These toxin delivery systems are fully analogous to and often rival the complexity of venom delivery systems found in animals such as venomous snakes, scorpions and spiders.
Four representative plant species showcasing venom delivery systems: (A) co-habiting ants that numerous ant-plants provide a home and food for; (B) haustria of parasitic plants that attack other plants; (C) stinging trichomes of stinging plants; and (D) raphides that penetrate the oral membranes of animals that browse on plants. Image credit: Hayes et al., doi: 10.3390/toxins17030099.
Toxinologists and other biologists have studied toxic organisms and their secretions for centuries.
Their interest has stemmed in large part from the frequently severe consequences resulting from human exposure.
Humans have also tapped the potential of toxins to explore treatments for human ailments and diseases.
In doing so, scientists have leveraged countless natural experiments involving interactions of the toxins with targeted cells and tissues.
The classification of biological toxins, in particular the distinction between poison and venom, features a colorful and sometimes contentious history.
Nevertheless, with a consensus opinion and the introduction of a third term, toxic biological secretions can be classified into three groups based on the mode of delivery to other organisms.
These include poisons transferred passively without a delivery mechanism (e.g., ingestion, inhalation, or absorption across the surface); toxungens delivered to the body surface without an accompanying wound (e.g., spitting, spraying, or smearing); and venoms conveyed to internal tissues via the creation of a wound (e.g., stinging, biting).
Organisms that possess these toxins are referred to as poisonous, toxungenous, and/or venomous, respectively.
These distinctions provide a meaningful framework for studying the evolution of these toxins, including their biochemical structure; associated structures for their synthesis, storage, and application; and their functional roles.
Discourse regarding venoms and venomous animals has focused almost exclusively on animals.
Venom usage has evolved independently in at least 104 lineages among at least eight animal phyla, which highlights the remarkable adaptive utility of the trait.
But do venom delivery systems exist in other entities?
“Our findings show that reliance on venom for solving problems like predation, defense, and competition is far more widespread than previously recognized,” said Professor William Hayes, a researcher at Loma Linda University.
“Venomous animals have long fascinated biologists that were seeking to understand their deadly secretions and the traits associated with their use, but have also contributed to numerous life-saving therapeutics.”
“Until now, our understanding of venom, venom delivery systems, and venomous organisms has been based entirely on animals, which represents only a tiny fraction of the organisms from which we could search for meaningful tools and cures.”
According to the study, plants inject toxins into animals through spines, thorns, and stinging hairs, and some also co-exist with stinging ants by providing living spaces and food in exchange for protection.
Even bacteria and viruses have evolved mechanisms, like secretion systems or contractile injection systems, to introduce toxins into their targets through host cells and wounds.
“I have a long history of researching venom in rattlesnakes, and began exploring a broader definition of venom over a decade ago while teaching special courses on the biology of venom,” Professor Hayes said.
“As my team and I were working on a paper to define what venom truly is, we found themselves encountering non-animal examples and decided to dig deeper to identify numerous examples that may have been overlooked.”
The study paves the way for new discoveries, and the authors hope it will encourage collaboration among specialists and scientists across disciplines to further explore how venom has evolved across diverse organisms.
“We’ve only scratched the surface in understanding the evolutionary pathways of venom divergence, which include gene duplication, co-option of existing genes, and natural selection,” Professor Hayes concluded.
The study was published in the journal Toxins.
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William K. Hayes et al. 2025. It’s a Small World After All: The Remarkable but Overlooked Diversity of Venomous Organisms, with Candidates Among Plants, Fungi, Protists, Bacteria, and Viruses. Toxins 17 (3): 99; doi: 10.3390/toxins17030099
