Transcriptomic and physiological analyses reveal the effects of exogenous selenium on organic selenium synthesis and quality in tea

• Camellia sinensis possess the ability to accumulate Se. • Compared to selenite, foliar application of nano-Se demonstrates higher absorption and conversion efficiency into SeCys 2 and SeMet in tea. • Treatment with 100 mg/L nano-Se significantly improved the quality of tea. • Genes such as ABC transporters, CBSX1, SAMDC, AMY2, GT3 , and GST responded to the induction by exogenous Se. Camellia sinensis has significant industrial value and economic benefits. An appropriate concentration of selenium (Se) can promote the growth of C. sinensis . Here, we conducted a comparative analysis of the effects of selenite and nano-Se on Se/sulfur (S) absorption and transformation, photosynthesis, accumulation of nutritional metabolites, and antioxidant capacity in tea. Compared with selenite, nano-Se was more readily absorbed and converted into SeCys 2 and SeMet. Nano-Se facilitated greater S participation in plant growth and development. Treatments with 100 mg/L selenite and nano-Se enhanced the contents of photosynthetic pigments, soluble sugars, soluble proteins, tea polyphenols, tea polysaccharides, and total flavonoids in tea. Furthermore, treatment with nano-Se significantly enhanced the activities of antioxidant enzymes, such as glutathione reductase, catalase, and peroxidase, thereby boosting the antioxidant capacity of tea. Transcriptome analysis revealed that, compared with selenite, nano-Se significantly upregulated the expression of genes including ABC transporters, CBSX1, GT3, GST , and MSR , which play pivotal roles in Se absorption and transformation, metabolic synthesis, and the glutathione metabolic pathway. In conclusion, nano-Se (100 mg/L) is more suitable than selenite as a low-toxicity, high-efficiency Se fertilizer for the biofortification of Se-enriched tea. These findings provide a theoretical basis for the application of Se fertilizers in improving tea quality and elucidate the molecular mechanisms by which Se regulates the growth and development of C. sinensis .