STOP1 regulates multiple genes that protect arabidopsis from proton and aluminum toxicities.

The Arabidopsis (Arabidopsis thaliana) mutant stop1 (for sensitive to proton rhizotoxicity1) carries a missense mutation at an essential domain of the histidine-2-cysteine-2 zinc finger protein STOP1. Transcriptome analyses revealed that various genes were down-regulated in the mutant, indicating that STOP1 is involved in signal transduction pathways regulating aluminum (Al)- and H(+)-responsive gene expression. The Al hypersensitivity of the mutant could be caused by down-regulation of AtALMT1 (for Arabidopsis ALUMINUM-ACTIVATED MALATE TRANSPORTER1) and ALS3 (ALUMINUM-SENSITIVE3). This hypothesis was supported by comparison of Al tolerance among T-DNA insertion lines and a transgenic stop mutant carrying cauliflower mosaic virus 35SAtALMT1. All T-DNA insertion lines of STOP1, AtALMT1, and ALS3 were sensitive to Al, but introduction of cauliflower mosaic virus 35SAtALMT1 did not completely restore the Al tolerance of the stop1 mutant. Down-regulation of various genes involved in ion homeostasis and pH-regulating metabolism in the mutant was also identified by microarray analyses. CBL-INTERACTING PROTEIN KINASE23, regulating a major K(+) transporter, and a sulfate transporter, SULT3;5, were down-regulated in the mutant. In addition, integral profiling of the metabolites and transcripts revealed that pH-regulating metabolic pathways, such as the gamma-aminobutyric acid shunt and biochemical pH stat pathways, are down-regulated in the mutant. These changes could explain the H(+) hypersensitivity of the mutant and would make the mutant more susceptible in acid soil stress than other Al-hypersensitive T-DNA insertion lines. Finally, we showed that STOP1 is localized to the nucleus, suggesting that the protein regulates the expression of multiple genes that protect Arabidopsis from Al and H(+) toxicities, possibly as a transcription factor.