Assessment of the potential of endophytic Bacillus thuringiensis producing nematicidal Cry5B protein for the control of Meloidogyne spp. in cucumber

Abstract

Bacillus thuringiensis (Bt) strains produce a variety of Cry proteins with nematicidal properties. Among these, Cry5B has shown toxicity against plant-parasitic nematodes of the genus Meloidogyne . In this study, the potential of a Bt strain producing Cry5B to colonize cucumber plants and suppress the root-knot nematode Meloidogyne javanica infestation was evaluated. The strain was applied by irrigation as a suspension of spores and crystals to cucumber seedlings. Bt was subsequently detected after plating extracts of surface-sterilized roots, cotyledons, stems, and leaves, demonstrating systemic movement and stable endophytic colonization for at least eight weeks, although Bt concentrations in leaves decreased progressively toward the most distal parts of the plant. The presence of vegetative cells in all tissues (assessed by comparing the CFU obtained from plant extracts before and after heat-shock treatments) further supported the ability of the strain to grow endophytically within cucumber. Bt-treated plants irrigated once or twice with 1 × 10 ¹⁰ spores and crystals challenged with 350  M. javanica juveniles failed to reduce nematode infection or reproduction. The number of egg masses and emerging J2 per plant were similar to those observed in plants irrigated with the Cry5B-deficient Bt strain. These results indicate that, although Cry5B displays potent nematicidal activity in vitro , endophytic colonization of cucumber plants by the applied producing strain did not confer effective protection against M. javanica under the experimental conditions tested. Understanding the factors that limit the in planta efficacy of Bt may help guide the development of more consistent Bt-based biological control strategies against plant-parasitic nematodes.