Magnesium alleviates aluminum toxicity by promoting polar auxin transport and distribution and root alkalization in the root apex in populus.

Aims: Populus can tolerant high concentration Al stress. However, the mechanisms of Mg alleviation to Al toxicity in populus remain unknown. Methods: In the present study, adequate Mg was supplied to poplar under high concentration Al toxicity stress, and the pH gradients of root surface, Mg and Al uptake from root apex were detected using electrophysiological analysis, fluorescence staining, etc. Furthermore, auxin distribution and transport dynamic kinetics in root tips were also detected. Results: In the present study, we found that adequate supply of Mg alleviated high concentration Al toxicity to populus root growth. Mg promoted the pH gradients of root surface in populus, leading to alkalization in the root transition zone, which prevented Al toxicity to root elongation. The auxin distribution in root tips of populus was detected using DR5:GFP reporter. The transport and distribution of polar auxin in root transition zone were inhibited by Al toxicity, but partially alleviated by Mg supply. Further electrophysiological experiments using auxin transporter mutant pin-formed 2 (pin2) showed that Mg alleviated Al toxicity by PIN2-based polar auxin transport, which regulated root surface alkalization in the transition zone. Transcriptomic analysis and yeast complementation experiments using mrs2 mutants also reveals same relationships among Mg, auxin and Al. Conclusions: The present study suggests that Mg promotes polar auxin transport and distribution, which leads to the elevated root surface pH regulation, and further alleviates the Al toxicity. Our results partially clarify the mechanism of Mg in alleviating the toxicity of high concentration Al in woody plants. [ABSTRACT FROM AUTHOR]