Terpenoids play vital roles in plant ecology—from repelling pests to attracting pollinators—and are produced by terpene synthases (TPSs) enzymes. These enzymes are unique for their fused domain structure, housing both Class I and Class II catalytic activities. Scientists have long hypothesized that this unusual architecture originated from a fused bacterial gene, but no such example had ever been found. This absence cast doubt on the bacterial origin theory and left a major evolutionary question unanswered. Due to these gaps in knowledge, further exploration into microbial genomes was essential to uncover whether the plant TPS gene family truly traces back to bacterial ancestors.
In a paper (DOI: 10.1093/hr/uhae221) published on August 3, 2024, in Horticulture Research, researchers from the University of Tennessee and Iowa State University report the discovery of five bifunctional diterpene cyclase/synthase (DCS) genes in bacteria. These bacterial enzymes mirror the structure and function of plant TPSs, and one, in particular, produces ent-kaurene—crucial for plant hormone synthesis. This finding supports a single gene fusion event in bacteria that may have been horizontally transferred into early land plants, shedding light on the evolutionary leap that led to one of the most versatile enzyme families in plant biology.
To uncover evidence of bacterial TPS-like genes, the team screened over 15,000 bacterial genomes and identified five genes with a fused tridomain structure—combining Class I and Class II activities. These genes, named DCSs, were from diverse bacterial phyla and shared domain architecture with plant TPSs. The researchers expressed these genes in engineered E. coli, and three—CseDCS, ChjDCS, and StrDCS—showed full bifunctional activity. CseDCS, from Candidatus Sericytochromatia, was particularly notable for synthesizing ent-kaurene, a vital precursor in gibberellin hormone biosynthesis. When split, CseDCS retained full activity in its separated domains, mimicking the evolutionary pathway seen in plants. Detailed mutation analysis revealed that critical catalytic residues in CseDCS match those found in both plant and bacterial hormone-producing enzymes. Furthermore, phylogenetic analysis placed bacterial DCSs in a unique position between major clades of plant TPS genes, suggesting that early gene duplication and neofunctionalization events followed this bacterial fusion event. The absence of similar fused genes in algae or simpler plant lineages supports a single horizontal gene transfer as the origin of the entire TPS family in plants.
“This is a rare and exciting example of evolutionary archaeology,” said Dr. Reuben J Peters, co-senior author of the study. “We’ve been searching for years for evidence that could bridge the gap between bacterial metabolism and plant biosynthetic complexity. Finding a bacterial enzyme that mirrors the plant’s hormonal pathway so precisely—and can even be split like ancestral proteins—is like discovering the missing chapter in a book we thought we’d never finish. It reveals how a single gene may have seeded one of the most diverse chemical repertoires in the plant world.”
By identifying a bacterial origin for the TPS gene family, this study not only clarifies a long-standing mystery in plant evolution but also opens doors for applied science. These enzymes could serve as novel biotechnological tools for engineering microbial systems to produce valuable terpenoids used in medicine, agriculture, and fragrance industries. The study also reinforces the role of horizontal gene transfer in plant adaptation to land environments. As genome sequencing continues to expand across bacterial lineages, researchers anticipate finding more DCS variants, offering deeper insight into the evolutionary plasticity of biosynthetic enzymes and new resources for synthetic biology and natural product innovation.
###
References
DOI
10.1093/hr/uhae221
Original Source URL
https://doi.org/10.1093/hr/uhae221
Funding information
This work was supported by an ASAP-SPRINT award from the University of Tennessee, AgResearch (to F.C.) and a grant from the NIH (GM131885 to R.J.P.).
About Horticulture Research
Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2023. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.