# Bacterial Semichemicals and Transkingdom Interactions with Insets and Plants
## 3 BVOC Biosynthetic Pathways and Their Intersections with Primary and Secondary Metabolism
### BVOCs and Secondary Metabolism
#### 3.2.2 Terpene and Terpenoid Metabolism in Bacteria  
Terpenes, terpenoids, and, more in general, isoprenoids, are a large class of compounds widespread among plants, yeasts, archaea, and bacteria with various functions, such as hormones [135], photosynthetic pigment [136], and major structural components in the archaeal membrane [137]. The isoprenoid class includes cholesterol and liposoluble vitamins A, D, E, and K [138]. In plants, many terpenes and terpenoids are multifunctional compounds. They protect plants against herbivores and pathogens, attract mutualists such as pollinators, and may act also as chemical signals for other plants or insects [139; 140]. Different species of _Dendroctous_ are parasites of conifers, which are plants that produce various terpenes useful for protecting themselves from herbivous insects [141]. The insect gut community affects tolerance to terpenes and terpenoids. Some bacterial strains metabolize these dangerous compounds, thereby reducing their toxicity and improving the insect survival [142]. On the other hand, in _Dendroctous valens_ multifunctional pheromone verbenoe is synthesized by the gut bacterial community by biotransforming conifer monoterenes ingested by diet [23; 24], showing the promiscuous nature of these compounds. Other terpen act as hormones in plants, such as in the case of gibberellins, a class of diterpenes. In plants, gibberellins control development at different levels including seed germination, stem elongation, and flower induction. Plants, bacteria, and fungi synthesize these compounds, but the biosynthetic pathways are distinct in the different organisms. Plants use two diterpene synthases to form _ent_-kaurene, the giberellin precursor, from geranylgeranyl diphosphate (GGPP), while fungi use only a single bifunctional diterpene synthase. In both plants and fungi, _ent_-kaurene is then oxidized to _ent_-kaureneic acid by cytochromes P450. _ent_-kaureneic acid is further metabolized by different sets of enzymes in plants (sequentially: Two cytochromes P450, two 2-oxoglutarate dependent dioxygenases) and fungi (sequentially: Two cytochromes P450, desaturase, cytochrome P450) to obtain the final product, gibberellin A (GA\({}_{3}\)) [143]. Bacteria have evolved a distinct pathway, as shown in _Bradyrhizobium japonicumum_, which contains an operon consisting of genes coding for a ferredoxin, a short-chain alcohol dehydrogenase, three cytochrome P450, a GGPP synthase, and two diterpene synthases (_ent_-copalyl diphosphate synthase and _ent_-kaurene synthase), which control giberellin biosynthesis. To synthesize terpen and terpenoids, two interconvertible building blocks are required: Isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). Biosynthesis of these precursors involves two pathways, i.e., the mevalonate pathway (MVA) and the non-mevalonate pathway, also known as 2-C-methyl-d-erythrol 4-phosphate (MEP) pathway [144; 145]. The MVA pathway is considered as the fundamental pathway for isoprenoid biosynthesis in archaea and eukaryotes, although it seems to be present also in several bacteria. There are three hypotheses about the evolution of MVA pathway [145]. The first hypothesis contends that an ancestral pathway appeared in a common ancestor of eukaryotes and archaea, and it was recently acquired by few bacteria by horizontal genetic transfer (HGT) from a eukaryotic or archaeal donor. The second one proposes a similar model in which the HGT occurred in a common ancestor of a group of modern-day bacteria. The third hypothesis proposes that the MVA pathway was ancestral not only to archaea and eukaryotes, but also to bacteria. However, it was lost by most of bacteria. In this scenario, the last universal common ancestor life (LUCA) possessed the MVA pathway [145]. A number of Actinobacteria, Bacterioidetes, Firmicutes, Chloroflexi, Proteobacteria, and Spirochetes genomes contain homologs of MVA pathway genes, but not the entire classic pathway [145].