Bacillus thuringiensis Cry5, Cry21, App6 and Xpp55 proteins to control Meloidogyne javanica and M. incognita

Abstract

The global imperative to enhance crop protection while preserving the environment has increased interest in the application of biological pesticides. Bacillus thuringiensis (Bt) is a Gram-positive bacterium that can produce nematicidal proteins and accumulate them in parasporal crystals. Root-knot nematodes are obligate root plant parasitic which are distributed worldwide, causing severe damages to the infested plants and, consequently, large yield reductions. In this work, we have evaluated the toxicity of the Bt crystal proteins Cry5, Cry21, App6, and Xpp55 against two root-knot nematodes belonging to the Meloidogyne genus (M. incognita and M. javanica). The results show that all four proteins, when solubilized, were highly toxic for both nematode species. To check the potential of using Bt strains producing nematicidal crystal proteins as biopesticides to control root-knot nematodes in the field, in planta assays were conducted, using two wild Bt strains which produced Cry5 or a combination of App6 and Cry5 proteins. The tests were carried out with cucumber or with tomato plants infested with M. javanica J2, irrigated with spore + cristal mixtures of the respective strains. The results showed that the effectiveness of the nematicidal activity was plant-dependent, as Bt was able to reduce emerged J2 in tomato plants but not in cucumber plants. In addition, the toxicity observed in the in planta assays was much lower than expected, highlighting the difficulty of the proteins supplied as crystals to exert their toxicity. This emphasizes the delivery of the Bt proteins as crucial for its use to control root-knot nematodes. KEY POINTS: • Solubilized Cry5, Cry21, App6 and Xpp55 Bt proteins are toxic to M. javanica. • Cry21 toxicity to M. incognita is similar to that of Cry5, App6, and Xpp55 proteins. • The Cry5 and App6 toxicities on M. javanica after Bt irrigation is crop dependent.

Keywords: In planta assay; Bio-control; Bioassay; Bt wild strains; In vitro assay.

Fig. 1

Scanning electron microscopy of V-CO3.3 and V-AB8.18 wild strains showing spores and parasporal crystal inclusions. a V-CO3.3 strain showing bipiramidal crystals. b V-AB8.18 strain, showing elongated bar–shaped crystals. In the images, white arrows point to crystals and empty arrows point to spores

Fig. 2

Impact of irrigation with V-CO3.3 or V-AB8.18 on cucumber plants infested with M. javanica J2. The parameters recorded were a egg masses per plant, b J2 per plant, and c dry foliar weight. V-CO3.3: plants treated with V-CO3.3 spore + crystal suspensions. V-AB8.18: plants treated with V-AB8.18 spore + crystal suspensions. Oxamyl: plants treated with oxamyl. Non-V-CO3.3: negative control plants (inoculated and non-treated with V-CO3.3). Non-V-AB8.18: negative control plants (inoculated and non-treated with V-AB8.18)

Fig. 3

Impact of irrigation with V-CO3.3 or V-AB8.18 on tomato plants infested with M. javanica J2. The parameters recorded were a egg masses per plant, b J2 per plant, and c dry foliar weight. V-CO3.3: plants treated with V-CO3.3 spore + crystal suspensions. V-AB8.18: plants treated with V-AB8.18 spore + crystal suspensions. Non-V-CO3.3: negative control plants (inoculated and non-treated with V-CO3.3). Non-V-AB8.18: negative control plants (inoculated and non-treated with V-AB8.18)