Harnessing biocompatible bio-based nanoprotectants for phytohormone modulation and heavy metal detoxification in sustainable crop production.

Bio-based nanomaterials (BNMs) have emerged as promising modulators of plant growth hormone (phytohormone) signaling pathways under combined heavy metal (HM) stress conditions. This review investigates how BNMs influence hormone-regulated stress signaling networks to improve plant tolerance to HM stress at biochemical, molecular and physiological levels. Plant extract and microbial-mediated synthesis strategies use biological metabolites as reducing and stabilizing agents, providing environmentally compatible routes for agricultural nanomaterials. BNMs may influence phytohormone-regulated stress responses through multiple, partly interconnected mechanisms, including surface-mediated interactions, redox modulation, metal chelation and transcriptional regulation of hormone-related pathways. By combining ROS attenuation, metal chelation and stress-responsive gene regulation, BNMs may help preserve phytohormone biosynthesis and signaling while reducing HM toxicity. This review highlights how BNMs reshape gene expression networks associated with phytohormone biosynthesis and signaling, including abscisic acid, ethylene, auxin and jasmonic acid. It also examines how these nanomaterials enhance plant defense systems by upregulating stress-responsive genes and antioxidant enzyme activities. Furthermore, we discuss current challenges in synthesis standardization, scale-up, delivery efficiency and field validation, together with future directions for optimizing BNM-phytohormone interactions under multiple metal stress. Finally, this review highlights BNMs as environmentally compatible tools for improving crop performance in HM-contaminated soils, while emphasizing that precise hormone-pathway targeting requires further mechanistic and field-level validation.