Bacillus pumilus enhances tolerance in rice (Oryza sativa L.) to combined stresses of NaCl and high boron due to limited uptake of Na+

• Without inoculation, accumulation of salt toxic ions was responsible for shoot growth reduction.
• PGPRconferred rice tolerance to individual stresses due to induced plant antioxidation activity.
• PGPR supply excluded Na+ from rice leaves which enhanced the plant tolerance to NaCl + high B.

Plant growth promoting rhizobacteria (PGPR) confer plant tolerance to abiotic stresses like salinity and high boron (B) due to limited uptake of toxic ions as well as increased production of antioxidants. The current study was aimed to investigate whether particular PGPR strain is responsible either for the decreased uptake of B together with salt toxic ions or to promote rice growth through an efficient antioxidative system under combined stresses of salinity and high B. Rice seedlings were maintained in pots according to completely randomized design (CRD) and stressed with high B (0.92 mmol L−1 or 10 ppm) and NaCl (150 mmol L−1 or ECw of 14.7 dS m−1) for 8 weeks. Half of the pots received Bacillus pumilus-inoculated rice seedlings, whereas the other half received un-inoculated ones. Subsequently, plants were harvested and analyzed for mineral composition and antioxidation activity either using atomic absorption spectrometer (AAS) or spectrophotometer. In the absence of PGPR, NaCl salinity significantly enhanced the leaf B and salt toxic ions concentrations, thereby resulting in the shoot growth reduction when compared with the control. Similarly, combined treatment increased the leaf and xylem sap B as compared to NaCl alone, however, remained insignificant for salt toxic ions. Contrary, NaCl + high B decreased the leaf B concentrations as compared to high B alone. Application of PGPR enhanced the plant growth under individual stresses due to enhanced activity of certain of antioxidative enzymes. In combined treatment, B. pumilus showed a positive potential for limiting the Na+ accumulation in rice leaves, but not for leaf B. Moreover, limited uptake of Na+ resulted in the decreased plant antioxidation activity irrespective of increasing leaf B concentrations which in turn enhanced the rice tolerance.