Mangrove-associated halotolerant bacteria augments salinity stress tolerance in Solanum lycopersicum

Abstract

Plant growth-promoting rhizobacteria (PGPRs) can effectively alleviate the adverse effects of salt stress on plants. This study isolated PGPRs from a high-saline mangrove ecosystem in Abu Dhabi, focusing on strains with ACC deaminase (ACCD) activity to reduce stress-induced ethylene and other plant growth-promoting (PGP) traits, such as phosphate solubilization (P + ). A total of 57 salt-tolerant bacterial isolates from the mangrove rhizosphere were initially screened for ACCD and P + activities. Of these, 24 isolates exhibited positive PGP traits and were further assessed for their ability to enhance salt stress tolerance in tomato seedlings. Nine selected isolates significantly improved seedling growth compared to uninoculated controls on agar plates with 75 mM NaCl. Following further screening, four isolates (B1-B4) were chosen for greenhouse trials. Under greenhouse conditions, Bacillus subtilis (B1) and Bacillus siamensis (B3) markedly mitigated salt stress effects and enhanced tomato plant tolerance, while Bacillus velezensis (B2) showed moderate benefits, and Priestia filamentosa (B4) had the least impact. Inoculation with B1 and B3 significantly increased shoot length, shoot fresh and dry weights, root fresh and dry weights, and flower count under saline conditions. Chlorophyll-a fluorescence analysis indicated the highest photosystem II (PSII) efficiency in B1- and B3-treated plants, moderate efficiency in B2-treated plants, and the lowest in B4-treated and uninoculated controls. The leaf spectral reflectance indices of tomato plants inoculated with B1 and B3 were the highest, followed by moderate increases in B2-treated plants, while the lowest values were observed in plants treated with B4 and the uninoculated controls. Variations in salt tolerance among tomato plants inoculated with B1-B4 correlated with the isolates' PGP traits, especially ACCD activity and IAA production. Overall, B1 and B3 show strong potential as biofertilizers to enhance tomato production under saline conditions.

Keywords: ACC deaminase; Bacillus; IAA production; Phosphate solubilization; Photosynthesis; Salt stress.