STOP1-regulated () is involved in proton/malate co-secretion for Al tolerance in Arabidopsis.

Proton (H) release is linked to aluminum (Al)-enhanced organic acids (OAs) excretion from the roots under Al rhizotoxicity in plants. It is well-reported that the Al-enhanced organic acid excretion mechanism is regulated by SENSITIVE TO PROTON RHIZOTOXICITY1 (STOP1), a zinc-finger TF that regulates major Al tolerance genes. However, the mechanism of H release linked to OAs excretion under Al stress has not been fully elucidated. Recent physiological and molecular-genetic studies have implicated the involvement of SMALL AUXIN UP RNAs (SAURs) in the activation of plasma membrane H-ATPases for stress responses in plants. We hypothesized that STOP1 is involved in the regulation of Al-responsive , which may contribute to the co-secretion of protons and malate under Al stress conditions. In our transcriptome analysis of the roots of the (sensitive to proton rhizotoxicity1) mutant, we found that STOP1 regulates the transcription of one of the , namely . Furthermore, we observed that the expression of was induced by Al and repressed in the STOP1 T-DNA insertion knockout (KO) mutant (-KO). Through in silico analysis, we identified a functional STOP1-binding site in the promoter of . Subsequent in vitro and in vivo studies confirmed that STOP1 directly binds to the promoter of . This suggests that STOP1 directly regulates the expression of under Al stress. We next examined proton release in the rhizosphere and malate excretion in the T-DNA insertion KO mutant of SAUR55 (), in conjunction with -KO. Both and -KO suppressed rhizosphere acidification and malate release under Al stress. Additionally, the root growth of was sensitive to Al-containing media. In contrast, the overexpressed line of enhanced rhizosphere acidification and malate release, leading to increased Al tolerance. These associations with Al tolerance were also observed in natural variations of Arabidopsis. These findings demonstrate that transcriptional regulation of by STOP1 positively regulates H excretion via PM H-ATPase 2 which enhances Al tolerance by malate secretion from the roots of Arabidopsis. The activation of PM H-ATPase 2 by SAUR55 was suggested to be due to PP2C.D2/D5 inhibition by interaction on the plasma membrane with its phosphatase. Furthermore, RNAi-suppression of in tobacco shows suppression of rhizosphere acidification under Al stress, which was associated with the suppression of orthologs, which are inducible by Al in tobacco. It suggests that transcriptional regulation of Al-inducible s by STOP1 plays a critical role in OAs excretion in several plant species as an Al tolerance mechanism.