Involvement of Holotrichia parallela odorant-binding protein 3 in the localization of oviposition sites.

Organic fertilizers-derived volatiles attract Holotrichia parallela during oviposition. However, the mechanisms underlying the perception of oviposition cues in H. parallela remain unclear. Here, H. parallela odorant-binding protein 3 (HparOBP3) was identified as a key OBP. Bioinformatics analysis showed that HparOBP3 clustered together with Holotrichia oblita OBP8. HparOBP3 was mainly expressed in the antennae of both sexes. Recombinant HparOBP3 exhibited distinct binding affinities towards 22 compounds released by organic fertilizers. After 48 h of RNA interference (RNAi), the expression of HparOBP3 in male and female antennae was decreased by 90.77 % and 82.30 %, respectively. In addition, silencing of HparOBP3 significantly reduced the electrophysiological responses and tropism of males to cis-3-hexen-1-ol, 1-hexanol, and (Z)-ß-ocimene as well as females to cis-3-hexen-1-ol, 1-hexanol, benzaldehyde, and (Z)-ß-ocimene. Molecular docking indicated that hydrophobic residues Leu-83, Leu-87, Phe-108, and Ile-120 of HparOBP3 were important amino acids for interacting with ligands. Mutation of the key residue, Leu-83, significantly diminished the binding ability of HparOBP3. Furthermore, acrylic plastic arena bioassays showed that the attraction and oviposition indexes of organic fertilizers to H. parallela were reduced by 55.78 % and 60.11 %, respectively, after silencing HparOBP3. These results suggest that HparOBP3 is essential in mediating the oviposition behavior of H. parallela.

PI3K/Akt/CncC signaling pathway mediates the response to EPN-Bt infection in Holotrichia parallela larvae.

BACKGROUND: Combining the entomopathogenic nematode (EPN), Heterorhabditis beicherriana LF strain, and Bacillus thuringiensis (Bt) HBF-18 strain is a practical strategy to manage the larvae of Holotrichia parallela Motschulsky (white grubs). However, the mechanisms underlying the larval defense response to this combined biocontrol strategy are unknown. RESULTS: The activities of some antioxidant enzymes (SOD, POD, CAT) and some detoxifying enzymes (AChE, P-450, CarE, GST) in grubs showed an activation-inhibition trend throughout the EPN-Bt exposure time course. Eight potentially key antioxidant and detoxifying enzyme genes in response to EPN-Bt infection were identified from the midgut of grubs through RNA sequencing. After silencing CAT, CarE18, and GSTs1, the enzyme activities were significantly decreased by 30.29%, 68.80%, and 34.63%, respectively. Meanwhile, the mortality of grubs was increased by 18.40%, 46.30%, and 42.59% after exposure to EPN-Bt for 1 day. Interestingly, the PI3K/Akt signaling pathway was significantly enriched in KEGG enrichment analysis, and the expression levels of phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt), cap 'n' collar isoform-C (CncC), kelch-like ECH-associated protein 1 (Keap1), and CarE18 were all up-regulated when exposed to EPN-Bt for 1 day. Furthermore, RNAi-mediated PI3K silencing showed a similar down-regulated trend between PI3K/Akt/CncC and CarE18. Moreover, silencing PI3K rendered grubs more susceptible to EPN-Bt and accelerated symbiotic bacteria multiplication in grubs. CONCLUSION: These results suggest that the PI3K/Akt/CncC pathway mediates the expression of CarE18 and participates in the defense response of H. parallela larvae against EPN-Bt infection. Our data provide valuable insights into the design of appropriate management strategies for this well-known agricultural pest. © 2022 Society of Chemical Industry.