Plant parasite matchmakers

Next comes a tale of deception, seduction and sexual exploitation, all by a parasitic bacterium called phytoplasma. This infects plants, into which it secretes a protein called SAP54. This turns flowers into leaves, making the plant a better food source for leaf-hopper insects, which spread the bacteria when they come to feed. But, even more incredibly, it also makes the plant even more attractive to females, if there are males present, further reinforcing the population of vectors to spread the infection! As Zigmunds Orlovskis, who’s from the Latvian Biomedical Research and Study Centre and unpicked all this, puts it, it’s effectively operating like a molecular matchmaker…

Zigmunds – Phytoplasmas are parasitic bacteria that infect plants. They are transmitted by insects—so when an insect feeds on a plant, it picks up the bacterium and transmits it to another plant. It's very similar to how malaria is transmitted by mosquitoes.

One of the things this bacterium does is change plant development—specifically, it turns flowers into leaf-like structures. You might wonder why it does that. One hypothesis we had was that these manipulated, leaf-like structures might be more attractive to the insect vectors. So we tested that, and it turned out that those structures aren’t necessary for the increased insect attraction.

Chris – If the plant no longer produces flowers and only makes more leaves, does that render it sterile? And is it possible the bacterium does this just to make the plant grow itself to death—producing loads of tissue for the parasites to feed on?

Zigmunds – Yes, that’s an obvious question—and it puzzled us too. The plant does indeed become sterile and can no longer reproduce. So then, why would the parasite do that—why kill its own host—unless it helps facilitate its own spread before the plant senesces and dies?

Chris – Do you think that’s what’s happening—that the bacterium makes the plant sterile so it becomes a bacterial factory, feeding more insect vectors and ensuring wider spread?

Zigmunds – Yes, that’s one potential explanation.

Chris – And have you figured out how it’s doing that? Do you know what signal the bacteria release that causes the plant to behave this way?

Zigmunds – The bacterium secretes a specific protein into the plant. This protein not only alters floral biology but also affects the leaves, making the plant more attractive to the insect vectors. It does this in a peculiar way—it mainly biases female insects towards plants producing this bacterial protein, compared to those that don’t.

Chris – The protein itself—perhaps you could tell us its name in a moment—but to summarise: the bacterium makes a protein that alters plant development and makes it more appealing to female insect vectors, helping spread the infection?

Zigmunds – Yes. Curiously, the bacterium also requires the plant to be exposed to male insects and for the protein to be present, in order for the host to become more attractive to females for reproduction.

Chris – It's incredible how intertwined it is—it’s integrating all these signals. But how does the bacterium know male insects are present?

Zigmunds – Male and female insects produce different elicitors, and the plant mounts different responses to each. Both feed on the plant, but only females lay eggs. It’s likely that the egg-laying stimulus creates additional responses that males don’t. When we compare male- and female-exposed plants that produce the SAP-54 protein, we find that male exposure is required for those plants to be more attractive to females.
Chris – How did this evolve in the first place? It’s such a complex system—a bacterial parasite secreting an effector that manipulates plant and insect behaviours in very precise ways. It’s almost unbelievable.

Zigmunds – It is a remarkable adaptation. Work from other labs, including Philippe Raymond’s group in Switzerland, has shown that insect eggs contain elicitors in their shells that trigger plant responses similar to bacterial infections. Plants have evolved to detect and react to these natural stimuli. This bacterium appears to hijack the insect sex-specific responses to manipulate plants for its own spread.

Chris – And it’s also manipulating the insects by luring in mates. It's like a dating app for vectors, drawing them in when there's most to gain from them mating.

Zigmunds – Yes, that’s why we think of this system as a kind of molecular matchmaker—making male-exposed plants more attractive to females. It’s a fascinating phenomenon.