Cobalt tolerant bacteria mobilize iron in garden pea ( <i>Pisum sativum</i> L.) to mitigate cobalt stress in iron deficit soils

Excess cobalt (Co) in plants induces oxidative stress and competes with iron (Fe), leading to Fe deficiency, leaf loss, and reduced chlorophyll content. Although Co is essential for some lower and leguminous plants, its toxicity hampers growth. In this study, seven previously isolated siderophore and 1-aminocyclopropane-1-carboxylate deaminase (ACCD) bacteria possessing PGP properties such as indole-3-acetic acid like substances production, and phosphate solubilization were screened for Co-tolerance. Pot study (2000 ppm Co stress) revealed enhanced root (108.10%-297.89%) and shoot length (28.99%-118.01%), and increased uptake of nitrogen (35.36-41.36 mg g^-1), phosphorous (3.54-4.21 mg g^-1), Co (3.09-5.2 µg g^-1) and Fe (34.08-41.02 µg g^-1), and chlorophyll (13.19-42.97 mg g^-1). Furthermore, inoculation of bacteria also significantly enhanced the soil siderophore units (96.21%-262.01%), ACCD production (1.74-4.99 µmol mL^-1) and the soil respiration activity such as fluorescein diacetate hydrolysis (11.33-48.57 µg g^-1), dehydrogenase enzyme (99.26-197.32 µg g^-1) and alkaline phosphatase (418.20-918.20 µg g^-1). In conclusion, strains IMN 4 (<i>Delftia</i> sp.) and SBTS 12 (<i>Rhodococcus</i> sp.) can be used to alleviate Co-stress <i>via</i> mobilizing Co and Fe in plants grown in Fe limited soils.