Bacillus Strains as Effective Biocontrol Agents Against Phytopathogenic Bacteria and Promoters of Plant Growth

Abstract

Modern crop production relies on the application of chemical pesticides and fertilizers causing environmental and economic challenges. In response, less environmentally impactful alternatives have emerged such as the use of beneficial microorganisms. These microorganisms, particularly plant growth-promoting bacteria (PGPB), have demonstrated their ability to enhance plant growth, protect against various stresses, and reduce the need for chemical inputs. Among the PGPB, Bacillus species have garnered attention due to their adaptability and commercial potential. Recent reports have highlighted Bacillus strains as biocontrol agents against phytopathogenic bacteria while concurrently promoting plant growth. We also examined Bacillus plant growth-promoting abilities in Arabidopsis thaliana seedlings. In this study, we assessed the potential of various Bacillus strains to control diverse phytopathogenic bacteria and inhibit quorum sensing using Chromobacterium violaceum as a model system. In conclusion, our results suggest that bacteria of the genus Bacillus hold significant potential for biotechnological applications. This includes developments aimed at reducing agrochemical use, promoting sustainable agriculture, and enhancing crop yield and protection.

Keywords: Bacillus; Phytopathogen biocontrol; Plant growth promotion; Quorum sensing inhibition.

Fig. 1

Detection and phylogenetic analysis of the lactonase gene in Bt strains. A PCR detection of aiiA (lactonase, 750 bp) and 16s rRNA (endogenous, 450 bp) genes from genomic DNA of different Bt strains. 1: Bt R1; 2: Bt R4; 3: Bt R3; 4: Bt serovar kenyae; 5: Bt serovar aizawai; 6: Bt strain 1; 7: Bt serovar tolworthi; 8: Bt serovar kurstaki strain 1; 9: Bt strain 2; 10: Bt serovar kurstaki strain 2; 11: Bt serovar kurstaki strain 3; 12: Bt R2; 13: Bt serovar alesti; 14: Bt serovar entomocidus; NTC, non template control. B Phylogenetic analysis based on the amino acid sequences of the lactonase with the neighbor-joining method using 1000 bootstrap replicates

Fig. 2

Confrontation assays of Bt strains against phytopathogenic bacteria. Inhibition area (mm²) of growth inhibition of phytopathogenic bacteria treated with different concentrations of Bt strains (1.2 × 10³, 2.4 × 10³, and 4.8 × 10³ cells/mL) in Petri dishes. Confrontation against A C. michiganensis, B R. solanacearum, and C X. campestris. Standard deviation bars are shown (three replicates were evaluated). Different letters indicate statistically significant differences between groups, as determined by the Tukey test (p < 0.05) for each cell concentration

Fig. 3

Quorum sensing inhibition evaluation in C. violaceum treated with Bt extracts. A Plates showing inhibition of violacein synthesis at 72 h post-inoculation (hpi). Ctrl, control; R1-3, replicates. B Quantification of violacein synthesis inhibition after 72 hpi treatment with 0.4 mg of Bt extracts, contained in 15 µL. Different letters indicate statistically significant differences determined by the Tukey test (p < 0.05). C Inset indicating the location of Bacillus assayed by triplicate: 1. Bt R1; 2. Bt kenyae; 3. Bt tolworthi; 4. Bt alesti; 5. Bt kurstaki strain 1; 6. methanol (negative control); 7. acyl-homoserine lactone (positive control)

Fig. 4

Effect of Bt strains on root system architecture in ARR5 Arabidopsis seedlings. A Petri dishes with the plant-bacteria interaction. Arabidopsis seedlings were inoculated with different Bt strains. Control indicates non-inoculated plates with Bt strains. B Primary root length (cm). C Lateral root number. Different letters indicate statistically significant differences determined by the Tukey test (p < 0.05)

Fig. 5

Effect on auxin and cytokinin accumulation in Arabidopsis seedlings inoculated with Bt strains. A Representative images illustrating the impact of different Bacillus strains on the growth of primary roots, secondary roots, and shoots in Arabidopsis seedlings. Afterwards, seedlings were stained for GUS activity. B Arabidopsis expressing the auxin inducible construct DR5::uidA treated with the Bt strains. C Arabidopsis expressing the cytokinin inducible promoter ARR5::uidA treated with the indicated Bt strains. Representative images of analyzed plants are shown