Characterization of the Complex Regulation of AtALMT1 Expression in Response to Phytohormones and Other Inducers

In Arabidopsis (Arabidopsis thaliana), malate released into the rhizosphere has various roles, such as detoxifying rhizotoxic aluminum (Al) and recruiting beneficial rhizobacteria that induce plant immunity. ALUMINUM-ACTIVATED MALATE TRANSPORTER1 (AtALMT1) is a critical gene in these responses, but its regulatory mechanisms remain unclear. To explore the mechanism of the multiple responses of AtALMT1, we profiled its expression patterns in wild-type plants, in transgenic plants harboring various deleted promoter constructs, and in mutant plants with defects in signal transduction in response to various inducers. AtALMT1 transcription was clearly induced by indole-3-acetic acid (IAA), abscisic acid (ABA), low pH, and hydrogen peroxide, indicating that it was able to respond to multiple signals, while it was not induced by methyl jasmonate and salicylic acid. The IAA-signaling double mutant nonphototropic hypocotyls4-1; auxin-responsive factor19-1 and the ABA-signaling mutant aba insensitive1-1 did not respond to auxin and ABA, respectively, but both showed an Al response comparable to that of the wild type. A synthetic microbe-associated molecular pattern peptide, flagellin22 (flg22), induced AtALMT1 transcription but did not induce the transcription of IAA- and ABA-responsive biomarker genes, indicating that both Al and flg22 responses of AtALMT1 were independent of IAA and ABA signaling. An in planta β-glucuronidase reporter assay identified that the ABA response was regulated by a region upstream (-317 bp) from the first ATG codon, but other stress responses may share critical regulatory element(s) located between -292 and -317 bp. These results illustrate the complex regulation of AtALMT1 expression during the adaptation to abiotic and biotic stresses.