Beneficial Microorganisms: Sulfur-Oxidizing Bacteria Modulate Salt and Drought Stress Responses in the Halophyte Plantago coronopus L.

Land degradation due to salinity and prolonged drought poses significant global challenges by reducing crop yields, depleting resources, and disrupting ecosystems. Halophytes, equipped with adaptive traits for drought and soil salinity, and their associations with halotolerant microbes, offer promising solutions for restoring degraded areas sustainably. This study evaluated the effects of halophilic sulfur-oxidizing bacteria (SOB), specifically Halothiobacillus halophilus, on the physiological and biochemical responses of the halophyte Plantago coronopus L. under drought and salt stress. We analyzed the accumulation of ions (Na, Cl, K) and sulfur (S), along with growth parameters, glutathione levels, photosynthetic pigments, proline, and phenolic compounds. Drought significantly reduced water content (nearly 10-fold in plants without SOB and 4-fold in those with SOB). The leaf growth tolerance index improved by 70% in control plants and 30% in moderately salt-stressed plants (300 mM NaCl) after SOB application. SOB increased sulfur content in all treatments except at high salinity (600 mM NaCl), reduced toxic sodium and chloride ion accumulation, and enhanced potassium levels under drought and moderate salinity. Proline, total phenolic, and malondialdehyde (MDA) levels were highest in drought-stressed plants, regardless of SOB inoculation. SOB inoculation increased GSH levels in both control and 300 mM NaCl-treated plants, while GSSG levels remained constant. These findings highlight the potential of SOB as beneficial microorganisms to enhance sulfur availability and improve P. coronopus tolerance to moderate salt stress.