Harnessing biostimulants to alleviate potentially toxic element stress in vegetable crops

Biostimulants sustainably mitigate potentially toxic element stress by reducing metal toxicity, enhancing antioxidant and molecular defenses, and limiting metal accumulation in edible tissues, ensuring safer vegetable production. Vegetable crops are vital to human nutrition but increasingly vulnerable to potentially toxic elements (PTEs) contamination from industrial effluents, wastewater irrigation, phosphate fertilizers, and vehicular emissions. Toxic metals such as cadmium (Cd), lead (Pb), and chromium (Cr) impair photosynthesis, disrupt nutrient uptake and hormonal signaling, and induce oxidative stress via reactive oxygen species (ROS), leading to reduced growth, yield, and food safety. These effects originate from primary interactions such as metal binding to enzymes and disruption of transporter systems, which trigger downstream oxidative and molecular stress responses. Conventional remediation approaches are costly and unsustainable, prompting a shift toward biostimulant-based mitigation strategies. This review critically examines the role of microbial, plant growth-promoting rhizobacteria (PGPR), arbuscular mycorrhizal fungi (AMF), organic (humic substances, seaweed extracts, protein hydrolysates), and inorganic (silicon, selenium) biostimulants in alleviating HM stress in vegetable crops. These agents reduce metal bioavailability through chelation and immobilization. Genetic regulators such as ZIP, NRAMP, and HMA mediate metal transport across cellular membranes, and the expression of their genes is regulated in response to PTE stress, thereby affecting metal uptake and transport. Additionally, they enhance the activity of antioxidant enzymes including, superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and support vacuolar sequestration via the synthesis of phytochelatins and metallothioneins. Additionally, biostimulants improve nutrient use efficiency, reshape root architecture, and regulate hormonal crosstalk under stress conditions. The review highlights underlying molecular mechanisms, including the transcriptional reprogramming of genetic regulators (e.g., WRKY, NAC, bZIP), the activation of MAPK signaling cascades, and epigenetic modifications. By reinforcing plant defense systems and limiting metal translocation to edible tissues, biostimulants offer a sustainable and scalable approach to producing safe vegetables in metal-contaminated environments, aligning with global goals for food security and eco-resilient agriculture.