Plants in the mint family are rich in chemical molecules that give distinctive scents and flavors to their leaves. The mint family (Lamiaceae) is the sixth largest family of flowering plants and includes many familiar species, apart from mint itself, such as basil, rosemary, peppermint, lavender, sage and coleus. The aromatic and flavorful compounds produced by these plants function to attract pollinators and to deter herbivores, and they have been essential in driving the diversification of this large group of plants.
Researchers from Michigan State University have been studying the great variety of metabolites found in mint plants for many years, and have recently focused on a group of molecules called terpenoids. These compounds are essential in protecting the plants from predators and pathogens, and are also common ingredients in green and sustainable agrochemicals, antioxidants, cosmetics and fragrances.
“People easily recognize members of the mint family for their specialized metabolites,” said Dr. James K. Billman, Jr. and Professor Björn Hamberger. “Metabolites are an efficient way for plants to defend themselves because they can’t run away.”
Professor Hamberger worked previously with Robin Buell, a former MSU genomics researcher, who sequenced the genomes from several different species in the mint family. This collaboration with Buell’s team led graduate students Abigail Bryson and Emily Lanier to discover how several genomes of the mint family have evolved over the past 60 million to 70 million years and how their chemistries have emerged.
“Over millions of years, plants have adapted and evolved for their particular niches where they thrive, and that means that these chemistries are diverse and have clearly adjusted to their environment,” said Professor Hamberger. “So, we try to identify and discover pathways to these specialized metabolites that plants make.”
According to the researchers, this family of plants have diversified the natural characteristics of their metabolites throughout their evolution, leading to the great variety of different chemical compounds found in today’s members of the group. In order to understand the mechanisms by which such diversity has evolved, the researchers took an interdisciplinary approach and analyzed the genetic and chemical pathways involved in terpenoid biosynthesis in different species in the mint family.
Bryson identified the genomic framework of terpenoid biosynthesis, and Lanier analyzed the chemical pathways. Together, Lanier and Bryson discovered something highly unusual in the beautyberry (Callicarpa americana) genome from the mint family. It has a large biosynthetic gene cluster (BGC), which is a group of genes, located close together in the genome, that are involved in the same metabolic pathways. Additionally, Bryson and Lanier found variants of this BGC in six other species in the mint family.
“We are learning that the physical location of genes within the genome is important,” Bryson said. “It can drive evolution of specialized metabolic pathways in the plant, leading to a vast diversity of interesting natural plant compounds.”
The BGC cluster of the beautyberry plant contains genes that encode two distinct terpenoid pathways. The team found these terpenoids accumulate in various parts of the plant, such as the leaves and roots, and may play distinct roles in helping the plant to adapt to its environment.
“It’s the same base molecule, but each species is making its own version and modifying it in different ways to fit their survival needs,” Lanier said.
Professor Hamberger describes it like a recipe that everyone has a copy of and changes to suit their requirements and preferences.
The researchers hope that a better understanding of the ways in which these compounds have been modified by different plant species will lead to potential future applications that could include new medicines and pesticides. We already use some of the compounds as flavor additives, fragrances, and anti-herbivory agents.
Previous research has led to unique medical uses for mint plants. For example, Indian Coleus can be used as a natural treatment for glaucoma and Texas sage is a natural antimicrobial that is effective against tuberculosis. The new molecular adaptations that have been found in the current study open the door for future applications of natural plant products from the mint family.
“Our team has been excited about the opportunities within the mint family,” said Professor Hamberger. “Those mint enzymes, as in the American beautyberry plant, give us the ability to make plant-natural products in the lab, including – hopefully in the future – natural good-smelling mosquito repellants.”
Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.