Magnesium (Mg(2+)) is an essential macronutrient for plant growth and development, and the CorA/MRS2/MGT-type Mg(2+) transporters play important roles in maintaining Mg(2+) homeostasis in plants. Although the MRS2/MGT genes have been identified in two model plant species, Arabidopsis and rice, a comprehensive analysis of the MRS2/MGT gene family in other plants is lacking. In this work, 12 putative MRS2/MGT genes (ZmMGT1- ZmMGT12) were identified in maize and all of them were classified into five distinct subfamilies by phylogenetic analysis. A complementation assay in the Salmonella typhimurium MM281 strain showed that five representatives of the 12 members possess Mg(2+) transport abilities. Inhibition of ZmMGT protein activity using the hexaamminecobalt (III) (Co-Hex) inhibitor indicated that the ZmMGT protein mediated both low-affinity and high-affinity Mg(2+) transport in maize. A semi-quantitative reverse transcription-PCR (RT-PCR) analysis revealed that eight genes were constitutively expressed in all of the detected tissues, with one being specifically expressed in roots and three having no detectable expression signals. A quantitative RT-PCR analysis showed that some ZmMGT members displayed differential responses to Mg(2+) deficiency and aluminum (Al) stress. Furthermore, root growth inhibition and Mg(2+) accumulation analyses in two maize inbred lines, which conferred different levels of Al tolerance, revealed that ZmMGT proteins contributed to the Al resistance of the Al tolerance genotype. We hypothesize that ZmMGT family members function as Mg(2+) transporters and may play a role in linking Mg(2+) deficiency and Al stress responses. Our results will be valuable in a further analysis of the important biological functions of ZmMGT members in maize.

Aluminum (Al) toxicity is the primary limiting factor that affects crop yields in acid soil. However, the genes that contribute to the Al tolerance process in maize are still poorly understood. Previous studies have predicted that ZmAT6 is a novel protein which could be upregulated under Al stress condition. Here, we found that ZmAT6 is expressed in many tissues and organs and can be dramatically induced by Al in both the roots and shoots but particularly in the shoots. The overexpression of ZmAT6 in maize and Arabidopsis plants increased their root growth and reduced the accumulation of Al, suggesting the contribution of ZmAT6 to Al tolerance. Moreover, the ZmAT6 transgenic maize plants had lower contents of malondialdehyde and reactive oxygen species (ROS), but much higher proline content and even lower Evans blue absorption in the roots compared with the wild type. Furthermore, the activity of several enzymes of the antioxidant system, such as peroxidase (POD), superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), increased in ZmAT6 transgenic maize plants, particularly SOD. Consistently, the expression of ZmSOD in transgenic maize was predominant upregulated by Al stress. Taken together, these findings revealed that ZmAT6 could at least partially confer enhanced tolerance to Al toxicity by scavenging ROS in maize.

Aluminum (Al) toxicity is a major factor limiting crop production and plant growth in acid soils. The complex inheritance of Al toxicity and tolerance mechanisms in maize has uncharacterized yet. In thsi study, the maize inbred line 178 seedlings were treated with 200 μmol L-1 CaCl 2 +0 μmol L-1 AlCl 3 (control) and 200 μmol L-1 CaCl 2 +60 μmol L-1 AlCl 3 (Al treatment) for 1 and 6 h, respectively. The experiment was repeated three times. Then a detailed temporal analysis of root gene expression was performed using an Agilent GeneChip with 34 715 genes, only the genes showing more than 2.0-fold difference (P<0.01) between the control and the Al treatment maize seedlings was analyzed further. Thus, a total of 832 different expression genes, 689 significantly up-regulated and 143 down-regulated, were identified after the seedlings were treated with Al for 6 h. And 60 genes, 59 up-regulated and 1 down-regulated, were also detected after the seedlings were treated for 1 h. Replicated transcriptome analyses further showed that about 61% of total significantly genes could be annotated based on plant genome resources. Quantitative real-time PCR (qRT-PCT) of some selected candidate genes was used to demonstrate the microarray data, indicating significant differences between the control and Al treated seedlings. Exposure to Al for 6 h triggered changes in the transcript levels for several genes, which were primarily related to cell wall structure and metabolism, oxidative stress response, membrane transporters, organic acid metabolism, signaling and hormones, and transcription factors, etc. After Al treated for 1 h, differential abundance of transcripts for several transporters, kinase, and transcription factors were specifically induced. In this study, the diversity of the putative functions of these genes indicates that Al stress for a short stage induced a complex transcriptome changes in maize. These results would further help us to understand rapid and early mechanisms of Al toxicity and tolerance in maize regulated at the transcriptional level.

ZmMGT10 was specifically expressed in maize roots and induced by a deficiency of magnesium. Overexpression of ZmMGT10 restored growth deficiency of the Salmonella typhimurium MM281 strain and enhanced the tolerance in Arabidopsis to stress induced by low magnesium levels by increasing uptake of Mg via roots. CorA/MRS2/MGT-type Mg transporters play a significant role in maintaining magnesium (Mg) homeostasis in plants. Although the maize CorA/MRS2/MGT family comprises of 12 members, currently no member has been functionally characterized. Here, we report the isolation and functional characterization of ZmMGT10 from the maize MRS2/MGT gene family. ZmMGT10 has a typical structure feature which includes two conserved TMs near the C-terminal end and an altered AMN tripeptide motif. The high sequence similarity and close phylogenetic relationship indicates that ZmMGT10 is probably the counterpart of Arabidopsis AtMGT6. The complementation of the Salmonella typhimurium mutated MM281 strain indicates that ZmMGT10 possesses the ability to transport Mg. ZmMGT10 was specifically expressed in the plant roots and it can be stimulated by a deficiency of Mg. Transgenic Arabidopsis plants which overexpressed ZmMGT10 grew more vigorously than wild-type plants under low Mg conditions, exhibited by longer root length, higher plant fresh weight and chlorophyll content, suggesting ZmMGT10 was essential for plant growth and development under low Mg conditions. Further investigations found that high accumulation of Mg occurred in transgenic plants attributed to improved Mg uptake and thereby enhanced tolerance to Mg deficiency. Results from this investigation illustrate that ZmMGT10 is a Mg transporter of maize which can enhance the tolerance to Mg deficient conditions by improving Mg uptake in the transgenic plants of Arabidopsis.

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