Molecular insights into Acinetobacter calcoaceticus JK40-mediated zinc availability and transporter gene modulation in rice.

Zinc (Zn) deficiency is a critical edaphic constraint that limits the productivity of rice (Oryza sativa L.). This study examined the mechanistic effectiveness of Acinetobacter calcoaceticus JK40 (Ac-NAU-JK40), a zinc-solubilizing bacterium (ZSB) isolated from the rice rhizosphere. Notably, JK40 was selected for this study due to its dual ability to solubilize both zinc (Zn) and phosphate (P), a trait believed to prevent P-induced Zn precipitation in the rhizosphere. In hydroponic assays, co-inoculation of Ac-NAU-JK40 with insoluble ZnO significantly improved morphological traits (shoot length + 24.7%; root volume + 207%) and biochemical markers (chlorophyll: 26.2 SPAD units; protein: 49.01 mg g⁻1), including growth regulators such as indole-3-acetic acid (IAA) and gibberellic acid (GA3) at 14 days after treatment (DAT). This was accompanied by increased Zn uptake (shoot: 56.40 µg g⁻1; root: 52.10 µg g⁻1) and decreased oxidative stress, indicated by lower SOD/CAT activity compared to uninoculated controls. Importantly, gene expression analysis revealed a distinct molecular mechanism. Zn-starved controls upregulated high-affinity transporter genes (OsZIP4, OsYSL14), whereas Ac-NAU-JK40 + ZnO inoculated plants showed significant downregulation of these genes. This acts as a molecular marker of physiological Zn sufficiency, indicating that bacterial solubilization effectively restored cellular Zn homeostasis and suppressed the plant's starvation response. These findings provide molecular evidence that native rhizospheric ZSBs can alleviate Zn deficiency, resulting in transcriptional downregulation of Zn transporter genes associated with nutrient stress.