Abstract
Transgenic plants
that produce Bacillus thuringiensis (Bt) crystalline (Cry) toxins are cultivated
worldwide to control insect pests. Resistance to B. thuringiensis toxins
threatens this technology, and although different resistance mechanisms have
been identified, some have not been completely elucidated. To gain new insights
into these mechanisms, we performed multiple back-crossing from a 3000-fold
Cry1Ac-resistant BtR strain from cotton bollworm (Helicoverpa armigera),
isolating a 516- fold Cry1Ac-resistant strain (96CAD). Cry1Ac resistance in
96CAD was tightly linked to a mutant cadherin allele (mHaCad) that contained 35
amino acid substitutions compared with HaCad from a susceptible strain (96S). We
observed significantly reduced levels of the mHaCad protein on the surface of
the midgut epithelium in 96CAD as compared with 96S. Expression of both cadherin
alleles from 96CAD and 96S in insect cells and immunofluorescence localization
in insect midgut tissue sections showed that the HaCAD protein from 96S
localizes on the cell membrane, whereas the mutant 96CAD-mHaCad was retained in
the endoplasmic reticulum (ER). Mapping of the mutations identified a D172G
substitution mainly responsible for cadherin mislocalization. Our finding of a
mutation affecting membrane receptor trafficking represents an unusual and
previously unrecognized B. thuringiensis resistance mechanism.