Assessment of plant growth-promoting activities of Cr(VI) tolerant endophytes isolated from <i>Phyllanthus amarus</i> Schum &amp; Thonn for their bioremediation potential

Plant growth-promoting endophytic bacteria (PGPEB) have emerged as promising candidates for the remediation of heavy metal-contaminated soils owing to their eco-friendly and cost-effective nature. However, information on hexavalent chromium [Cr(VI)]-tolerant endophytes associated with Phyllanthus amarus and their potential role in Cr(VI) bioremediation remains limited. In the present study, Cr(VI)-tolerant PGPEB were isolated from P. amarus growing at a long-term waste disposal site in Varanasi, India, and evaluated for their plant growth-promoting (PGP) and Cr(VI)-remediating capabilities. All the isolates had shown variable levels of PGP traits and tolerated Cr(VI) concentrations ranging from 100 to 700 mg L−1. Among them, strain PaR9, identified as Agrobacterium pusense, showed the highest indole-3-acetic acid production (8.514 µg mL−1) and removed 67.21% of Cr(VI) within 72 h under in vitro conditions. SEM and FTIR analyses revealed Cr(VI)-induced alterations on the bacterial surface, indicating metal adsorption and interaction with cellular functional groups. Furthermore, inoculation with A. pusense PaR9 significantly enhanced biomass accumulation, root and shoot growth, and photosynthetic pigment content in spinach, red amaranth, and green amaranth cultivated in soil containing 100 mg kg−1 Cr(VI). These findings highlight the potential of A. pusense PaR9 as a dual-function endophyte for promoting plant growth and mitigating Cr(VI) stress, supporting its application in sustainable bioremediation strategies.