How do plants defend themselves?

Plants use their immune system to defend against pathogens. In order to activate that immune system, they first need to identify the enemy. Understanding this is vital for future healthier crops.

But how do they activate this cellular defense system? The answer can result in genetically modified crops with an improved immune response and increased resistance to pathogens.

Progress is being made in that direction. Researchers at the Technical University of Munich in Germany have discovered that receptors in plant cells identify bacteria through simple molecular building blocks.

“The immune system of plants is more sophisticated than we thought,” said Stefanie Ranf of the university.

Her international research team of biochemists has discovered substances that activate plants’ defense systems.

Until this recent discovery, scientists had assumed that plant cells recognize bacteria through complex molecular compounds from the bacterial cell wall. Certain molecules composed of fat-like particles and sugar molecules called LPS were suspected of triggering the immune response.

In 2015, Ranf’s team successfully identified the respective receptor protein called LORE. All experiments indicated that LORE activates the plant cell’s immune system when it detects LPS molecules from the cell wall of certain bacteria.

“The surprise came when we wanted to study this receptor protein more closely,” Ranf said.

“Our goal was to find out how LORE distinguishes different LPS molecules. For this we needed high-purity LPS.”

The researchers found that only LPS samples with certain short fatty acid constituents triggered plant defense. They also found all these active LPS samples also contained extremely strong adhering free fatty acid molecules. Only after months of experimentation was the team able to separate these free fatty acids from the LPS.

“When we finally succeeded in producing highly purified LPS, it became apparent that the plant cell did not respond to them at all,” she said.

“Thus it was clear that the immune response is not triggered by LPS, but instead by these short fatty acids.”

The 3-hydroxy fatty acids are very simple chemical building blocks compared to the much larger LPS. They are indispensable for bacteria and are produced in large quantities for incorporation into diverse cellular components.

“The strategy of plant cells to identify bacteria through these basic building blocks is extremely sophisticated. The bacteria require these 3-hydroxy fatty acids and therefore cannot bypass the immune response.”

In the future, these results could help in breeding or genetically modifying plants with an improved immune response. The goal will be to breed plants with 3-hydroxy fatty acids to have increased resistance to pathogens.

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