Catmint, or catnip, is a flowering plant in the mint family belonging to the genus Nepeta Lamiaceae that is well-known for its intoxicating effect on cats. This “feline” phenomenon is caused by the volatile compound nepetalactone.
Interestingly, the main function of nepetalactones in Nepeta is not to attract cats, but to protect plants against herbivorous insects by producing a defensive chemical that repels insects with efficiencies comparable to the synthetic repellant DEET.
Iridoids are present in many Lamiaceae species but were lost in the ancestor of the Nepetoideae, the sub-family containing Nepeta, and then somehow re-emerged in the evolution of the Nepeta lineage. Scientists are now trying to understand the chronology of events that led to the re-emergence of this major class of defensive compounds.
In a new study published in "Science Advances," an international team of researchers, including Michigan State University plant biologist Robin Buell, investigated how and why catmint makes nepetalactone and how the biosynthetic pathways for the formation of this unique chemical molecule have evolved.
The team, led by Sarah O’Connor, director of the Department of Natural Product Biosynthesis at the Max Planck Institute for Chemical Ecology in Jena, Germany, and including Pamela and Douglas Soltis at the University of Florida and Natalia Dudareva at Purdue University, began its study into these questions by sequencing the genome of catmint. What they found provided a significant clue to the production of nepetalactone in catmint.
“We discovered a suite of unusual enzymes that generate nepetalactone molecules,” said plant biologist Benjamin Lichman of the department of biology at the University of York and the study’s first author. “These enzymes are not found in any related plant species and have evolved uniquely in catmint. When we first saw the genome sequence of catmint, we realized that the important genes that we hypothesized were active in the formation of nepetalactone were next to each other in the genome. This allowed us to solve the problem more easily.”
The scientists next compared the genome of two catmint species that are both able to produce nepetalactone with the closely related medicinal plant, hyssop, which doesn’t produce nepetalactone or any other iridoids.
This comparative approach – the reconstruction of ancient genes and comprehensive phylogenetic analyses – enabled the researchers to understand the chronology of events that led to the emergence of nepetalactone biosynthesis and to determine the mechanisms for the loss and subsequent re-evolution of iridoid biosynthesis in catmint.
These new discoveries provide novel insights into the interplay between enzyme and genome evolution in the origins, loss and re-emergence of plant chemical diversity.
“Catmint provides a great model example for studying these processes,” said O’Connor, whose research focuses on the biosynthesis of plant metabolic products. “We are now trying to modify the chemicals present in the catmint plants to help us better understand all aspects of the pathway and the ecological functions of nepetalactone. This, in turn, can help us uncover the selective pressures that led to the loss and regaining of this pathway.”
The team is also looking at other Nepeta species that produce unusual iridoids.
“Plants are constantly evolving new chemistry,” O’Connor said. “With our research, we would like to get snapshots of this evolution in action.”
The research was supported by the NSF-funded Mint Genome Project led by Buell at Michigan State University.
“This project has been so exciting because we have unraveled how catnip evolved to produce a chemical that makes cats crazy,” said Buell, who is also an MSU AgBioResearch faculty member. “Now we need to find out why cats go crazy over catnip.”