Bacillus thuringiensis Cry6A exhibits nematicidal activity to Caenorhabditis elegans bre mutants and synergistic activity with Cry5B to C. elegans

The group of nematicidal crystal protein Cry6A shares very low identity and exhibits different structure with Cry5B, another well-studied group of nematicidal crystal protein produced by Bacillus thuringiensis. In this study, we assayed the susceptibility of bre mutants (Caenorhabditis elegans with resistance to Cry5B) to Cry6Aa2 and examined the synergistic activity between Cry6Aa2 and Cry5Ba2. Our results show that all bre mutants are susceptible to Cry6Aa2 on the lethal activity, growth inhibition, fertility and exhibit no cross-resistance to Cry6Aa2. Moreover, all combinations of Cry6Aa2 and Cry5Ba2 with serial ratios exhibit significant synergism to C. elegans, and the highest synergistic effect was observed when Cry6Aa2 and Cry5Ba2 were mixed with a ration of 4 : 1. The susceptibility of bre mutants to Cry6A and synergistic activity between Cry6A and Cry5B may be attributed to the diverse action mode, because of different structure of the two nematicidal crystal protein toxins.

Significance and impact of the study: Bacillus thuringiensis nematicidal crystal proteins Cry6A and Cry5B share notable structural differences. Our study confirmed that there is no cross-resistance of Caenorhabditis elegans bre mutants (with resistance to Cry5B) to Cry6A, and combination of Cry6A and Cry5B shows synergistic activity against C. elegans. This result provides a more effective strategy for biocontrol of plant-parasitic nematodes using a combination of Cry6A and Cry5B. Combination of Cry6A and Cry5B could enhance nematicidal toxicity of Bt and supply an alternative to overcome the potential resistance of plant-parasitic nematodes when large-scale and long-term use of Bt toxins.