Decreased Cry1Ac activation by midgut proteases associated with Cry1Ac resistance in Helicoverpa zea

Min Zhang, Jizhen Wei, Xinzhi Ni, Jie Zhang, Juan L. Jurat-Fuentes, Jeffrey A. Fabrick, Yves Carrière, Bruce E. Tabashnik, Xianchun Li

Research output: Contribution to journalArticle

4 Scopus citations

Abstract

BACKGROUND: Field-evolved resistance of Helicoverpa zea to Bacillus thuringiensis (Bt) toxin Cry1Ac was first reported more than a decade ago, yet the underlying mechanisms remain elusive. Towards understanding the mechanisms of resistance to Cry1Ac, we analyzed a susceptible (LAB-S) and two resistant (GA and GA-R) strains of H. zea. The GA strain was derived from Georgia and exposed to Bt toxins only in the field. The GA-R strain was derived from the GA strain and selected for increased resistance to Cry1Ac in the laboratory. RESULTS: Resistance to MVPII, a liquid formulation containing a hybrid protoxin similar to Cry1Ac, was 110-fold for GA-R and 7.8-fold for GA relative to LAB-S. In midgut brush border membrane vesicles, activity of alkaline phosphatase and aminopeptidase N did not vary significantly among strains. The activity of total proteases, trypsin-like proteases and chymotrypsin-like proteases was significantly lower for GA-R and GA than LAB-S, but did not differ between GA-R and GA. When H. zea midgut cells were exposed to Cry1Ac protoxin that had been digested with midgut extracts, toxicity was significantly lower for extracts from GA-R and GA relative to extracts from LAB-S, but did not differ between GA-R and GA. Transcriptional analysis showed that none of the five protease genes examined was associated with the decline in Cry1Ac activation in GA-R and GA relative to LAB-S. CONCLUSION: The results suggest that decreased Cry1Ac activation is a contributing field-selected mechanism of resistance that helps explain the reduced susceptibility of the GA-R and GA strains. Relative to the LAB-S strain, the two Cry1Ac-resistant strains had lower total protease, trypsin and chymotrypsin activities, a lower Cry1Ac activation rate, and Cry1Ac protoxin incubated with their midgut extracts was less toxic to H. zea midgut cells.

Original languageEnglish (US)
Pages (from-to)1099-1106
Number of pages8
JournalPest management science
Volume75
Issue number4
DOIs
StatePublished - Apr 2019

Keywords

  • Bacillus thuringiensis
  • Bt crops
  • Cry1Ac protoxin
  • Helicoverpa zea
  • bollworm
  • cotton
  • genetically engineered

ASJC Scopus subject areas

  • Agronomy and Crop Science
  • Insect Science

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