Loss of function of Arabidopsis NADP-malic enzyme 1 results in enhanced tolerance to aluminum stress.

In acidic soils, aluminum (Al) toxicity constitutes a significant limitation to crop production worldwide. Given its Al binding capacity, malate allows internal as well as external detoxification strategies to cope with Al stress, but little is known about the metabolic processes involved in this response. Here, we analyzed the relevance of NADP-dependent malic enzyme (NADP-ME), which catalyzes the oxidative decarboxylation of malate, in Al tolerance. Plants lacking NADP-ME1 (nadp-me1) display lower inhibition of root elongation after Al treatment, compared to the wild type (wt). Moreover, wt roots exposed to Al show a drastic decrease in NADP-ME1 transcript level. Although malate levels in seedlings and root exudates are similar in nadp-me1 and wt, a significant increase of intracellular malate is observed in roots of nadp-me1 after a long exposure to Al. The nadp-me1 plants also show lower H₂ O₂ content in the root apices treated with Al and no inhibition of root elongation when exposed to glutamate, an amino acid implicated in Al signaling. Proteomic studies showed several differentially expressed proteins involved in signal transduction, primary metabolism, protection against biotic and other abiotic stimulus and redox processes in nadp-me1, which may participate directly or indirectly in Al tolerance. The results indicate that NADP-ME1 is involved in adjusting the malate levels in the root apex, and its loss results in an increased content of this organic acid. Furthermore, the results suggest that NADP-ME1 affects signaling processes, such as the generation of reactive oxygen species and those that involve glutamate, which could lead to root growth inhibition.