Bacillus thuringiensis translocation in citrus scion/rootstock combinations and binding of cry toxins with the Asian citrus psyllid gut receptors

Abstract

Bacillus thuringiensis (Bt) can be effective in controlling insect pests either in spray products or by their toxins expressed in transgenic crops. The discovery of Bt as endophytic in citrus opened new perspectives for the control of Diaphorina citri Kuwayama, the vector of the bacteria Candidatus Liberibacter spp. associated with Huanglongbing. In this study, the endophytic translocation of Bt in citrus seedlings (small and large plants) and in different scion-rootstock combinations were investigated. We also assessed the lethal concentration of D. citri nymphs caused by systemic Bt in citrus and, moreover, the binding of Cry toxins with the Asian citrus psyllid (ACP) gut receptors. Endophytic translocation of Bt in citrus seedlings and nursery trees was confirmed by colony isolation, PCR detection, and Btk-GFP microscopy analyses, which demonstrated that, after Bt inoculation into the substrate, the bacteria enter through lateral root emergence sites and are localized in stem xylem vessel elements. Mean lethal concentrations for D. citri third instar nymphs were 4.92 × 104 and 2.19 × 104 spores mL−1 for S1450 and S1302 strains, respectively. Nymphal mortality induced by the bacteria was higher in citrus seedlings than in citrus nursery trees. All Cry-toxins tested were able to bind to D. citri brush border membrane vesicles, suggesting the potential pathogenicity of Bt strains against psyllid. This study clearly demonstrates the penetration and Bt translocation in the xylem of small and large citrus plants and the potential of the bacteria to cause ACP nymphs mortality that fed on shoots from Bt-treated citrus.