Siderophore-mediated iron enrichment in the biofilm matrix enhances plant iron nutrition

Plant-beneficial microorganisms are frequently reported to enhance iron (Fe) nutrition in plants, yet the precise underlying mechanisms remain largely unknown. Although both bacterial siderophore production and biofilm formation are beneficial for microbial plant growth promotion, these two bacterial traits have been studied separately. Here, we reveal a strong coupling between these two bacterial traits in enhancing plant Fe uptake using the biofilm-forming rhizobacterium Bacillus velezensis SQR9. We demonstrate that SQR9 biofilms accumulate Fe on plant roots and serve as an Fe reservoir. Crucially, the siderophore bacillibactin enables biofilm Fe accumulation from the environment, while simultaneously stimulating Fe acquisition mechanisms in plants. Field experiments confirmed the ability of SQR9 to boost crop yields in alkaline soils, highlighting its potential for improving iron-limiting plant performance. Our findings emphasize a key role of rhizobacterial siderophores and biofilms in Fe uptake and offer mechanistic insights for microbial biofortification strategies against Fe deficiency in crops.