Altered cell wall properties are responsible for ammonium-reduced aluminium accumulation in rice roots

The phytotoxicity of aluminium (Al) ions can be alleviated by ammonium (NH4+) in rice and this effect has been attributed to the decreased Al accumulation in the roots. Here, the effects of different nitrogen forms on cell wall properties were compared in two rice cultivars differing in Al tolerance. An in vitroAl-binding assay revealed that neither NH4+ nor NO3- altered the Al-binding capacity of cell walls, which were extracted from plants not previously exposed to N sources. However, cell walls extracted from NH4+-supplied roots displayed lower Al-binding capacity than those from NO3--supplied roots when grown in non-buffered solutions. Fourier-transform infrared microspectroscopy analysis revealed that, compared with NO3--supplied roots, NH4+-supplied roots possessed fewer Al-binding groups (-OH and COO-) and lower contents of pectin and hemicellulose. However, when grown in pH-buffered solutions, these differences in the cell wall properties were not observed. Further analysis showed that the Al-binding capacity and properties of cell walls were also altered by pHs alone. Taken together, our results indicate that the NH4+-reduced Al accumulation was attributed to the altered cell wall properties triggered by pH decrease due to NH4+ uptake rather than direct competition for the cell wall binding sites between Al3+ and NH4+. Ammonium (NH4+) is known to alleviate Al toxicity by reducing Al accumulation in rice roots, but the responsible mechanisms are poorly understood. In this study, we found that the NH4+-reduced accumulation of Al was associated with decreased Al-binding groups (-OH and COO-) and contents of pectin and hemicellulose in both Al-tolerant and Al-sensitive rice cultivars. Furthermore, these alterations of cell wall properties were caused by pH decrease due to uptake of NH4+.