Biofertilizer and biostimulant potentials of phosphate-solubilizing Bacillus subtilis subsp. subtilis M1 strain and silicon in improving low phosphorus availability tolerance in rosemary.

The present study aimed to evaluate the single and combined effects of Si exogenous treatment and Bacillus subtilis subsp. subtilis M1 strain inoculation on rosemary tolerance to low phosphorus (P) availability. Hence, rosemary plants were fertilized with 250 µmol Ca3HPO4 (stressed plants) or 250 µmol KH2PO4 (control plants) under Si treatment and B. subtilis M1 strain inoculation. P starvation negatively affected rosemary growth and its P nutrition. However, exogenous Si supply or B. subtilis M1 strain inoculation significantly (P < 0.001) alleviated the deficiency-induced effects and significantly improved rhizogenesis, acid phosphatase activity, P uptake, and eventually dry weight of shoot and root. Moreover, Si-treatment and/or B. subtilis M1 strain inoculation significantly (P < 0.001) reduced the oxidative damage, in terms of malondialdehyde and hydrogen peroxide accumulation. This was found positively correlated with the higher superoxide dismutase activity, and the elevated non-enzymatic antioxidant molecules accumulation, including total polyphenols in Si-treated and inoculated P-deficient plants. Taken together, Si supplementation and/or B. subtilis M1 strain inoculation could be a good strategy to sustain rosemary plant growth under P starvation conditions.

Synergistic effect of Pseudomonas alkylphenolica PF9 and Sinorhizobium meliloti Rm41 on Moroccan alfalfa population grown under limited phosphorus availability.

This study looked at the synergistic effect of Pseudomonas alkylphenolica PF9 and Sinorhizobium meliloti Rm41 on the Moroccan alfalfa population (Oued Lmaleh) grown under symbiotic nitrogen fixation and limited phosphorus (P) availability. The experiment was conducted in a growth chamber and after two weeks of sowing, the young seedlings were inoculated with Sinorhizobium meliloti Rm41 alone or combined with a suspension of Pseudomonas alkylphenolica PF9. Then, the seedlings were submitted to limited available P (insoluble P using Ca3HPO4) versus a soluble P form (KH2PO4) at a final concentration of 250 µmol P·plant-1·week-1. After two months of P stress, the experiment was evaluated through some agro-physiological and biochemical parameters. The results indicated that the inoculation of alfalfa plants with Sinorhizobium strain alone or combined with Pseudomonas strain significantly (p < 0.001) improved the plant growth, the physiological and the biochemical traits focused in comparison to the uninoculated and P-stressed plants. For most sets of parameters, the improvement was more obvious in plants co-inoculated with both strains than in those inoculated with Sinorhizobium meliloti Rm41 alone. In fact, under limited P-availability, the co-inoculation with two strains significantly (p < 0.01) enhanced the growth of alfalfa plants evaluated by fresh and dry biomasses, plant height and leaf area. The results indicated also that the enhancement noted in plant growth was positively correlated with the shoot and root P contents. Furthermore, the incensement in plant P contents in response to bacterial inoculation improved cell membrane stability, reflected by low malonyldialdehyde (MDA) and electrolyte leakage (EL) contents, and photosynthetic-related parameters such as chlorophyll contents, the maximum quantum yield of PS II (Fv/Fm) and stomatal conductance (gs). Our findings suggest that Pseudomonas alkylphenolica PF9 can act synergistically with Sinorhizobium meliloti Rm41 in promoting alfalfa growth under low-P availability.