A highly accumulated secretory protein from cotton bollworm interacts with basic helix–loop–helix transcription factors to dampen plant defense

Summary Caterpillar oral secretion (OS) contains active molecules that modulate plant defense signaling. We isolated an effector‐like protein (Highly Accumulated Secretory Protein 1, HAS1) from cotton bollworm ( Helicoverpa armigera ) that is the most highly accumulated secretory protein of the nondigestive components in OS and belongs to venom R‐like protein. Elimination of HAS1 by plant‐mediated RNA interference reduced the suppression of OS on the defense response in plants. Plants expressing HAS1 are more susceptible to insect herbivory accompanied by the reduced expressions of multiple defense genes. HAS1 binds to the basic helix–loop–helix (bHLH) transcription factors, including GoPGF involved in pigmented gland formation and defense compounds biosynthesis in cotton and MYC3/MYC4 the main regulators in jasmonate (JA) signaling in Arabidopsis. The binding activity is required for HAS1 to inhibit the activation of bHLHs on plant defense gene expressions. Together with our previous study that another venom R‐like protein HARP1 in cotton bollworm OS blocks JA signaling by interacting with JASMONATE‐ZIM‐domain repressors, we conclude that the venom R‐like proteins in OS interfere with plant defense in a dual suppression manner. Considering the venom proteins in parasitic wasp assault the immune system of its host animal, our investigation reveals their conserved function in carnivorous and herbivorous insects.