The zinc-finger protein STOP1 (sensitive to proton rhizotoxicity 1) regulates transcription of multiple genes critical for tolerance to aluminum (Al) and low pH in Arabidopsis. We evaluated the contributions of genes that are suppressed in the stop1 mutant to Al- and low pH-tolerance using T-DNA-inserted disruptants, and transgenic stop1 mutants expressing each of the suppressed genes. STOP2, a STOP1 homolog, partially recovered Al- and low pH-tolerance by recovering the expression of genes regulated by STOP1. Growth and root tip viability under proton stress were partially rescued in the STOP2-complemented line. STOP2 localized in the nucleus and regulated transcription of two genes (PGIP1 and PGIP2) associated with cell wall stabilization at low pH. GUS assays revealed that STOP1 and STOP2 showed similar cellular expression in the root. However, the expression level of STOP2 was much lower than that of STOP1. In a STOP1 promoter::STOP2-complemented line, Al tolerance was slightly recovered, concomitant with the recovery of expression of ALS3 (aluminum sensitive 3) and AtMATE (Arabidopsis thaliana multidrug and toxic compound extrusion), while the expression of AtALMT1 (aluminum-activated malate transporter 1) was not recovered. These analyses indicated that STOP2 is a physiologically minor isoform of STOP1, but it can activate expression of some genes regulated by STOP1.

Aluminum (Al) and proton (H + ) tolerances are essential traits for plants to adapt to acid soil environments. In Arabidopsis (Arabidopsis thaliana), these tolerances are mediated by a zinc-finger transcription factor, SENSITIVE TO PROTON RHIZOTOXICITY1 (AtSTOP1), which regulates the transcription of multiple genes critical for tolerance to both stressors. Here, the functions of orthologous proteins (STOP1-like proteins) in other plant species were characterized by reverse genetics analyses and in planta complementation assays. RNA interference of a gene for NtSTOP1 repressed Al and H + tolerances of tobacco (Nicotiana tabacum) roots. Tobacco roots released citrate in response to Al, concomitant with the up-regulated transcription of an ortholog of an Al tolerance gene encoding a citratetransporting multidrug and toxic compound extrusion protein. The RNA interference repression of NtSTOP1 blocked this process and also repressed the transcription of another orthologous gene for Al tolerance, ALUMINUM SENSITIVE3, which encodes a prokaryotetype transporter. These results demonstrated that NtSTOP1 regulates Al tolerance in tobacco through the transcriptional regulation of these genes. The in planta complementation assays revealed that other plant species, including woody plants, a legume, and a moss (Physcomitrella patens), possess functional STOP1-like proteins that can activate several H + and Al-tolerance genes in Arabidopsis. Knocking out the gene encoding the STOP1-like protein decreased the Al tolerance of P. patens. Together, our results strongly suggest that transcriptional regulation by STOP1-like proteins is evolutionarily conserved among land plants and that it confers the ability to survive in acid soils through the transcriptional regulation of Al-and H + -tolerance genes.