Enhancing Pea Plant Growth, Nutrient Acquisition, and Symbiosis in Cobalt-Stressed Soil Using Metal-Tolerant Klebsiella sp.

Excessive cobalt (Co) levels in agricultural soil cause significant toxicity, reducing crop growth and yield. This study aimed to assess the potential of Klebsiella sp. SRB-5 (Accession no. OR715782), in mitigating cobalt toxicity and enhancing the growth of garden peas under cobalt stress. Strain SRB-5, tolerant to 4000 ppm of Co(II), was evaluated for producing growth-regulating substances, including indole-3-acetic acid (IAA), ammonia, siderophore, ACC deaminase, and solubilized phosphate, under cobalt stress. The optimal conditions for Co-(II) biosorption by SRB-5 were determined to be 25°C, pH 6.0, and an incubation time of 72 h. The strain's ability to mitigate Co-(II) toxicity was tested by inoculating peas grown in soil treated with 1000, 2000, and 3000 ppm Co-(II). Inoculation with Co-tolerant SRB-5 alleviated cobalt toxicity and significantly enhanced the physiological and biochemical properties of plants. Notably, SRB-5 increased root length (19.2%), root biomass (29%), seedling vigor index (18.4%), total chlorophyll (52%), nodule biomass (41%), leghaemoglobin content (38%), root nitrogen (27%), and phosphorous content (19.3%) in 1000 ppm Co-stressed peas. Additionally, bacterial inoculation reduced proline, malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and membrane injury by 85%, 57.3%, 90%, and 75%, respectively, in 1000 ppm Co-exposed plants. Priming with SRB-5 also reduced cobalt uptake in roots (88%), shoots (53.7%), and grains (79.6%) compared to uninoculated treatments. Metal-tolerant beneficial soil bacteria, such as Klebsiella sp. strain SRB-5, could serve as an effective alternative for enhancing pea production in metal-contaminated soils. The use of Co-tolerant PGPR strains holds potential for development as biofertilizers in future agricultural practices.