Modulation of external and internal aluminum resistance by ALS3-dependent STAR1-mediated promotion of STOP1 degradation

The ALMT1 transporter aids malate secretion, chelating Al3+ ions to form nontoxic Al-malate complexes, believed to exclude Al from the roots. However, the extent to which malate secreted by ALMT1 is solely used for the exclusion of Al3+ or can be reutilized by plant roots for internal Al tolerance remains uncertain. In our investigation, we explored the impact of malate secretion on both external and internal Al resistance in Arabidopsis thaliana. Additionally, we delved into the mechanism by which the tonoplast-localized bacterial-type ATP-binding cassette (ABC) transporter complex STAR1/ALS3 promotes the degradation of the Al resistance transcription factor STOP1 to regulate ALMT1 expression. Our study demonstrates that the level of secreted malate influences whether the Al-malate complex is excluded from the roots or transported into root cells. The nodulin 26-like intrinsic protein (NIP) subfamily members NIP1;1 and NIP1;2, located in the plasma membrane, coordinate with STAR1/ALS3 to facilitate Al-malate transport from root apoplasm to the symplasm and eventually to the vacuoles for the internal Al detoxification. ALS3-dependent STAR1 interacts with and promotes the degradation of STOP1, regulating malate exudation. Our findings demonstrate the dual roles of malate exudation in external Al exclusion and Al absorption for internal Al detoxification.