STOP2 Activates Transcription of Several Genes for Al- and Low pH-Tolerance that Are Regulated by STOP1 in Arabidopsis

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.