Supplying adequate iron (Fe) to greenhouse crops is challenging, particularly for iron-sensitive plants grown in soilless substrates under alkaline conditions. High pH levels reduce Fe bioavailability by promoting the formation of insoluble iron hydroxide (Fe(OH)₃), which limits plant uptake. Plant growth-promoting bacteria (PGPB) enhance plant growth and stress tolerance through diverse mechanisms, including siderophore production. Siderophores are small molecules that have a high affinity for metal ions like ferric iron (Fe³⁺). Once the siderophore chelates iron, the complex is soluble, and plant roots can readily absorb it. This study identified siderophore producing bacteria (SPB) capable of enhancing plant growth under Fe-limiting conditions. A rhizosphere bacterial collection from greenhouse plants was screened for siderophore production using the chrome azurol S (CAS) assay. Genomic analysis of 12 isolates identified in the CAS assay revealed diverse siderophore-related genes and other plant growth-promoting traits. Five of the isolates (C2G2, C5A12, C6E3, C8G7, and C10A8) were identified as strong candidates based on their siderophore-related genes and Fe uptake pathways. A ferrozine-based assay confirmed Fe solubilization in a modified Hoagland’s solution containing goethite (Fe³⁺O(OH)) as an insoluble Fe source, with bacterial treatments yielding 0.57 mg Fe L⁻¹ versus 0 mg Fe L⁻¹ in controls (no bacteria). Subsequent in planta experiments evaluated the impact of SPB on French marigold ‘Durango Yellow’ (Tagetes patula) growth in hydroponic systems under Fe-limiting conditions. French marigold proved to be a good model plant for this screening system. Digital phenotyping showed that plants inoculated with isolates C5A12, C8G7, or C10A8 had 53.0%, 45.7%, or 50.3% more green leaf tissue, respectively, compared to untreated control plants. Additionally, treated plants had increased root biomass. These findings demonstrate the potential of SPB to enhance Fe availability and plant growth under iron-limiting conditions and offer a novel strategy for improving greenhouse crop production. The incorporation of SPB into greenhouse practices may benefit Fe-sensitive crops or systems where pH fluctuations reduce Fe availability, ultimately improving crop quality and sustainability.
