The clone wars on plant planet

Why are aphids among the worst enemies of our gardens? 

On the battlefield, what qualities define a formidable enemy? Is it the ability to fend off attacks, rapidly multiply one's forces, conquer new territories, or ally with more powerful adversaries? Don't be deceived by small sizes: when it comes to plants, tiny aphids are among the worst foes, embodying all these strategic traits – and more.

• Evasion tactics: Aphids can weaken plant defenses with proteins contained in their saliva, enhancing their feeding and reproductive success.
• Rapid reproduction: With their prolific reproduction and quick life cycle, a population counting a few aphids can explode into hundreds or thousands in just weeks.
• Strategic dispersal: Under stress, winged aphids appear, and they can spread over long distances and colonize new plants.

• Defensive alliances: The honeydew of some aphids can attract ants, which, in turn, protect them from predators and parasitoids. Read more about this in our article “Two-faced ants“.
• Disease transmission: Aphids are key vectors* of plant viruses. While non-winged aphids can effectively transmit viruses to nearby plants, winged aphids are capable of dispersing viruses across long distances.
• Secret weapons: Aphids can harbor specific bacteria that, in exchange for shelter and nutrients, can help to weaken plant defenses, to transmit viruses more efficiently or to survive attack of other enemies, such as parasitoid wasps. 

The aphid’s unseen allies

Within each aphid lurks a multitude of microscopic allies – endobacteria. Every aphid is mandatorily associated with a bacterial species, typically Buchnera aphidicola, which synthesizes essential amino acids that are scarce in the aphid's primary diet of plant sap. In return, aphids provide a safe home, including specialized cells called bacteriocytes, as well as a steady supply of nutrients to these tiny symbionts.

This relationship is the result of a long co-evolutionary process, so successful that aphids can also host other endobacteria for various purposes. These are often called "facultative endosymbionts" and may help aphids deal with plant defenses, stimulate immunity against pathogens and parasitoids, modulate traits such as color and the production of winged offspring, and even mediate aphid behaviors and interactions with many other organisms.

These hidden players are fundamental to aphid biology, showcasing the intricate effects of the microbial world on broader ecological processes.

It takes two to tango

My research explores how aphids’ unseen allies, the endosymbiotic bacteria, influence plant virus transmission. We discovered that specific facultative endobacteria (for example, Hamiltonella defensa) can influence aphid characteristics in ways that promote disease spread. For example, when aphids harboring this specific endobacterium are free of a plant virus, they prefer to feed on virus-infected plants, which leads to virus acquisition. But once they acquire the virus, these aphids’ preferences shift to healthy plants, a behavioral change that promotes virus spread. Additionally, these aphids can infect many more plants than aphids with other endobacteria, indicating that specific hidden partners play a major role in the aphids' abilities to sicken plants. In other words, it takes an aphid to transmit certain plant viruses, but aphids associated with specific bacteria can be much better at it!

This finding has led to many more questions that we are exploring about the fascinating effects of the microbial world on disease transmission: How do certain endobacteria enhance plant virus transmission? Can plants also counteract their own unseen allies? What about the effects on other organisms in the environment, such as parasitoids and predators? If aphids with specific endobacteria are better protected against natural enemies, can the virus help certain aphid-endobacteria associations become nearly invincible?

*Glossary

Endobacteria: Symbiotic bacteria that live inside another organism.

Endosymbiont: A symbiotic organism that lives inside the body or cells of another organism.

Symbiont: An organism that lives in close association with another organism of a different species (symbiosis), which can result in mutual benefits, neutral effects, or negative effects on one or both symbionts.

Vector: An organism that carries and transmits an infectious pathogen to another living organism.