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

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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)
JournalPest Management Science
DOIs
Publication statusAccepted/In press - Jan 1 2018

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Keywords

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

ASJC Scopus subject areas

  • Agronomy and Crop Science
  • Insect Science

Cite this

Decreased Cry1Ac activation by midgut proteases associated with Cry1Ac resistance in Helicoverpa zea. / Zhang, Min; Wei, Jizhen; Ni, Xinzhi; Zhang, Jie; Jurat-Fuentes, Juan L.; Fabrick, Jeffrey A.; Carriere, Yves; Tabashnik, Bruce E; Li, Xianchun.

In: Pest Management Science, 01.01.2018.

Research output: Contribution to journalArticle