Physiological and Transcriptomic Insights into Melatonin-Mediated Alleviation of Cadmium Stress in the Endangered Plant Plantago fengdouensis

Plantago fengdouensis is an endangered species threatened by habitat loss, population fragmentation, and cadmium (Cd) contamination. This study evaluated the effects of exogenous melatonin (MT) on Cd tolerance from seed germination to the seedling stage. In Petri-dish assays, 50–100 mg L−1 Cd markedly inhibited germination; 50 mg L−1 was selected as the lowest concentration producing severe inhibition while retaining sufficient germination for mitigation testing, and 100 μM MT provided the strongest alleviation. In pot experiments, MT promoted seedling growth, shoot and root biomass accumulation, and root and leaf development while reducing Cd accumulation and root-to-shoot translocation. Physiological analyses showed that 200–250 μM MT most effectively alleviated Cd-associated damage by maintaining photosynthesis, enhancing antioxidant and osmotic-adjustment responses, and modulating endogenous hormones. Transcriptome analysis identified 4871 differentially expressed genes (DEGs) between C50M200 and C50M0, with prominent enrichment in plant hormone signaling, cell-wall/plasma-membrane functions, secondary metabolism, and photosynthesis. Exploratory weighted gene co-expression network analysis (WGCNA) further identified treatment-associated modules and candidate hub genes. Overall, the data support coordinated MT-associated responses involving stronger antioxidant defense, reduced Cd accumulation/translocation, and hormone-signaling remodeling. Because transcriptomes were obtained from leaves and WGCNA included nine samples, the inferred molecular mechanisms should be regarded as candidate pathways pending targeted expression and functional validation.