Aluminum (Al) stress, a prevalent constraint in acidic soils, inhibits plant growth by inhibiting root elongation through restricted cell expansion. The molecular mechanisms of Al-induced root inhibition, however, are not fully understood. This study aimed to elucidate the role of Small Auxin-up RNAs (SlSAURs), which function downstream of the key Al stress-responsive transcription factor SENSITIVE TO PROTON RHIZOTOXICITY 1 (SlSTOP1) and its enhancer STOP1-INTERACTING ZINC-FINGER PROTEIN 1 (SlSZP1), in modulating root elongation under Al stress in tomato (Solanum lycopersicum). Our findings demonstrated that tomato lines with knocked-out SlSAURs exhibited shorter root lengths when subjected to Al stress. Further investigation into the underlying mechanisms revealed that SlSAURs interact with Type 2C Protein Phosphatases (SlPP2Cs), specifically D-clade Type 2C Protein Phosphatases (SlPP2C.Ds). This interaction was pivotal as it suppresses the phosphatase activity, leading to the degradation of SlPP2C.D's inhibitory effect on plasma membrane H+-ATPase. Consequently, this promoted cell expansion and root elongation under Al stress. These findings increase our understanding of the molecular mechanisms by which Al ions modulate root elongation. The discovery of the SlSAUR-SlPP2C.D interaction and its impact on H+-ATPase activity also provides a perspective on the adaptive strategies employed by plants to cope with Al toxicity, which may lead to the development of tomato cultivars with enhanced Al stress tolerance, thereby improving crop productivity in acidic soils.
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