Amazing Mint and its Interesting Evolution         

By Dr. DorothyBelle Poli

Professor of Biology at Roanoke College, Certified Aromatherapist

When someone says the word “mint” to me, the first thing that pops into my head is the fact that the mint species has square stems, and they smell like menthol. But the scientist in me immediately questions if this is actually accurate. To me, the mint species are herbs which are defined as “a leafy and green part in non-woody plants of temperate climate zone.”1 From an aromatic perspective, herbs are more fragrant than spices as fresh plants. This isn’t a group that I am that familiar with and so in writing this paper, I had to dive deep into the literature to wrap my head around what “mint” really meant. Scouring that literature, it became obvious that mints were much more complicated than I gave them credit. They comprise the Mentha genus, and they belong to the Lamiaceae family. This family is the sixth largest family of angiosperms. I was in for some serious schooling by this group of plants!

Starting with the Basics: Let’s Meet the Family!

Medicinally, herbs are the stronger category than spices in numbers of species providing relief for a symptom. The Lamiaceae family contains some of the biggest and most influential herbs around. Basil (Ocimum spp.), rosemary (Salvia rosmarinus), and peppermint (Mentha × piperita) are family members used as flavor additives, but they also have strong aromatic properties. Add English lavender (Lavandula angustifolia) and sage (Salvia officinalis) to that aromatic family and you start to become impressed! There are numerous ornamental plants, too.

Fun Fact: You most likely know that the Lamiaceae family, as a whole, does not all have square stems. But the Mentha genus does! 

The Lamiaceae family has a varied and diverse secondary metabolite catalogue. Members possess antimicrobial, antifungal, anti-herbivory, and pollinator attracting compounds mainly in the monoterpene, sesquiterpenes, and iridoids (non-canonical monoterpenes) categories. These compounds have been useful for human health, food, and agricultural purposes.2 When we drill down to the specifics of the Mentha genus, we see similar trends.

Mint (Mentha spp.) plants have a long history as traditional medicine.3 For example, horsemint (Mentha longifolia), fresh or dried, is mainly used to treat indigestion, menstrual pain, coughs, asthma, fever, and headaches.4,5 The Mentha genus has “aerial parts that will produce large numbers of aroma chemicals like menthol, menthone, isomenthone, menthofuran, carvone, linalool, linalyl acetate, and piperitenone oxide which are useful in pharmaceutical, food, flavor, cosmetics, beverages and allied industries.”6 In an interesting twist, 13,000 distinct diterpenes have been found in plants, and about 3000 of those are in at least one species from the Lamiaceae family.7  And, making matters more complicated, the yield and essential oil composition are influenced by the interactions between the plant and its environment.8 But the interesting part, to me, is that these chemicals all derive from the same precursors. But what does this mean and what can we learn from it? Experts say that the evolution of chemical complexity “has been a major driver of plant diversification, with novel compounds serving as key innovations.”9 Unique and multiple mechanisms most likely contributed to the evolution of the chemistry found in the mint group and the larger Lamiaceae family.

Dissecting the Evolution of the Mint Chemistry

In the mid-1980s, scientists started to understand that mints were able to house bacteria in their roots.10 Quickly the understanding was that some of these bacteria were non-pathogenic and others were  – similar trends were being observed in other species.11,12 Throughout the investigations, some bacteria did not change anything with respect to growth, development, or chemical composition, but others could impact plant health and vigor. In these cases, the result was that plant harvest would be less than optimal. Economical loss caused us to focus on growing plants in tissue culture, a sterile and controlled situation rather than in soils where the plant would be exposed to bacteria. By the 1990s, the need to eradicate bacteria from mint (Mentha spp.) cultures became a real issue.13 Since some of these bacteria could remain undetected for some time, plants would be transplanted to a culture situation and the secretive bacteria inhabiting roots would cause the media to become a perfect place for bacteria proliferation and the plantlets wouldn’t grow.

But what is really occurring in these roots? What can we learn from these infections?

In the 1990s, we learned that mint (Mentha spp.) plants might develop microbial colonization through wounds and natural openings via the root systems.14 Some believed that these bacteria were crucial for metabolism but at the time there was no evidence for this. Yet microbes were thought to help with disease resistance to some plant pathogens. In other words, the plants would bring in some bacteria and the bacteria would help them fight disease. But other bacteria would cause disease.15 When mints (Mentha spp.) were examined, it was found that twenty-two bacteria colonized them, and most were Gram-negative in nature providing clues on how to eradicate them with antibiotics if desired.16 Interestingly, when mint (Mentha spp.) plants produce antibacterial components, they are reacting to the pressure from those negative microbe interactions.

Aromatic plants are often found growing in low fertility soils/environments.17 Arbuscular mycorrhizal fungi have been added to plants in agriculture to improve cultivation of many crops; in oregano (Origanum vulgare) and mint (Mentha spp.) plants supplemented with these fungi, each developed higher content of essential oils and nutrient elements.18 This suggests that the fungi may allow plant growth in low fertility soils.19 But one of the properties of mints (Mentha spp.) is their anti-fungal properties. If fungi help mint (Mentha spp.) plants, why would the plant produce anti-fungal chemistry? Like the bacterial examples, plants may try to kill off some disease-causing fungi while allowing symbiotic or potentially beneficial fungi to survive.

Heavy metal contamination of agricultural lands is becoming a serious threat to sustainable agroecosystems.20 Cadmium can be toxic to corn mint/wild mint/field mint (Mentha arvensis) by causing the plant to have retarded growth and photosynthetic capacity.21 However, another study using Arbuscular mycorrhizal fungi shows that it can aid peppermint (Mentha × piperita) to uptake metal from wastewater irrigated soils; specifically led, cadmium, iron, manganese, zinc, and copper are stored in the leaves without causing severe problems.22 While this is problematic for humans who want to use these plants for medicines, it does provide insight into the plants’ ability to survive low fertility soils and poor environments with the aid of beneficial fungi. An interesting side note is that mint (Mentha spp.) plants are so good at extracting iron and manganese, they are good choices to reclaim heavy metal contaminated soils.

Mint (Mentha spp.) plants are more hostile to animals, particularly insects. The species commonly known as pennyroyal (Mentha pulegium) and spearmint (Mentha spicata) were explored to see how their main components, pulegone, methone, and carvone faired as insecticidal and genotoxic activities in fruit flies (Drosophila melanogaster). Both plants possessed insecticidal ability, but spearmint (M. spicata) added the extra mutagenic ability on the genome of the fruitfly!23 Therefore, we should always be cautious of essential oil plant species that are able to mutate another organism’s genome.

In Conclusion

This old plant biologist took a journey to learn more about the mint (Mentha spp.) species. What seemed like a simple walk around teas, toothpaste, and gastrointestinal trauma, turned into a walk down a complicated evolutionary system of chemical changes pressured by microbes, fungi, and animals. Each change resulted in stronger ways to manage the plant environment which was usually poor soil and stress-filled environments (like lack of water and too many salts in the soils). The mints (Mentha spp.) are resilient and full of determination to survive. I cannot do anything but admire their tenacity and creativity in using what chemistry they had to make it work. Small changes to structures over many years created novel compounds perfect for a particular challenge. The result was over 3000 terpenes waiting for the aromatherapist to understand and use. Exploring how and why the plant uses them provides clues on how we should use them. I am never less than surprised by how a plant – an organism without a brain and relying on evolutionary mechanisms to impact its survival or defeat – seems to understand the world better than we do!   

About Dr. DorothyBelle Poli:

Dr. DorothyBelle (DB) Poli is a Full Professor of Biology at Roanoke College in Salem, Virginia, and a Research Associate at the Virginia Museum of Natural History. She is an evolutionary plant physiologist who explores how plant hormones influenced axis formation. DB received her Plant Biology PhD. from the University of Maryland in 2005. In addition, she is a certified Aromatherapist from the Aromahead Institute. She is the co-Director of the Dragon Research Collaborative, a transdisciplinary research group that explores how plant fossils may have contributed to the dragon folklore around the world.  You can contact Dr. Poli at poli@roanoke.edu

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