Exploiting Siderophore-Producing Rhizobacteria for Enhanced Iron Acquisition and Growth Promotion in Oryza sativa (L.) and Eleusine coracana (L.) Gaertn.

Iron deficiency is a major nutritional constraint limiting plant growth and crop productivity in many agricultural soils. Plant growth-promoting rhizobacteria (PGPR) capable of producing siderophores can enhance iron availability and improve plant performance under iron-limiting conditions. In the present study, fifty rhizobacterial isolates obtained from diverse rhizosphere soils were screened for plant growth-promoting traits, including indole-3-acetic acid (IAA), ammonia, hydrogen cyanide (HCN), and siderophore production. Among the isolates, 36 exhibited siderophore-producing ability, and the most efficient strain was identified through 16S rRNA gene sequencing as Pseudomonas fluorescens MR8. Optimization studies revealed maximum siderophore production (82% siderophore units) in succinic acid medium after 72 h of incubation at pH 7.0 and 30°C. Spectroscopic characterization using UV-visible and FTIR analyses indicated that the siderophore possessed pyoverdine-like characteristics. The growth-promoting efficacy of P. fluorescens MR8 was evaluated in Oryza sativa (rice) and Eleusine coracana (finger millet) under iron-deficient conditions through in vitro co-cultivation and greenhouse experiments. Inoculation with the bacterial strain significantly enhanced seed germination, shoot and root growth, chlorophyll content, and iron accumulation compared with uninoculated controls. Rice exhibited a marked increase in lateral root development, while finger millet showed no significant change in root branching. Iron accumulation increased to 65.78 ppm and 81.0 ppm in rice and finger millet, respectively, under in vitro conditions, and further increased to 72.24 ppm and 95.3 ppm under greenhouse conditions. The findings demonstrate that siderophore-producing P. fluorescens MR8 effectively enhances iron acquisition and plant growth, highlighting its potential as a sustainable bioinoculant for improving crop productivity and nutritional quality in iron-deficient soils.