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An effector from cotton bollworm oral secretion impairs host plant defense signaling

Insects have evolved effectors to conquer plant defense. Most known insect effectors are isolated from sucking insects, and examples from chewing insects are limited. Moreover, the targets of insect effectors in host plants remain unknown. Here, we address a chewing insect effector and its working mechanism. Cotton bollworm (Helicoverpa armigera) is a lepidopteran insect widely existing in nature and severely affecting crop productivity. We isolated an effector named HARP1 from H. armigera oral secretion (OS)

Chun-Yu Chen, Yao-Qian Liu, Wei-Meng Song, Dian-Yang Chen, Fang-Yan Chen, Xue-Ying Chen, Zhi-Wen Chen, Sheng-Xiang Ge, Chen-Zhu Wang, Shuai Zhan, Xiao-Ya Chen, and Ying-Bo Mao

PNAS July 9, 2019 116 (28) 14331-14338

Significance

Plants recognize insect-derived molecules and make the accurate defense response during herbivory. However, insects release effectors that disturb host plant defense responses for fitness. Effectors are crucial components in biotic interactions. We identified a caterpillar-derived effector (HARP1) from oral secretion of cotton bollworm, a devastating agricultural pest. HARP1 is released from larvae to plant leaves during feeding and is able to migrate from wounding site into plant cells automatically. HARP1 interacts with JASMONATE-ZIM-domain (JAZ) proteins, the suppressor of Jasmonate (JA) pathway, and blocks signaling transduction by preventing JAZ degradation. HARP1-like proteins are widely distributed and have conserved function in noctuids, and they may contribute to insect adaptation to host plants during coevolution.

Abstract

Insects have evolved effectors to conquer plant defense. Most known insect effectors are isolated from sucking insects, and examples from chewing insects are limited. Moreover, the targets of insect effectors in host plants remain unknown. Here, we address a chewing insect effector and its working mechanism. Cotton bollworm (Helicoverpa armigera) is a lepidopteran insect widely existing in nature and severely affecting crop productivity. We isolated an effector named HARP1 from H. armigera oral secretion (OS). HARP1 was released from larvae to plant leaves during feeding and entered into the plant cells through wounding sites. Expression of HARP1 in Arabidopsis mitigated the global expression of wounding and jasmonate (JA) responsive genes and rendered the plants more susceptible to insect feeding. HARP1 directly interacted with JASMONATE-ZIM-domain (JAZ) repressors to prevent the COI1-mediated JAZ degradation, thus blocking JA signaling transduction. HARP1-like proteins have conserved function as effectors in noctuidae, and these types of effectors might contribute to insect adaptation to host plants during coevolution.

 

See https://www.pnas.org/content/116/28/14331

 

Figure 1: HARP1 in H. armigera OS migrates into the leaf cells through the wounding damage sites. (A) qRT-PCR analysis of HARP1 transcripts in indicated tissues of fifth-instar larvae. The expression level in midgut was set to 1. Error bars represent ± SD (n = 3 biological replicates). (BD) Immunoblot detection of HARP1 protein level. The protein amount in each loading was quantified by Bradford assay and visualized by Coomassie Brilliant Blue (CBB) staining. All of the experiments were repeated at least two times, and the results were consistent. In B, the fourth instar larvae were fed on artificial diet supplemented with (+) or without (−) 0.1% gossypol for 1 d, and total proteins were collected from midgut, gut fluid and OS. In C, the OS was collected from the fourth instar larvae fed on artificial diet (AD), glandless (GL), or glanded (GD) cotton and A. thaliana (AT) leaves for 1 d. In D, the OS was collected from the indicated instar larvae that were fed on AT leaves for 1 d. (E) Whole amount immunohistochemistry detection of HARP1 at the chewing sites (red arrows) of Arabidopsis leaves. The mechanical wounding leaves were used as negative control. Anti-HARP1 antibody was used to detect HARP1 in BE. (F and G) Translocation of the Venus-HARP1 fusion protein into plant cells through damage sites. The Arabidopsis leaves were punched and incubated with the protein solutions of Venus-HARP1 or Venus for 1 h and washed for three to four times to remove the extra proteins that adhered on the leaf surface. The boxes indicate the location as shown in G. (F) Venus-HARP1 but not Venus was detected at the wounded sites. (G) A portion of Venus-HARP1 was located in the nucleus of leaf cell. Fluorescence intensity in cross-section (white arrow) is shown. (Scale bars: E, 100 μm; F, 500 μm; G, 5 μm).

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