A battlefield in every lettuce leaf

A lettuce plant may look defenceless, but every leaf is the scene of a constant battle. Fungi, bacteria and other pathogens, including downy mildew, are always searching for ways to invade and eventually cause disease. At the same time, the plant is trying to detect them before it is too late. Biologists at Utrecht University have now uncovered how lettuce does exactly that. Their discovery, published in the journal PNAS, sheds new light on the remarkable immune system of plants and could eventually contribute to the development of stronger, more resilient crops.

A lettuce plant cannot run away from danger. It cannot hide from disease-causing microbes or chase away harmful insects. Yet plants have survived for hundreds of millions of years in a world full of enemies. How?

“That’s one of the most fascinating questions in plant biology,” says Guido Van den Ackerveken, Professor of Translational Plant Biology at Utrecht University. “Plants may appear passive, but in reality they are constantly monitoring their surroundings.”

In a study published recently in the scientific journal PNAS, researchers from Utrecht University reveal how lettuce recognizes an important warning signal produced by plant pathogens.

The study was led by PhD candidate Iñigo Bañales together with Guido Van den Ackerveken and Dmitry Lapin. Besides providing fundamental new insights into the evolution of plant immunity, the discovery could eventually help develop crops that are naturally more resistant to disease.

A smoke detector for pathogens

Like humans, plants possess an innate immune system that can recognize invading organisms. Covering the surface of every plant cell are thousands of receptors: molecular sensors that constantly scan for molecular patterns of bacteria, fungi, oomycetes (such as downy mildew) and other pathogens.

“You can think of such a receptor as a smoke detector,” says Van den Ackerveken. “It doesn’t detect the fire itself, but a signal that something is wrong. As soon as the receptor is triggered, the plant's immune system switches on.”

The Utrecht researchers wanted to understand how this alarm system works in lettuce.

Screening two hundred lettuce varieties

Their search capitalises on a collection of nearly two hundred different lettuce varieties. Bañales and his colleagues tested how each responded to the nlp24 peptide, a tiny fragment of a protein that many pathogens produce during their attack on plants. Some varieties immediately activated their defence system, while others showed no response.

That difference turned out to be the crucial clue. By comparing DNA of the different lettuce varieties, the researchers identified a single gene responsible for recognizing nlp24. The gene encodes a receptor on the surface of plant cells, which they named LNR (Lettuce nlp24 Receptor).

After screening nearly two hundred lettuce varieties, the final piece of the puzzle fell into place.

To confirm their discovery, the researchers introduced the gene into a lettuce plant that naturally lacked the receptor. The result was exactly what they had hoped for: the plant became capable of recognizing the pathogen signal.

“That was the decisive proof that we had found the right receptor,” says Van den Ackerveken.

Evolution invented the same solution twice

The next discovery was perhaps even more surprising. Biologists already knew that the much-studied plant Arabidopsis thaliana can detect the same pathogen signal. However, the Utrecht team discovered that lettuce uses an entirely different type of receptor to achieve the same result. 

Evolution came up with two different ways to defend against proteins that are essential for pathogens. This highlights just how important it is for plants to recognize them.

In other words, two distantly related plant species (whose last common ancestor lived more than 100 million years ago) independently evolved different solutions to the same biological challenge.

“In a sense, evolution came up with a similar invention twice, but in two completely different ways,” says Van den Ackerveken. “That shows just how important the recognition of these NLP proteins must be.”

For evolutionary biologists, the finding provides a fascinating glimpse into the long-running arms race between plants and the pathogens that infect them.

Targeting the pathogen’s weak spot

The researchers then took a closer look at exactly what the lettuce receptor recognises.

It turned out to be a very small part of the pathogen protein: a chain of just seven amino acids known as a heptapeptide motif. This tiny fragment is essential for the pathogen to infect plants.

“If pathogens wanted to escape detection by changing this part of the protein, they would also damage one of their own key weapons,” Van den Ackerveken explains. “In other words, they can’t simply change it without losing their ability to infect the plant.”

That makes the newly discovered receptor especially interesting. It targets a part of the pathogen that evolution has left with very little room to change.

Stronger crops start with fundamental research

Does this mean we will soon see disease-resistant lettuce in supermarkets? Not quite.

According to Van den Ackerveken, this discovery should not be seen as a silver bullet. “It’s like securing a house,’ he says. “One lock helps, but multiple locks combined with a good alarm system make it much harder for burglars to get in. Plants work in much the same way. By combining different defence mechanisms, you make it much more difficult for pathogens to invade and cause disease.”

The receptors discovered in lettuce could one day be used to strengthen the natural immune systems of many other crops.

Nor are the researchers focusing solely on lettuce. They have already demonstrated that the receptor can function in other crops, including potato. This suggests that similar receptors could likely be used to strengthen the natural immunity of many economically important crop species.

To help bridge the gap between fundamental research and agricultural applications, the researchers have filed a patent covering the receptor’s use. Together with industry partners, they are now exploring how the discovery can be developed further.

Unlocking nature’s own defences

For the researchers, their publication in PNAS marks only the beginning of a much larger story.

Every plant species possesses its own collection of immune receptors, many of which are still waiting to be discovered. Each could contribute to developing crops that rely more on their own natural defence mechanisms.

perhaps the most beautiful aspect of this discovery is that it reminds us how incredibly sophisticated plants really are.

“The better we understand how plants recognize pathogens, the better we will be able to harness those natural defence systems,” says Van den Ackerveken. “But perhaps the most beautiful aspect of this discovery is that it reminds us how incredibly sophisticated plants really are. They may seem silent and vulnerable, yet at the molecular level they are constantly monitoring their surroundings and defending themselves.”