Synergistic effects of Bacillus amyloliquefaciens strains CBa_BFL2 and CBa_RA37 from the desert triangular spurge on the bacteria wilt disease in tomato

Consortia-based inoculants, such as the microbiome method, are advocated over individual biocontrol agents (BCAs), owing to partner reinforcement and rapid rhizospheric establishment. Herein, we screened the sole and dual suppressive effects of selected desert spurge (Euphorbia antiquorum) derived Bacillus spp. vis-à-vis Ralstonia solanacearum.

The conventional agar diffusion method was used to examine the BCAs' capacity to release antibiotics. Thereafter, the compatibility, the ability to form biofilm and the phytotoxicity of the BCAs towards tobacco leaves were screened. The Box-PCR fingerprinting technique was adopted to ascertain the endophytism of the BCAs, and the wilt suppression test followed by the specific activity of defence-related enzymes was performed.

R. solanacearum was completely inhibited by the extracts from Bacillus velezensis CBv_BE1, Bacillus amyloliquefaciens CBa_BFL2, and Bacillus amyloliquefaciens CBa_RA37 at 0.625 mg/mL. The outstanding candidates could form biofilm and colonize into tomato seedlings, with a plateau at log10 CFU = 4.29/g fresh weight. The consortium from CBa_BFL2 and CBa_RA37 (CBa_BFL2/CBa_RA37) yielded vigorous plants (up to a 319% increase in biomass). Likewise, a 90% and 89% reduction in wilt incidence and severity was recorded by the CBa_BFL2-CBa_RA37 combination. The Guaiacol peroxidases (GPX), phenylalanine ammonia lyase (PAL), and superoxide dismutase (SOD) specific activity were significantly and inversely correlated with wilt severity via Pearson's test. This alludes to a greater innate defence system in tomatoes in response to bacterization.

Our data suggest that tomato priming with a consortium of B. amyloliquefaciens CBa_BFL2 and CBa_RA37 resulted in vigorous and healthier tomato seedlings as referred to their sole effects, with altered activities of key plant distressing enzymes. However, further studies are warranted to reveal its full potential as a pioneering alternative to agrochemicals for the control of bacterial wilt.