SB24

Fusarium root rot is an important disease of soybean in Ontario, Canada. This study is to select antagonistic bacterial agents as effective alternatives to chemical pesticides for the control of root rots caused by Fusarium oxysporum and F. graminearum. Twenty-two Bacillus subtilis strains from soybean and corn roots were tested in dual cultures for inhibition of mycelial growth of F. oxysporum and F. graminearum. All strains significantly reduced mycelial growth of F. oxysporum by approximately 17 to 48% and of F. graminearum by 10 to 32%. Ten B. subtilis strains selected based on their larger fungal inhibition zones were evaluated against macroconidial germination. These strains inhibited the spore germination of F. oxysporum by 20 to 48% and of F. graminearum by 14 to 32% in cell-free filtrates. Under greenhouse conditions, the efficacy of seed and soil treatments with B. subtilis strains against the two Fusarium root rot pathogens was evaluated based on root rot severity, seedling emergence, plant height, and root dry weight. Six B. subtilis strains (SB01, SB04, SB23, SB24, SB28, and SB33) from soybean roots and two strains (CB01 and CH22) from corn roots significantly reduced the severity of the two Fusarium root rots in seed or soil treatments. Strains SB01, SB04, SB23, and SB24 were the most effective treatments against both pathogens in either seed or soil treatment. When applied as seed treatments, these four strains reduced root rot severity by 43 to 63% and increased emergence by 13 to 17%, plant height by 9 to 18%, and root dry weight by 8.4 to 19%. When used as soil treatments, they reduced root rot severity by 68 to 74% and increased emergence by 14 to 18%, plant height by 11 to 23%, and root dry weight by 16 to 24%. These results suggest that the novel strains of B. subtilis identified in this research can be effective alternatives to fungicides in managing Fusarium root rots of soybean, and a greater level of efficacy may be achieved when they were used as soil treatments than seed treatments.

Soil salinization is a critical constraint on global rice production, posing a serious threat to food security and underscoring the urgent need for sustainable strategies to fortify plant stress resilience. In this context, this study demonstrates that SB24, a halotolerant PGPR isolated from saline rice fields, markedly improved the salinity tolerance of rice genotype Pusa 44 through coordinated physiological, biochemical, and molecular responses. This strain, identified by 16S rDNA sequencing, exhibited high salt tolerance (up to 16% NaCl) and possessed multiple plant growth-promoting traits. Under 125 mM NaCl stress, SB24 inoculation enhanced germination and growth, with significant increases in shoot and root biomass compared to uninoculated controls. SB24-treated plants showed elevated photosynthetic pigments (Chl a, Chl b, carotenoids), soluble sugars, and soluble proteins, alongside enhanced antioxidant enzyme activities (SOD, CAT, APX, PPO) significantly. These changes were accompanied by reductions in oxidative stress markers, including electrolyte leakage, H₂O₂ accumulation, lipid peroxidation, and lipoxygenase activity. SB24 also promoted osmolyte accumulation (proline, glycine betaine and maintained ion homeostasis by reducing Na⁺ uptake while increasing K⁺ and Ca^2⁺ retention. At the transcriptional level, SB24 upregulated the expression of key salt-responsive genes, including OsSOS1 (salt overly sensitive 1) 1.35-fold, OsNHX1 (vacuolar Na⁺/H⁺ antiporter 1) 1.5-fold, OsHKT1;5 (high-affinity K⁺ transporter 1;5) 1.23-fold, OsFeSOD (iron superoxide dismutase) 1.22-fold, and OsAPX (ascorbate peroxidase) 1.18-fold, validating the observed physiological, and biochemical responses to improved salt stress tolerance. Taken together, these findings establish SB24 as a potent bioinoculant with strong potential for mitigating salinity stress through integrated, multi-level mechanisms.