Plants often experience aluminum (Al) toxicity in acidic soils, where the transcription factor SENSITIVE TO PROTON RHIZOTOXICITY1 (STOP1) plays a pivotal role in regulating transcriptional responses to Al stress. While posttranscriptional regulation of STOP1 under Al toxicity has been extensively studied, the mechanisms linking Al stress signals to STOP1 protein stability remain unclear. In this study, we employed multiscale pH imaging and noninvasive microtest (NMT) techniques to demonstrate that Al stress induces cytosolic acidification in the root apex of tomato (Solanum lycopersicum), which promotes the accumulation of SlSTOP1. This finding suggests that cytosolic acidification serves as a critical intermediate connecting Al stress to SlSTOP1 stabilization. Comparative transcriptomic analysis revealed that a significant subset of Al-responsive genes, including the known Al-resistance gene SlHAK5, are coregulated by both Al stress and low pH. Further functional characterization showed that SlHAK5 not only contributes to Al resistance but also plays a key role in maintaining cytosolic pH homeostasis under Al stress. In Slhak5 mutants, the expression of Al-induced genes was dysregulated, concomitant with attenuated cytosolic acidification. Correspondingly, SlSTOP1 accumulation was significantly reduced in Slhak5 mutants compared to wild-type (AC) plants under Al stress, indicating that SlSTOP1-mediated SlHAK5 expression feedback regulates cytosolic acidification. Additionally, Slhak5 mutants exhibited heightened sensitivity to proton stress. Collectively, our findings uncover a novel regulatory circuit involving SlSTOP1 and SlHAK5, which modulates SlSTOP1 stability through cytosolic acidification, thereby enhancing plant adaptation to proton and Al toxicity.

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