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

Abstract

• 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.