NH4+-N Promotes Fluoride Transport and NO3−-N Increases Fluoride Fixation in Roots of Camellia sinensis

Tea plants (Camellia sinensis) uniquely hyperaccumulate fluoride (F) and concurrently exhibit a preference for ammonium nitrogen (NH4+-N) over nitrate nitrogen (NO3−-N). However, the mechanistic basis for co-existence of NH4+-N preference and F hyperaccumulation in C. sinensis remains unexplored. Here, we investigated F accumulation and translocation with varying N supplies (0 mM and 2.854 mM N with NH4+-N:NO3−-N ratios of 3:1, 4:0 and 0:4) and F concentrations (0, 8 and 16 mg·L−1 NaF) to reveal the mechanism driving NH4+-N preference and F hyperaccumulation in C. sinensis. Results show that NH4+-N supply enhanced H+ efflux, mobilizing aluminum (Al) to form mobile Al-F complexes for translocation to shoots, thereby alleviating F toxicity in roots. This process was facilitated by transporters including CsCLCd, CsCLCe, CsCLCf2 and CsFEX. In contrast, NO3−-N promoted root sequestration of F as immobile calcium (Ca)-F complexes, exacerbating damage. Under NO3−-N supply, CsCLCb primarily mediated NO3− transport, while CsCLCc, CsCLCe, CsCLCf1, CsCLCf2 and CsFEX were involved in F transport. In leaves, CsCLCd, CsCLCe, CsCLCf1, CsCLCf2, CsCLCg and CsFEX mediated vacuolar sequestration under both N conditions. These findings elucidate that NH4+-N preference is mechanistically linked to F hyperaccumulation through an Al-assisted translocation pathway, which confers tolerance by exporting F from roots.