Structural Insight Into Jasmonic Acid Signalling Repression by Insect HARP1 Effector.

Through long-term natural selection, a co-evolutionary relationship has formed between plants and pests. However, pathogens and pests can also undermine plant resistance by releasing certain substances such as effectors. Helicoverpa armigera R-like protein 1 (HARP1), an effector in oral secretions, is capable of interacting with JASMONATE-ZIM DOMAIN (JAZ) protein. This interaction inhibits the degradation of JAZ and prevents the activation of jasmonic acid (JA) signalling in response to biotic stress. Nevertheless, the mechanism by which HARP1 interacts with JAZ to suppress JA signalling remains elusive. In this study, we first confirm that the ZIM domain within JAZ is sufficient for the HARP1-JAZ interaction. To gain mechanistic insight, we determined the crystal structure of HARP1 and utilised AlphaFold2 to predict its binding mode with JAZ3. The structure analysis reveals that HARP1 is a β-sandwich fold composed of seven strands, which directly binds to JAZ homo- or hetero-dimers. This binding prevents the degradation of the JAZ repressor, consequently ensuring the repressed JA signalling pathway in the plant. Our structural and functional studies provide new insights into the JA signalling transcriptional repression mechanism by effectors released by pests that suppress JA signalling.