Indium-Induced Toxicity Disrupts Redox Balance, Impairs Photosynthesis, and Induces Cell Wall Lignification in Brassica napus

Indium pollution from high-tech industries and e-waste is an emerging environmental concern, posing risks to crop health and food safety. Despite its increasing presence in agricultural ecosystems, the phytotoxic effects of indium and the underlying tolerance mechanisms in crops remain largely unknown. This study investigates physiological, biochemical, and molecular responses of two Brassica napus L. genotypes, ZS-11 (tolerant) and ZZ-1510 (susceptible), under escalating indium exposure. Indium toxicity significantly inhibited growth, more severely in ZZ-1510, due to greater root accumulation and restricted translocation. High indium levels induced oxidative stress through excessive ROS production, disrupted nutrient uptake (Mg, K, Zn, Mn, and Fe), and caused ultrastructural damage in guard cells, mesophyll, and root tips, as evidenced by elevated MDA. Antioxidant enzyme activities (GSH, SOD, CAT, APX, and POD) increased at 50-100 mg L^-1 but declined at 200 mg L^-1, with moderate downregulation of stress-responsive genes (BnGR, BnPAL, and BnMT-1). Interestingly, ZS-11 showed early upregulation of lignin biosynthesis genes (BnF5H, BnCAD5, BnCCR2, and Bn4CL), enhancing cell wall reinforcement. Conversely, ZZ-1510 exhibited downregulation of chlorophyll biosynthesis genes (BnCAO, BnCHLG, BnPOR), and upregulation of senescence marker BnSAG12, leading to impaired photosynthesis (lower chlorophyll, impaired gas exchange, and diminished fluorescence) and premature wilting. These genotype-specific responses reveal key mechanisms and biomarkers for developing indium-resilient crops, supporting risk assessment and food safety in contaminated environments.