Se uptake, transport and transformation in tea plants (Camellia sinensis) supplied with selenite or selenate under hydroponic conditions

Selenium (Se) is an essential micronutrient for humans and a beneficial element for plants. Tea plants ( Camellia sinensis ) efficiently accumulate Se and convert inorganic Se into organic Se species, making Se-enriched tea an important dietary source of bioavailable Se. However, the molecular mechanisms underlying Se uptake, transport, and transformation in tea plants remain largely unknown, especially the dynamic conversion process of inorganic Se into different organic Se species. A robust analytical method was established for the simultaneous determination of the five major Se species in tea plants, including Se(IV), Se(VI), selenocystine (SeCys 2 ), methylselenocysteine (MeSeCys), and selenomethionine (SeMet). Distinct metabolic patterns were observed in tea plants under different exogenous Se treatments. Specifically, absorbed Se(VI) is quickly translocated to the above-ground parts and subsequently converted into organic Se via a Se(IV) intermediate, while Se(IV) is completely retained in the roots of tea plants and rapidly transformed into organic Se, mainly transported to the leaves in the MeSeCys and SeMet species. Integrated transcriptome analysis identified multiple differentially expressed candidate genes from the SULTR , PHT , AQP , NHX , NRT , APS , APR , SIR , CYS , CGS , HMT , and MTR gene families, implying their potential roles in Se uptake, translocation, and transformation in tea plants, although their specific functions remain to be validated. In addition, the first day after Se(IV) treatment was identified as a critical stage for Se speciation transformation, accompanied by rapid accumulation of MeSeCys and SeMet and significant induction of genes involved in Se transport, and transformation. Overall, a putative molecular model describing Se uptake, transport, and transformation in tea plants was proposed. These findings provide both technical and theoretical foundations for elucidating the mechanisms underlying Se accumulation in tea plants, while also offering valuable insights for the development of Se-enriched tea production strategies.