Frontiers in Plant Science · 9 authors, 7 centres
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This study isolated five halotolerant Bacillus strains from rhizosphere soil that significantly improved tomato plant growth under salt stress (4 dS/m NaCl). PGPR-treated plants showed increased germination, biomass, chlorophyll, proline, and antioxidant enzyme activity, along with reduced sodium, chloride, and MDA levels. These findings support the use of halotolerant PGPR as a sustainable strategy to mitigate salinity stress in tomato cultivation in arid and semi-arid regions.
**Background:** Soil salinity is a major abiotic stress affecting over 833 million hectares globally, with approximately 6.727 million hectares affected in India, particularly in Gujarat. Salinity induces water deficiency, ion toxicity, nutrient imbalance, and oxidative stress in plants, reducing crop productivity. Tomato (Lycopersicon esculentum Mill.) is the second most important vegetable crop worldwide and is highly sensitive to salinity. Plant growth-promoting rhizobacteria (PGPR), especially halotolerant Bacillus species, offer a sustainable alternative to mitigate salt stress by producing phytohormones, solubilizing nutrients, and modulating plant antioxidant systems.
**Methods:** Rhizosphere soil samples were collected from 'Kesudo' (Butea monosperma), 'Kawaria' (Cassia tora), and 'Arjun' (Terminalia arjuna) plants in the Little Rann of Kutch, Gujarat (23°58′30″N, 70°12′19″E). A total of 107 bacterial strains were isolated on 5% NaCl-supplemented media. All isolates were screened for plant growth-promoting traits including indole-3-acetic acid (IAA) production, phosphate solubilization, siderophore production, and ACC deaminase activity. Five potent strains were selected and identified via 16S rRNA sequencing: Bacillus pumilus NCT4 (KF853108), Bacillus firmus NCT1 (KF853131), Bacillus licheniformis LCT4 (KF853123), Bacillus cereus LAT3 (KF853105), and Bacillus safensis LBM4 (KJ883295). A greenhouse experiment was conducted at Hemchandracharya North Gujarat University, Patan, using tomato S-22 seeds. The experimental design included 12 treatments with 10 replicates each, under non-saline (0 dS/m) and saline (4 dS/m NaCl) conditions. Seeds were surface-sterilized with 0.1% HgCl2, bioprimed in bacterial culture for 30 minutes, and sown in 3 kg of autoclaved vertisol soil. Physiological parameters (germination, shoot/root length, fresh/dry weight, leaf area) and biochemical parameters (chlorophyll, proline, soluble sugars, MDA, SOD, CAT, APX, GR) were measured after two months. Mineral analysis (Na, Cl, Mg, Ca, K, P, Fe) was performed using atomic absorption spectroscopy and titration methods. Statistical analyses included two-way ANOVA, Pearson correlation, principal component analysis, and UPGMA clustering.
**Key Results:** Among the five selected strains, NCT1 produced the highest IAA (45.7 μg/ml), NCT4 showed the greatest phosphate solubilization (11.69 mg P released from 100 mg tricalcium phosphate), and LCT4 exhibited the highest siderophore production (56.1% SU) and ACC deaminase activity (641.2 nmol α-ketobutyrate mg⁻¹ h⁻¹). Under 4 dS/m salinity, PGPR-treated plants showed significantly improved growth compared to non-inoculated controls. NCT4 treatment produced the highest shoot height (57.35 cm vs 37.9 cm in controls), root length (16.15 cm vs 10.68 cm), leaf area (7.18 cm² vs 5.72 cm²), and germination (76% vs 63%). Shoot fresh weight increased from 5.74 g (control) to 9.35 g (NCT4), and shoot dry weight from 2.05 g to 3.35 g. Under saline conditions, NCT4, NCT1, and LCT4 significantly increased soluble sugar and proline content. Chlorophyll content was significantly higher in NCT4-, NCT1-, LCT4-, and LAT3-treated plants under salinity. MDA levels were significantly reduced in NCT4-, NCT1-, LCT4-, and LAT3-treated plants under salt stress. Antioxidant enzyme activities were elevated: SOD activity increased significantly with NCT4, NCT1, and LCT4; CAT activity increased with all five strains; APX activity increased with all five strains under salinity; GR activity increased significantly only with NCT4. Mineral analysis under salinity showed significant reductions in Na content with all five PGPR strains, and significant reductions in Cl content with NCT4. Mg and P content increased significantly with all five strains under both saline and non-saline conditions. Ca content increased significantly with NCT4 and NCT1; K content increased with NCT4, NCT1, LCT4, and LAT3; Fe content increased significantly with NCT4 and NCT1 under salinity. Correlation analysis revealed positive correlations between soluble sugar/proline and antioxidant enzyme activities, and between germination rate and Fe/K content. Negative correlations were observed between germination rate and MDA/Cl content, and between shoot/root length and Cl content.
**Clinical Implications:** This study demonstrates that halotolerant Bacillus strains, particularly B. pumilus NCT4 and B. firmus NCT1, can effectively mitigate salt stress in tomato plants by enhancing physiological growth, boosting antioxidant defenses, improving nutrient uptake, and reducing sodium and chloride accumulation. These findings have direct agricultural relevance for tomato cultivation in salt-affected soils, especially in arid and semi-arid regions like Gujarat, India, where approximately 2.23 million hectares are salt-affected. The use of these PGPR strains offers a cost-effective, environmentally sustainable alternative to chemical amendments for improving crop productivity under salinity stress. Future research should focus on identifying gene expression and mRNA expression patterns associated with tomato tolerance mechanisms.