STOP1, a Cys2/His2 type zinc-finger protein, plays critical role in acid soil tolerance in Arabidopsis.

Under acid soil condition, rhizotoxic ions such as aluminum and protons cause severe yield loss in food and biomass production via inhibition of root growth and/or enhancement of sensitivity to drought stress. Therefore, improvement of crop tolerance to rhizotoxic ions would be an important target for crop breeding, and identification of key genes that regulate tolerance to both aluminum and proton ions should provide a solution to this limitation. We recently isolated a mutant that shows hyper-sensitivity to protons in root growth, namely stop1 (sensitive to proton rhizotoxicity). The stop1 mutant was isolated from an ethyl methanesulfonate mutagenized population by measuring root growth under low pH (pH 4.3). Interestingly, stop1 also shows hypersensitivity to aluminum ion, but not to other rhizotoxic ions. Cloning of the STOP1 gene revealed that it encodes a Cys2/His2 zinc-finger type transcription factor, and a conserved His residue was replaced with Tyr in the predicted amino acid sequence for the mutant gene. This indicates that STOP1 is involved in the signal transduction pathway of proton and aluminum tolerance. Indeed, under acidic condition, stop1 failed to induce the AtALMT1 gene, which encodes one of the major factors for aluminum tolerance. By searching a public database, we have identified genes homologous to STOP1 in rice, maize and some other plants. Thus, the STOP1 family genes may act as a key factor for acid tolerance in a wide variety of plants.