TY - JOUR
T1 - Decreased Cry1Ac activation by midgut proteases associated with Cry1Ac resistance in Helicoverpa zea
AU - Zhang, Min
AU - Wei, Jizhen
AU - Ni, Xinzhi
AU - Zhang, Jie
AU - Jurat-Fuentes, Juan L.
AU - Fabrick, Jeffrey A.
AU - Carrière, Yves
AU - Tabashnik, Bruce E.
AU - Li, Xianchun
N1 - Funding Information:
This work was supported by Biotechnology Risk Assessment Grant Program competitive grants No. 2011-33522-30729 and 2014-33522-22215 from the USDA National Institute of Food and Agriculture and Agricultural Research Service, National Natural Science Foundation of China (Grant 31228019, 31321004) and China Scholarship Council scholarship to Min Zhang. We thank Dr Zhifan Yang (College of Life Sciences, Hubei University, Wuhan, China) and Dr Qingli Shang (College of Plant Science, Heping Campus, Jilin University, Changchun, China) for technical support. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.
Publisher Copyright:
© 2018 Society of Chemical Industry
PY - 2019/4
Y1 - 2019/4
N2 - 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.
AB - 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.
KW - Bacillus thuringiensis
KW - Bt crops
KW - Cry1Ac protoxin
KW - Helicoverpa zea
KW - bollworm
KW - cotton
KW - genetically engineered
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U2 - 10.1002/ps.5224
DO - 10.1002/ps.5224
M3 - Article
C2 - 30264537
AN - SCOPUS:85055880887
VL - 75
SP - 1099
EP - 1106
JO - Pest Management Science
JF - Pest Management Science
SN - 1526-498X
IS - 4
ER -