A team of researchers from Tokyo University of Science, Ehime University, Okayama University, The University of Tokyo, and Iwate Biotechnology Research Center studied how some plants sense "herbivore-derived danger signals" (HDS). These signals are specific chemicals in oral secretions of insects which activates a cascade of events in the plant's defense machinery, leading to the development of plant resistance (or immunity against) to a predator. Despite decades of research, exactly how plants recognize these signals has remained a mystery.
Led by Prof Gen-ichiro Arimura, the research team studied membrane proteins called "receptor-like kinases" (RLKs) found in soybean leaves. They based their study on previous evidence from plants such as Arabidopsis, tobacco, and cowpea, in which RLKs play a major role in HDS systems. The team focused on soybean RLK genes that are similar in structure and function to an RLK gene known to trigger danger response by recognizing oligosaccharides during pathogen attacks. They speculated that owing to these similarities, soybean genes might also show a mechanism similar to that seen in pathogen resistance.
The researchers found 15 such genes, from which they generated 15 types of Arabidopsis plants, each plant uniquely expressing only one of the 15 individual soy genes. When these plants were tested 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. The team's findings are unprecedented as the role of these RLKs in soybean HDS systems had never been revealed before. Moreover, when the scientists looked further 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. This confirmed the scientists' initial expectation that soybean and Arabidopsis possess similar mechanisms for danger response.