Mitigating chromium-induced oxidative stress in plants: a cross-talk between phytohormones and rhizospheric microbes

Hexavalent chromium [Cr(VI)], released from tanning, electroplating, and metallurgical industries, is a Group 1 human carcinogen that induces severe oxidative stress in plants through excessive reactive oxygen species (ROS) production, disrupting antioxidant defence and causing damage to lipids, proteins, and DNA, ultimately reducing growth and productivity. This review synthesizes evidence from Google Scholar, Web of Science, Scopus, and ScienceDirect, emphasizing publications from 2020 onwards, covering Cr(VI)-induced oxidative stress, molecular alterations, PGPR mechanisms, phytohormone cross-talk, and multi-omics and synthetic biology approaches. Three core conclusions emerge: the ACC deaminase AP2/ERF pathway is the best-characterized PGPR-mediated Cr(VI) tolerance route, linking bacterial enzyme activity to plant transcriptional reprogramming and physiological recovery. No single phytohormone addresses all stress dimensions simultaneously, making complementary combinations like GA3 with IAA-producing PGPR, ABA with cytokinin, and JA with ACC deaminase strains more effective than single-hormone strategies. Finally, Cr(VI) molecular responses are governed by a four-module gene network coordinating uptake restriction, antioxidant defence, vacuolar sequestration, and transcriptional regulation. Two critical unresolved issues remain: the phytoextraction-food safety conflict, where PGPR traits enhancing soil Cr removal also increase shoot Cr translocation in food crops. Secondly, the absence of field-scale validation makes all current effectiveness metrics laboratory estimates, requiring multi-season agronomic confirmation.