Melatonin and hydrogen peroxide alleviate cadmium toxicity in tomato via regulation of transporter genes, antioxidant activity, and photosynthetic efficiency

Cadmium (Cd) is a naturally occurring heavy metal in the Earth’s crust; but its increasing environmental contamination poses significant risks to tomato plants. Nevertheless, the effects of Cd exposure on tomato plant growth and the underlying physiological and biochemical mechanisms remain largely undocumented. Melatonin (Mel) is a renowned multifunctional biomolecule that enhances plant resilience to abiotic stresses and functions as a strong antioxidant. Additionally, the signaling molecule hydrogen peroxide (H₂O₂) is capable of regulating plant growth and development, even in very small concentrations. We examined the effects of Mel and H₂O₂ supplementation on the growth of tomato plants under Cd stress (12 mg kg⁻¹) as well as the photochemical reflectance index (PRI), anthocyanin reflectance index (ARI), water band index (WBI), photosynthetic efficiency, antioxidant capacity, Cd accumulation, and transporter gene expression. Cd exposure significantly reduced plant growth, chlorophyll indices, and photosynthetic performance. Nevertheless, the application of Mel plus H₂O₂ notably enhanced WBI, PRI, ARI, and leaf photosynthesis compared with Cd exposure alone. The tolerance to Cd stress was attributed to increased proline accumulation, enhanced antioxidant enzyme activity, reduced accumulation of reactive oxygen species, and prevention of lipid peroxidation and electrolyte leakage. Additionally, Mel and H₂O₂ reduced Cd accumulation in the roots. Moreover, Cd exposure markedly upregulated the transporter genes LeIRT1, LeNRAMP1, LeMRAMP3, and LeFER, confirming their central roles in Cd uptake and metal homeostasis. Particularly, Mel alone slightly attenuated LeIRT1 and LeNRAMP1 expression, indicating a protective effect against excessive Cd influx, whereas combined Mel and H₂O₂ treatments further amplified expression, with the Cd, Mel, and H₂O₂ combination producing the highest expression across all four genes. Overall, these results demonstrate that Mel and H₂O₂ supplementation mitigates Cd-induced stress in tomato by improving photosynthetic and antioxidant performance while fine-tuning transporter gene expression to balance Cd uptake and metal homeostasis.