Characterization of AtALMT1 Expression in Aluminum-Inducible Malate Release and Its Role for Rhizotoxic Stress Tolerance in Arabidopsis

Abstract

Malate transporters play a critical role in aluminum (Al) tolerance responses for some plant species, such as Arabidopsis (Arabidopsis thaliana). Here, we further characterize AtALMT1, an Arabidopsis aluminum-activated malate transporter, to clarify its specific role in malate release and Al stress responses. Malate excretion from the roots of accession Columbia was sharply induced by Al, which is concomitant with the induction of AtALMT1 gene expression. The malate release was specific for Al among rhizotoxic stressors, namely cadmium, copper, erbium, lanthanum, sodium, and low pH, which accounts for the specific sensitivity of a null mutant to Al stress. Al-specific malate excretion can be explained by a combined regulation of AtALMT1 expression and activation of AtALMT1 protein, which is specific for Al. Although low pH treatment slightly induced gene expression, other treatments did not. In addition, malate excretion in Al-activated seedlings was rapidly stopped by removing Al from the solution. Other rhizotoxic stressors were not effective in maintaining malate release. Protein kinase and phosphatase inhibitor studies indicated that reversible phosphorylation was important for the transcriptional and posttranslational regulation of AtALMT1. AtALMT1 promoter-β-glucuronidase fusion lines revealed that AtALMT1 has restricted expression within the root, such that unnecessary carbon loss is likely minimized. Lastly, a natural nonsense mutation allele of AtALMT1 was identified from the Al-hypersensitive natural accession Warschau-1.

Root-Secreted Malic Acid Recruits Beneficial Soil Bacteria

“…In plants, increased accumulation of L-MA can be triggered by elevated biosynthesis of one or more of the tricarboxylic acid MA precursors, fumarate, oxaloacetate, or pyruvate, by fumarase, malate dehydrogenase, and malic enzyme, or by reduced MA turnover to pyruvate or oxaloacetate by malic enzyme and malate dehydrogenase, respectively (Casati et al, 1999). Alternatively, upregulation of the malate transporter (AtALMT1) augments root MA secretions (Kobayashi et al, 2007). Future lines of study will investigate the involvement of MA secretion on induction of bacterial volatile signaling in B. subtilis, which has been reported to be involved in the induction of ISR in B. subtilis-colonized plants .…”

“…To check whether aerial leaf infection with Pst DC3000 transcriptionally regulates AtALMT1 expression, we employed an Arabidopsis transgenic line carrying an AtALMT1 promoter::GUS fusion construct (Kobayashi et al, 2007). The 20-d-old AtALMT1 promoter::GUS Arabidopsis lines were leaf infiltrated with Pst DC3000, NPS3121, Pseudomonas aeruginosa PAO1, Escherichia coli OP50, and also roots treated with 4 mM AlCl 3 as a positive control.…”

“…The 20-d-old plants were leaf infiltrated with Pst DC3000, NPS3121, OP50, and PAO1 and a positive control of AlCl 3 root treatment was also included in addition to an untreated control. After 12 h of treatment, the roots were stained and imaged for the expression of AtALMT::GUS according to published protocol (Kobayashi et al, 2007).…”

Microbe-Associated Molecular Patterns-Triggered Root Responses Mediate Beneficial Rhizobacterial Recruitment in Arabidopsis

“…Alternatively, up-regulation of ALMT1 elicits root MA secretions (Kobayashi et al, 2007). It has been argued that most of the Gram-positive and Figure 6.…”

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

“…This possibility would be supported by a recent study on AtALMT1, in which the involvement of irreversible protein phosphorylation during AtALMT1 induction and expression was revealed. 10 In that study, both H + and Al 3+ promoted AtALMT1 expression, but this gene is not involved in H + tolerance because its disruption did not affect H + sensitivity. This fact, together with our previous finding of the hyper-sensitivity of the stop1 mutant to H + indicates that other genes may play a critical role in H + tolerance, which is regulated by STOP1.…”