Study reveals how pollinators evade plant toxins

New research is shedding light on the remarkable ability of pollinators such as honeybees to detoxify defense chemicals produced by plants. 

Scientists from the University of Exeter and Bayer AG have discovered that these insects, which belong to the Hymenoptera order, have a unique set of enzymes allowing them to break down harmful alkaloid toxins found in plant nectar and pollen. This critical trait has been preserved across nearly 300 million years of evolution and is shared by various species within this order, including bees, wasps, ants, and sawflies.

Alkaloids are chemical compounds that many plants produce as a defense mechanism against herbivores. However, these toxins can also be found in the nectar and pollen that pollinators rely on for nourishment. 

To better understand how these insects can tolerate such substances, the researchers examined the genes of several hymenopteran species. They found that all of the tested species produce the same group of enzymes, known as the CYP336 family of cytochrome P450 enzymes, which helps them tackle alkaloid toxins.

Dr. Angie Hayward, from Exeter’s Penryn Campus in Cornwall, explained the significance of this discovery: “These species differ greatly, but one thing they share is this ability to detoxify alkaloids. We were fascinated to discover this family of genes has been preserved across almost 300 million years of evolution by a whole order of insects with very diverse lifestyles.”

Interestingly, the research also revealed that even species with minimal contact with certain key alkaloids, such as nicotine, have retained the ability to metabolize them. Dr. Hayward compared this to the human tailbone or appendix, which are remnants of our evolutionary past.

To further investigate the enzyme’s capabilities, the researchers extracted the enzymes produced by the hymenopteran species and placed them in a cell-line to observe their reaction with alkaloids. The results confirmed that these enzymes do indeed detoxify the toxins.

Dr. Bartek Troczka, also from the University of Exeter, emphasized the importance of understanding how insects react to specific toxins: “Understanding how insects react to specific toxins is vital – it should inform how we produce any new chemicals such as pesticides and insecticides. To avoid environmental damage, we need very specific compounds that do very specific things.”

This study contributes to the broader attempt to understand how chemicals are broken down by insects and the extent to which the genes responsible for this process persist across insect groups. 

Dr. Julian Haas, insect toxicologist at Bayer AG, praised the multidisciplinary nature of the research, stating that it “highlights the promise of multidisciplinary teamwork to better understand the molecular and evolutionary basis of detoxification mechanisms in insects, which will ultimately aid with the understanding of their interaction with other toxins, including insecticides.”

The study received funding from the Biotechnology and Biological Sciences Research Council (BBSRC) and Bayer AG.

Types of plant toxins

Plants have developed various types of toxins as defense mechanisms against herbivores and pathogens. Some of the most common types of plant toxins include:

1. Alkaloids: These are nitrogen-containing compounds found in various plants, such as nightshade, poppy, and tobacco. Examples of alkaloids include nicotine, caffeine, morphine, and atropine. Alkaloids often have pharmacological effects on humans and animals, which may be toxic at high concentrations.
2. Glycosides: These are compounds that consist of a sugar molecule bonded to another functional group, often with toxic properties. Examples of toxic glycosides include cyanogenic glycosides, which release toxic hydrogen cyanide when broken down, and cardiac glycosides, which can have toxic effects on the heart.
3. Terpenoids: These are a large and diverse group of compounds derived from isoprene units. Some terpenoids, such as pyrethrins found in chrysanthemum flowers, have insecticidal properties, while others like limonene, found in citrus fruits, have insect-repellent qualities.
4. Phenolics: These compounds are derived from phenol and include a wide range of chemical structures, such as flavonoids, tannins, and lignin. Phenolic compounds can have various toxic effects on herbivores, including reduced digestion and absorption, inhibited enzyme activity, and oxidative stress.
5. Protease inhibitors: These are proteins that interfere with the digestive enzymes of herbivores, making it difficult for them to digest plant material effectively. Protease inhibitors can lead to reduced growth rates and nutrient absorption in herbivores.
6. Lectins: These are carbohydrate-binding proteins that can disrupt the metabolism and digestive processes of herbivores by binding to their gut lining, causing nutrient malabsorption and potential cell damage.
7. Oxalates: These are compounds that can form insoluble crystals in the digestive system of herbivores, leading to irritation and potential damage to tissues.

It is essential to note that the toxicity of these compounds may vary significantly depending on the concentration, plant species, and the animal or organism consuming the plant. Many of these compounds also have beneficial properties at lower concentrations or specific contexts, such as medicinal applications or antioxidants.

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