Zinc solubilizing rhizobacteria as bioinoculant for turmeric: transcriptomic insights into acid stress adaptation, zinc mobilization and growth promotion.

The application of a zinc (Zn) solubilizing PGPR strain, particularly in combination with zinc oxide (ZnO), enhanced Zn bioavailability, thereby improving turmeric (Curcuma longa L.) growth, yield, and increased Zn accumulation in the rhizomes. Expression patterns of differentially expressed genes (DEGs) of a high Zn solubilizing PGPR strain (Bacillus safensis) in comparison to a low Zn solubilizing (Phytobacter diazotrophicus) PGPR strain were studied through comparative RNA-Seq transcriptomics. During ZnO solubilization, B. safensis secreted high levels of organic acids (gluconic, keto gluconic, oxalic, citric, malonic and tartaric) and activated acid tolerance responses involving cell wall reinforcement, membrane permeability modulation, proton influx regulation, and metabolic adjustments. qPCR validation of 16 key differentially expressed genes under ZnO treatment confirmed an enhanced acid tolerance response. Our findings underscore that the enhanced Zn solubilization potential of the strain is not merely organic acid production but also from its ability to withstand low pH through multiple adaptive mechanisms. Both greenhouse and field experiments demonstrated that inoculation with B. safensis, either alone or combined with ZnO, significantly boosted soil Zn levels, while concurrently promoting turmeric productivity and nutritional quality.