevalonate (MVA) pathway. Plants like cannabis transport the precursor molecules, IPP and DMAPP, to trichomes where they are transformed by enzymes known as terpene synthases.
But we already knew this. The present study presents more recent and deeper findings.
For example, in the MVA pathway, isopentenyl phosphate kinase (IPK) catalyzes the formation of IPP and DMAPP. The Nudix hydrolases AtNudx1 and AtNudx3 appear to decatalyze IPP and DMAPP, thus facilitating the regulation of these precursor molecules. More expression of IPK increases terpene production, and more expression of the Nudix hydrolases decreases terpene production.
Precursor molecules are not always catalyzed into terpenes by terpene synthases. Cited examples include a monoterpene alcohol from lavender known as lavandulol; in this case, lavandulyl diphosphate synthase, a prenyltransferase, condenses the two requisite DMAPPs. The intermediate molecule for pyrethrins from chrysanthemum flowers (pyrethrins form a potent terpenoid pesticide) also come from the condensation of two DMAPPs, in this case catalyzed by chrysanthemyl diphosphate synthase, a prenyltransferase. In rose, Nudix hydrolase RhNudx1 appears to catalyze the formation of geraniol.
Reprinted from: Zhou F, Pichersky E. More is better: the diversity of terpene metabolism in plants. Curr Opin Plant Biol. 2020;55:1-10. doi:https://doi.org/10.1016/j.pbi.2020.01.005. License: CC BY-NC-ND 4.0.
Some plants—particularly “early land plants”—house microbial terpene synthase genes due to “horizontal gene transfer from bacteria and fungi.”
Typical terpene synthases can be divided into Type I and Type II. The Type I synthase knocks off diphosphate (e.g., from geranyl diphosphate, GPP) to create a carbocation intermediate (with positively charged carbon) molecule for biosynthesis. This single step is more common for monoterpenes and sesquiterpenes. Diterpenes, however, more commonly require that a Type II synthase first catalyzes geranylgeranyl diphosphate (GGPP) into an intermediate suitable for the Type I synthase to remove the diphosphate. A prominent example of this scenario can be found in Salvia divinorum, where a Type II synthase catalyzes GGPP into kolavenyl diphosphate (KPP), the intermediate for salvinorin A.
The research also touches on new discoveries in terpene emission. This generally relates to pinpointing specific transporter molecules responsible for specific terpenes. One poignant finding regards Petunia flowers, where sesquiterpenes (such as germacrene D) fumigate in closed bud headspace, resulting in lower pistil weight and lower seed yield. The original study refers to this as “inter-organ aerial transport” and notes that the purpose may be to protect the stigma from pathogens .
It’s fair to say that we’ve just scratched the surface of terpenes and their metabolism. The researchers aptly conclude, “more complexities are being discovered with no end in sight.” 
- Zhou F, Pichersky E. More is better: the diversity of terpene metabolism in plants. Curr Opin Plant Biol. 2020;55:1-10. doi:https://doi.org/10.1016/j.pbi.2020.01.005. [Impact Factor: 7.834; Times Cited: 18 (Semantic Scholar)]
- Boachon B, et al. Natural fumigation as a mechanism for volatile transport between flower organs. Nature Chemical Biology. 2019;15(6):583–588.doi:10.1038/s41589-019-0287-5. [Impact Factor: 12.587; Times Cited: 13 (Semantic Scholar)]