Bacillus cereus (Accession no. KR232400) was selected on the basis of its plant growth promoting potential (Shahzad et al. 2020). The strain was obtained from phyto-hormone laboratory at Quaid-i-Azam University, Pakistan. Bacterial strain was previously identified by 16S rRNA gene sequencing (Shahzad et al. 2016).

Three different concentrations of Zn solution (20, 40 and 60 mg kg^-1) for maize plant treatments were prepared in pure distilled water by dissolving zinc sulfate (ZnSO₄). We selected these three concentrations on the basis of previous scientific data (Long et al. 2003, Yang et al. 2005). Pure distilled water was used as the control for the experiment. 200 mL of each solution was added in 1 kg of potted soil.

For the preparation of inoculum, five ml of nutrient broth medium (Oxoid, UK) was autoclaved in test tubes for cultivation of bacterial strain. Bacteria were incubated in a shaker incubator (EXCELLA E24 Germany) at 37 ºC and 150 rpm for 48-72 h. Thereafter, the culture was centrifuged at 3000 rpm for 10 min. The pellet was re-suspended in autoclaved distilled water and the optical density (OD) was adjusted to 0.100 at 660 nm with UV-VIS spectrophotometer (UV-VIS Double Beam Spectrophotometer, Model NO: AE-S90-2D A & E Lab (UK) Co., Ltd.). Bacterial density was 10⁶ cells mL^-1 at optical density of 0.10 at 660 nm. The optical density was adjusted by adding about 200 ml sterile water to pure bacterial pellets.

Kashmir Gold variety maize (Zea mays L) seeds were collected from the National Agricultural Research Centre, Islamabad, Pakistan. The seeds were surface sterilized by washing with 95% ethanol, following by soaking in 10% Clorox for 2-3 minutes. The seeds were washed successively 2-3 times in autoclaved distilled water.

For experiment preparation, seeds were soaked in the B. cereus inoculum for 2-3 hrs, after that about 10 seeds were sown in each pot. Different concentrations of Zn solutions (100, 200 and 300 mg L^-1) and distilled water was added to each pot. Eight different treatments with three replicates were made. Detailed explanation of treatments is given in Table 1

The percentage germination was noted after seven day of sowing. Maize seedlings were uprooted after 21 days of sowing, after which root length (cm), plant height (cm), leaf length (cm), number of leaves and fresh plant weight parameters were recorded. Each treatment contained three replicates and 5 plants from each replicate were measured.

Determination of macro- and micro-nutrients (Na, Ca, Mg, K, P, N, Fe, Cu, Cr, Co, Zn, and Mn) was carried out by following the ammonium bicarbonate and diethylenetriamine pentaacetic acid (DTPA) method as described by Soltanpour and Schwab (1977). The 0.005 M ammonium bicarbonate and (DTPA) solution was prepared by adding 1.97 g of DTPA to 800 mL of distilled water. About 2 mL of ammonium hydroxide (NH₄OH) was added to facilitate dilution and to prevent effervescence due to ammonium bicarbonate addition. When DTPA was dissolved completely, 79.06 g of ammonium bicarbonate (NH₄HCO₃) was added. The pH of the solution was adjusted to 7.6 by adding 2 mL of ammonium hydroxide. Final volume was adjusted to 1 L with the addition of distilled water. For the analysis of different nutrient, 10 g of soil sample was mixed with 15 mL of distilled water. The suspension was stirred for 30 minutes on a magnetic stirrer. After stirring step, suspension was filtered. Two mL of filtrate was removed and 18 mL of distilled water was added for nutrient analysis.

The perchloric-acid digestion scheme was used to determine the content of various nutrients in maize plants (Allen et al. 1974). A solution of nitric acid, sulfuric acid, and perchloric acid in the ratio 5:1:0.1 (6.5 mL) was added to 0.25 g of plant material in a 50 mL flask and the mixture was heated on the hot plate in the fume hood to complete the digestion process. The formation of white fumes in the mixture confirmed the completion of digestion of the plant extract. Thereafter, distilled water (few drops) was poured into the flask and the mixture was allowed to cool. The digested plant extract samples were transferred to the volumetric flasks (50 mL) and the volume was raised up to 50 mL by the addition of distilled water. The samples were filtered with Whatman no.42 filter paper and were stored for further element analysis. The concentration of different elements/nutrients present in the plant samples were measured by atomic absorption spectrophotometer (Shimadzu AA-670 Germany).

The experiment was conducted in a completely randomized design (CRD) by using Statistic 8.1.1. (https://statistix.informer.com/8.1/). The results are the compare means and standard error of means of three replicates of a treatment. Five plants from each replicate were measured.

Percentage germination of maize seeds has shown variations among inoculated and uninoculated seeds (Fig. 1a). Maximum seed germination (%) was observed in B. cereus-inoculated seeds in combination with 20 mg kg^-1 ZnSO₄ (treatment T3), which was significantly higher than control (39%) and the majority of the treatments. High germination percentage was also recorded in B. cereus-inoculated seeds in combination with 40 mg kg^-1 ZnSO₄ (treatment T4), which was significantly higher than the treatment of uninoculated seeds with 60 mg kg^-1 ZnSO₄ (treatment T8), where the lowest values were obtained (germination reduction of 9% as compared to control).

The root length was only mildly affected by investigated treatments (Fig. 1b). The maximum increase in the root length (15%) was observed in B. cereus inoculated seeds in combination with 20 mg kg^-1 ZnSO₄ (treatment T3), although it was not significant compared to control. Similar increase in root length (14%) was also recorded for B. cereus-inoculated seed (treatment T2 treatments). On the contrary, the 19% reduction in root length was observed in the treatment of uninoculated seeds exposed to60 mg kg^-1 ZnSO₄ (treatment T8), which was not significant compared to control, but was significantly reduced in comparison to treatments T2 and T3. Other treatments exhibited values similar to those of control

Maximum increase in leaf length (17%) was observed in B. cereus-inoculated seeds in combination with 20 mg kg^-1 ZnSO₄ (treatment T3), although it was not significant as compared to control and other treatments of inoculated seeds; however, it was significantly higher in comparison to result obtained for uninoculated seeds (Fig. 1c). Namely, all treatments of uninoculated seeds resulted in reduction in leaf length, which was particularly after exposure to 20 mg kg^-1 ZnSO₄ (treatment T6).

Maximum number of leaves (1% increase compared to control) was observed in B. cereus-inoculated seeds in combination with 40 mg kg^-1 ZnSO₄ (treatment T4), although this value was not significantly different as compared to the majority of the treatments; the only exception was the treatment of uninoculated seeds exposed to 20 mg kg^-1 ZnSO₄ (treatment T6), which exhibited the lowest value (7% decrease compared to control) (Fig. 1d).

The inoculation of maize seeds with B. cereus significantly increased the fresh weight and height of maize plants (Fig. 2). A significantly higher increase in fresh weight was observed upon exposure of inoculated seeds to 20 mg kg^-1 ZnSO₄ (treatment T3, 80% increase) and 40 mg kg^-1 ZnSO₄ (treatment T4, 62% increased) in comparison to control and the treatments of uninoculated seeds. On the contrary, a significant reduction in plant fresh weight as compared to control was observed in all treatments of uninoculated seeds, with the maximum reduction of 27% recorded upon exposure to 60 mg kg^-1 ZnSO₄ (treatment T8) (Fig. 2b).

Exposure of inoculated seeds to ZnSO₄ slightly improved the plant height (treatments T3, T4 and T5) as compared to control; the maximum value (12% increase as compared to control) was obtained upon exposure to 20 mg kg^-1 Zn (treatment T3). On the other hand, exposure of uninoculated seeds to the same Zn concertation (20 mg kg^-1) resulted in a reduction in plant height up to 19% (Fig. 2a).

Zinc uptake was reduced in the majority of the treatments with inoculated seeds as compared to control; significantly for treatments T2 and T3 (exposure to 100 mg L^-1 ZnSO₄), and notably for treatment T4 (exposure to 49 mg kg^-1); reduced Zn uptake was 49%, 44% and 22% in T2, T3 and T4 respectively (Fig. 2c). However, the exposure of inoculated seeds to the highest tested Zn concentration (treatment T5) resulted in a significantly higher Zn uptake as compared to control. Among treatments with uninoculated seeds, exposure to 20 mg kg^-1 (treatment T6) resulted in a value similar to that of control, while upon treatment with 40 and 60 mg kg^-1 Zn (treatments T7 and T8, respectively) Zn uptake was significantly higher; the maximum increase (75% as compared to control) was observed in treatment T8 (Fig. 2c).

Different nutrients including Ca, Mg, K, Na, Fe, Cd, Cu, Cr, Zn, Co and Ni were detected in soil samples (Tab. 2). The plant nutrient status (Tab. 3) showed that the Cu content decreased in majority of the treatments as compared to control; the only exceptions were treatments T3 and T8. The Mn content was significantly elevated in the treatment of plants from inoculated seeds exposed to 20 mg kg^-1 ZnSO₄ (treatment T3), while the exposure of plants from uninoculated seeds to 40 mg kg^-1 ZnSO₄ (treatment T7) resulted in significantly lower Mn content as compared to control. Reduced Ni and Co contents were recorded in the majority of treatments as compared to control. Seed inoculation with B. cereus in general decreased the Na content as compared to values obtained in uninoculated plants, since the higher values were obtained in treatments of plants from uninoculated seeds (treatments T6, T7 and T8) as compared to treatments of plants from inoculated seeds to the corresponding Zn concentrations. Seed inoculation with B. cereus increased Cr content in plants as compared to control as well as to the treatments of plants from uninoculated seeds upon exposure to 20 and 40 mg kg^-1 ZnSO₄ (treatments T3 and T4) as compared to treatments T6 and T7, respectively). Fe content was significantly elevated in plants from inoculated seeds upon exposure to 40 and 60 mg kg^-1 ZnSO₄ compared to control as well as to the exposure of plants from uninoculated seeds to the same Zn concentration. Seed inoculation with B. cereus in general increased the Ca and Mg uptake in all investigated treatments as compared to control, with higher values obtained in plants from inoculated seeds compared to the uninoculated ones. K content was significantly higher in plants from inoculated seeds as compared to the control as well as to plants from uninoculated seeds exposed to the corresponding Zn concentrations.