Aluminium Alleviates Manganese Toxicity by Regulating the CsNRAMP5 Transporter in Tea Plant ( <i>Camellia sinensis</i> )

Manganese (Mn) is an essential micronutrient for plant growth. However, tea plants (Camellia sinensis) cultivated in acidic soils are at high risk of Mn toxicity due to elevated Mn bioavailability. Although aluminium (Al) alleviates Mn stress in Al-accumulating tea plants, the underlying mechanism remains unclear. Here, we demonstrate that high Al treatment effectively alleviates Mn toxicity symptoms by inhibiting Mn uptake and accumulation under the high Mn condition. Transcriptome analysis identified CsNRAMP5 as a key Mn transporter gene whose expression is induced by high Mn treatment but significantly suppressed by high Al treatment. Tissue-specific expression and subcellular localisation analysis indicated that CsNRAMP5 is a plasma membrane-localised protein with predominant expression in root. In situ hybridisation further showed that CsNRAMP5 is mainly expressed in cortical cells of the mature zone of tea roots. Heterologous expression in yeast and Arabidopsis confirmed that CsNRAMP5 mediates Mn transport. Furthermore, in vivo silencing of CsNRAMP5 significantly reduced Mn content in tea roots under both high Mn and combined Mn + Al treatment. Our findings demonstrate that Al alleviates Mn uptake in tea plants by down-regulating CsNRAMP5 expression, which provides novel molecular insight into Al-Mn antagonism and offers a potential target for breeding Mn-tolerant tea cultivars.