Venom is everywhere: Study examines hidden toxin delivery systems across life forms

By Tarun Sai LomteReviewed by Susha Cheriyedath, M.Sc.Feb 23 2025

Venom is not exclusive to animals—plants, fungi, protists, bacteria, and even viruses use venom-like mechanisms for defense, predation, and competition.

Dodder - Cuscuta on a potato bush. Review Article: It’s a Small World After All: The Remarkable but Overlooked Diversity of Venomous Organisms, with Candidates Among Plants, Fungi, Protists, Bacteria, and Viruses. Image Credit: EVGEIIA / Shutterstock

A recent review article published in the journal Toxins explored the diversity of venom delivery systems among plants, animals, fungi, protists, viruses, and bacteria.

Biologists and toxinologists have studied toxins and toxic organisms for centuries. Interest in toxins has been mainly due to the severe consequences of toxin exposure. Toxic biological secretions are classified into three classes based on their delivery mode: poisons, toxungens, and venoms. Poisons are passively transferred without delivery mechanisms.

Venoms are transferred to internal tissues through wounds, and toxungens are delivered to the body’s surface without wounds. Discourse on venoms and venomous organisms has primarily focused on animals. However, recent research suggests that venom-like mechanisms are much more widespread, evolving independently in multiple lineages beyond the animal kingdom. As such, the present study explored the diversity of venom delivery systems across biological entities and proposed a broader definition of venom that accounts for these overlooked systems.

Characteristics of venom delivery systems, venomous organisms, and venoms

All venomous animals share a common feature of injecting a toxic secretion into another organism via a wound. Nevertheless, storage, acquisition, function, and delivery of the secretion are markedly heterogeneous among animals. Autoglandular venomous animals produce and store toxins within their glands, e.g., scorpions. By contrast, autoaglandular venomous animals produce toxins but lack storage glands, e.g., jellyfish, anemones, and corals.

Besides, heteroaglandular venomous organisms acquire toxins from others and lack storage glands, whereas heteroglandular venomous animals have glands to store exogenous toxins. Venoms often consist of numerous toxins that act synergistically or individually. Venoms also serve other purposes besides defense and predation, such as offspring care, mating, intraspecific competition and communication, ectoparasite deterrence, and habitat creation. Similar principles apply to venomous organisms beyond animals, with plants, fungi, protists, and even bacteria showing comparable functional adaptations.

Four representative plant species showcasing proposed venom delivery systems. (A) Acacia (Vachellia) cornigera. Inset shows specializations for hosting colonies of symbiotic ants, including domatia (living quarters for the ants), extrafloral nectaries (nectar-producing glands), and Beltian bodies (providing food rich in lipids, sugars, and proteins and often red in color). The ants are venomous and protect the plant from herbivores, providing an effective defense analogous to that of facultatively venomous animals which co-opt the toxins of others. (B) Viscum album. Inset shows a cross-section of the specialized haustorium root structure invading the host plant’s vascular cambium. The haustorium secretes enzymes that degrade the protective bark layer and stimulate growth of new xylem tissue to connect with the parasite’s own vasculature. (C) Urtica dioica. Inset shows stinging trichomes, which comprise hollow, hypodermic needle-like structures which penetrate and break off in an animal’s skin upon physical contact, releasing irritating toxins. (D) Dieffenbachia sp. Inset portrays specialized calcium oxalate crystals (raphides) which penetrate the mucous membranes of animals that feed on the plant, causing irritation and potentially introducing proteolytic enzymes or pathogenic bacteria and fungi. Artwork: M. Benjamin Streit.

Plants

Plants are mainly autotrophic and do not usually capture prey; as such, they primarily employ toxins for defense. Plants from several families have pointed, hair-like structures known as stinging trichomes or hairs to protect against herbivores. There are two types of stinging trichomes: Urtica-type and Tragia-type. The Urtica-type stinging trichomes expel only liquid via the hypodermic syringe mechanism. The Tragia-type stinging trichomes expel a sharp crystal along with a liquid.

When herbivores brush against these plants, these hairs penetrate their skin and release toxins, leading to immediate, intense pain, partly due to the variable presence of neurotransmitters. Besides, inorganic potassium salts may further aggravate pain. These effectors quickly cause itching, burning, arterial dilation, tingling, burning, local sweating, rash, and neuropathy. While the intense pain peaks after 20–30 minutes, some symptoms may linger for days or months.

Trees from the Dendrocnide genus contain neurotoxins and other effectors, which are implicated in the deaths of dogs, livestock, and humans. Further, different plants inject pathogenic fungi and bacteria into herbivores via spines, prickles, and thorns. Besides, many plants have pointed, microscopic structures called raphides and silica needles within their tissues.

While toxins induce immediate pain, the pathophysiology of injected pathogenic microbes requires more time to develop. Furthermore, certain plants engage in a unique form of venom defense by forming mutualistic relationships with venomous ants. A large group of plants from 50 families coexist with stinging ant colonies. These ants provide effective venomous defense for the plants. These plants, called myrmecophytes, provide living quarters and food rewards to attract and sustain the ants. This arrangement functions similarly to heteroaglandular venomous animals that acquire toxins from other organisms for self-defense.

Additionally, some parasitic plants, such as Cuscuta (dodder), inject enzymes through specialized structures called haustoria. These enzymes break down the host plant’s defenses, facilitating nutrient extraction. This mechanism mirrors venomous interactions, as the host experiences biochemical disruption through an injected secretion.

Fungi and protists

Some fungi are parasites that cause pathogenesis, while others are predators. As such, many fungi produce toxins to support these forms of nutrition. Many toxins comprise non-volatile secondary metabolites that are toxic when ingested, while other toxins are delivered via hyphae into hosts for defensive and nutritional purposes.

Phytopathogenic fungi use specialized non-hyphal cells called appressoria to penetrate plant cuticles via turgor pressure and secrete toxins. The toxin destroys plant cells, and the fungi derive nutrition from dead cells to protect themselves from host defenses. Further, entomopathogenic fungi use adhesives, cuticle-degrading enzymes, or appressoria to penetrate insect cuticles and deliver toxins. Nematophagous fungi possess diverse capture and penetration devices to prey upon nematodes.

These (devices) include specialized hyphae that form adhesive knobs/networks, sticky balls, and non-constricting and constricting rings. Many of the nematophagous fungi synthesize toxins to kill or paralyze nematodes. These mechanisms closely resemble venom injection strategies observed in animals.

Protists are eukaryotic microorganisms that thrive in almost all environments. Most predatory forms of protists acquire nutrition using mechanisms resembling those of venomous animals. For example, some protists, such as Coleps, use toxicysts—specialized organelles that function like harpoon-like stylets—to inject toxins into prey.

Extrusomes are membrane-bound, ejectable bodies present in the cell cortex of protists. They generally serve predatory and defensive functions and are positioned near the oral apparatus, where they make initial contact with prey and discharge toxins within milliseconds. These toxins paralyze or kill the target. While the composition of toxins remains poorly defined due to challenges with acquisition and purification, acid phosphatase has been identified in some.

Two proposed venom delivery systems in unicellular eukaryotes. Both are offensive extrusomes that discharge their contents outside of the cell. (A) A group of ciliates (Coleps) attacking a Paramecium using venom. Inset shows the toxicysts, specialized organelles that penetrate the cell membrane of the target and deliver toxins. (B) The dinoflagellate Polykrikos displaying a discharged nematocyst, a harpoon-like organelle that potentially delivers venom into target prey and structurally resembles the nematocysts of venomous animals in the phylum Cnidaria (e.g., anemones, corals, jellyfishes). Artwork: M. Benjamin Streit.

Bacteria and viruses

Predatory bacteria occur in five phyla and 15 families. Bacteria cause pathogenic conditions by releasing substrates that enter host cells and modulate host biological processes. Some bacteria, such as Pseudomonas aeruginosa, engage in "chemical warfare" using venom-like Type VI secretion systems (T6SS), injecting toxins directly into competing microbes.

At least 10 secretion systems and 350 effector proteins have been described in bacteria. The effector proteins can alter almost all physiological aspects of host cells, often leading to devastating diseases in animals, plants, and humans.

Further, viruses invade host cells by penetrating their membranes via endocytosis, fusion, or pore formation. Plant viruses, unable to penetrate host cell walls, manipulate insects to inject them. While these mechanisms are not reminiscent of venom delivery, one cell entry mechanism, exhibited by bacteriophages, requires the virus to attach to the host cell surface and inject its genome and virion proteins using a hypodermic needle-like apparatus. This system is structurally and functionally analogous to venom injection in animals.

Concluding remarks

Taken together, many non-animal venom delivery systems have a functional role in predation, defense, competition, or reproduction. The study proposes that venom, traditionally viewed as an animal adaptation, is far more ancient and widespread. There is overwhelming evidence to support that many plants, protists, bacteria, viruses, and fungi possess toxin delivery systems analogous to animal venom delivery systems. This challenges conventional definitions of venom and highlights a previously overlooked aspect of evolutionary biology.

This new diversity of venomous organisms will take some time to be adopted. Overall, animals have much in common with other forms of life in terms of reliance on toxic secretions for defense, competition, and predation.