Zinc oxide biotransformation by mineral-dissolving rhizobacteria stimulated plant growth, physiochemical attributes, and mineral uptake in sunflower (Helianthus annuus L.)

Background

The rhizobacterial association with minerals and root surfaces is imperative for understanding the processes of biodissolution, biomineralization, and nutrient bioavailability. A profound understanding of the bacterial enticement of mineral dissolution and nutrient uptake in sunflowers is lacking. Rationale: The rationale of this study is to envisage how rhizobacterial inoculants transform zinc oxide (ZnO) into bioavailable form to promote crop growth and physiology. Aims: This study aims to identify rhizobacterial strains capable of biotransforming ZnO and assess their impact on sunflower growth, physiochemical attributes, and nutrient uptake. Methodology: Potential ZnO-dissolving rhizobacterial strains were isolated from the sunflower rhizosphere, and the possible mechanism of dissolution and acidification of the tris-minimal salt medium containing ZnO was explored by determining organic acid production. These strains were screened for dissolution of phosphate, mica, and pyrolusite insoluble minerals and were assessed for in vitro plant growth-promoting traits and enzymatic activities. They were evaluated for their capability to promote sunflower growth, antioxidant activities, and nutrient uptake under ZnO conditions. Results: The superlative rhizobacterial strains were identified as Bacillus sp. HMA4, Bacillus sp. HMA23, Bacillus sp. HMA26, Chryseobacterium sp. HMA31 and Pseudomonas sp. HMA35 through 16S rRNA gene sequencing. These strains demonstrated the bioreduction of ZnO into elemental zinc (Zn) through acidification mechanisms. They significantly produced acetic acid, formic acid, and lactic acid as major metabolites, causing biotransformation of Zn into organic acid-bound soluble Zn complexes. These strains were capable of stimulating the growth and biomasses of root and shoot, chlorophyll a and b contents, catalase activity, peroxidase activity, superoxide dismutase activity, ascorbate peroxidase activity, and uptake of nitrogen, phosphorus, potassium, and Zn contents in roots and shoots of sunflower seedlings in ZnO presences. Bacillus sp. HMA23 and Chryseobacterium sp. HMA31 promoted Zn contents in sunflower root with increases up to 1272 % and 1224 %, respectively, and shoots up to 455 % and 687 % rise over uninoculated control under ZnO conditions. Conclusion: Bacillus sp. HMA23 and Chryseobacterium sp. HMA31 demonstrated their possible applications in promoting phytohormones, biomineralization, and nutrient bioavailability and were well-capable to promote Zn uptake in sunflowers. These bacterial strains could be recommended as potential bioinoculants to launch biofertilizers, but further extensive testing must be performed under various environmental conditions.