Investigating the phytohormone–antioxidant interplay mediated by soil amendments and <i>Trichoderma harzianum</i> for enhanced cadmium remediation in <i>Cosmos bipinnatus</i> and <i>Amorpha fruticosa</i>

Heavy metal contamination, particularly cadmium, poses a significant threat to plant health and agricultural productivity. While phytohormones modulate stress responses, the combined effects of chemical amendments and beneficial microbes on this regulatory network remain poorly understood. This study evaluated the integrated application of ethylenediaminetetraacetic acid, fertilizers, and Trichoderma harzianum T22 inoculation to mitigate cadmium toxicity in Cosmos bipinnatus and Amorpha fruticosa. The combined treatment significantly improved soil properties, increasing pH from 4.32 to 6.55 and organic matter by 150%, while reducing soil cadmium bioavailability by 74.9%. Plant growth was enhanced, with height increasing by 31% in C. bipinnatus and 29% in A. fruticosa. The amendments restricted cadmium translocation to shoots by up to 68% and boosted photosynthetic efficiency, elevating chlorophyll content by 102% and photosynthetic rate by 102.1%. Phytohormonal analysis revealed up to a 100% increase in jasmonic acid and indole-3-acetic acid, supported by the upregulation of associated biosynthetic and transporter genes. Antioxidant defense systems were strengthened, with activities of superoxide dismutase and catalase increasing by 98% and 105%, respectively, and levels of glutathione and proline rising by 89% and 127%. Furthermore, the treatment promoted a beneficial shift in the rhizosphere microbial community. These results demonstrate that the synergistic strategy effectively enhances plant resilience and cadmium remediation by activating interconnected physiological and molecular pathways. This approach presents a sustainable model for rehabilitating contaminated soils, with promising potential for field-scale application and integration with other phytotechnologies.