How a molecular 'alarm' system protects plants from predators

In nature, every species must be equipped with a strategy to survive in response to danger. Plants, too, have innate systems that are triggered in response to a particular threat, such as insects feeding on them. For example, some plants sense herbivore-derived danger signals (HDS), which are specific chemicals in oral secretions of insects. This activates a cascade of events in the plant's defense machinery, which leads to the plant developing resistance to (or immunity against) the predator. But despite decades of research, exactly how plants recognize these signals has remained a mystery.

In a new study published in Communications Biology, a research team from Tokyo University of Science, Ehime University, Okayama University, the University of Tokyo, and Iwate Biotechnology Research Center, led by Prof Gen-ichiro Arimura, attempts to shed light on exactly how plant HDS systems work. They chose to study membrane proteins called receptor-like kinases (RLKs), which are found in soybean leaves. They based their study on previous evidence from plants like Arabidopsis, tobacco and cowpea, in which RLKs play a major role in HDS systems. Prof Arimura says, "Scientists have been trying to understand the molecular mechanism of plant resistance for years, but the 'sensors' involved in plant recognition of insect pests are still not known. Thus, we wanted to get a detailed understanding of these mechanisms."

To begin with, the scientists focused on soybean RLK genes that were structurally and functionally similar to a RLK gene, which is known to trigger a danger response by recognizing oligosaccharides (small carbohydrate molecules) during pathogen attack. They speculated that owing to these similarities, soybean genes might also show a mechanism similar to that seen in pathogen resistance. They found 15 such genes through genetic analysis. Next, the scientists generated 15 types of Arabidopsis plants, each plant uniquely expressing only one of the 15 individual soy genes. When they tested these plants using oral secretions from the pest, they uncovered genes for two novel RLKs that showed a defense response specific to the oral secretions called GmHAK1 and GmHAK2. These findings were unprecedented: The role of these RLKs in soybean HDS systems had never been revealed before. Moreover, when the scientists dug deeper into the mechanism of these regulatory factors in Arabidopsis, they found two proteins, a HAK homolog and PBL27 (which play a role in intracellular signaling), to be involved in this pathway. Accordingly, this confirmed what the scientists had initially expected—soybean and Arabidopsis possess similar mechanisms for "danger response."

In agriculture, it is crucial to develop strategies for pest control in crop plants to avoid incurring losses. This study takes a massive step in this direction by uncovering an important cellular mechanism that triggers defense response in plants. Manipulating this innate cellular system may even help scientists to fuel the development of new agricultural products, potentially making life easier for farmers. Prof Arimura concludes, "It has been challenging to find new pest control methods that are effective and do not harm the ecosystem in any way. Our study offers a potential solution to this problem by uncovering the details of how certain plants develop resistance."