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.

Both insects and pathogens release effectors that are transferred into plant cells and weaken the host defense or immune response. While the imports of some bacterial and fungal effectors into plants have been reported, how caterpillar effectors enter plant cells remains a mystery. Here, using live cell imaging and real-time protein tracking, we show that HARP1, an effector from oral secretions of cotton bollworm (Helicoverpa armigera) which is previously reported to interfere with defense hormone jasmonate (JA) signaling output in host plants, enters plant cells via protein-mediated endocytosis. The interactions of HARP1 with vesicle trafficking components including CTL1, PATL2 and TET8 are essential for its entry. Notably, JA restricts HARP1 import by inhibiting endocytosis and HARP1 loading on endosomes. Taken together, the effector and JA set up a defense and counter-defense loop in the arm race between plants and insects. Our study unveils a new paradigm for the plant-insect interaction.