SlSAURs, targeted by SlSTOP1, inhibit SlPP2C.Ds to modulate H+-ATPase and tomato root elongation under aluminum stress

Aluminum (Al) stress, a prevalent constraint in acid soils, is known to inhibit 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 SlSAUR (Small auxin up-regulated RNA) proteins, which were downstream of the key Al stress-responsive transcription factor SlSTOP1 and its enhancer SlSZP1, in modulating root elongation under Al stress in tomato (Solanum lycopersicum). Our findings demonstrated that tomato lines with SlSAURs knockout exhibited shorter root lengths when subjected to Al stress. Further investigation into the underlying mechanisms revealed that SlSAURs interact with D-clade type 2C protein phosphatases, specifically SlPP2C.Ds. This interaction was pivotal as it suppresses the phosphatase activity, leading to the derepression of SlPP2C.Ds inhibitory effect on H+-ATPase. Consequently, this promoted cell expansion and root elongation under Al stress conditions. Our research significantly contributes to the current 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 provides a novel perspective on the adaptive strategies employed by plants to cope with Al toxicity. This knowledge may pave the way for the development of tomato cultivars with enhanced Al stress tolerance, thereby improving crop productivity in acid soils.