Root acid phosphatases and microbial biomass phosphorus induced Cd tolerance and P acquisition in wheat inoculated with P solubilizing bacteria

Microbial bioremediation has emerged promisingly to improve crop tolerance to cadmium (Cd). Moreover, Cd tolerance and phosphate acquisition in plants positively correlated under P solubilizing bacteria inoculation, yet there is no evidence on specific mechanisms influencing Cd tolerance and plant P acquisition. The present study evaluates Cd tolerance in rock P-amended durum wheat in response to inoculation with P solubilizing bacteria (PSB) [three individual isolates Bacillus siamensis, Rahnella aceris, Bacillus cereus and their consortium (PSB_Cs)] and consequently reveals key rhizosphere mechanisms involved in both Cd tolerance and P use efficiency. Results show that inoculation overall improved plant growth, rhizosphere parameters and nutrient uptake (P, N, K) under increasing Cd concentrations [8 (Cd₈) and 16 (Cd₁₆) ppm Cd²⁺]. Under Cd₁₆, Rahnella aceris induced the most significant plant responses in terms of biomass [shoots (31 %), roots (40 %), and spikes (92 %)], rhizosphere available P (234 %) and root inorganic P (109 %) compared to uninoculated plant. Microbial biomass P (MBP) and root acid phosphatases (APase) were 33-and 13-times higher, respectively, than in uninoculated plants. In addition, inoculation (particularly using PSB_Cs) significantly decreased Cd translocation factor (TF) (Cd₈: -17 % and Cd₁₆: -22 %) and Cd bioaccumulation factor (BAF) (Cd₈: -6 % and Cd₁₆: -40 %) concomitantly to enhanced root morphological traits and P contents in shoots and spikes. Furthermore, PSB inoculation under Cd constraint increased (rhizosphere available P / MBP) and (Root APase / Rhizosphere Apase) ratios that significantly (p < 0.05) correlate with plant P uptake in shoots and spikes. Increase in both ratios was concomitant to a significant decrease in TF and BAF of Cd exemplified by negatively significant correlations (r²=0.70 and r²=0.57, p < 0.05). This finding elucidates the key role of bacterial inoculation that presumably triggered Cd tolerance and aboveground P owing to increased (rhizosphere available P / MBP) and (Root / Rhizosphere APase) ratios in PSB-inoculated wheat.