ZmMATE6 From Maize Encodes a Citrate Transporter That Enhances Aluminum Tolerance in Transgenic Arabidopsis Thaliana

Background: The yields of cereal crops grown on acidic soils are often reduced by aluminum (Al) toxicity because the prevalence of toxic Al 3+ cations increases as pH falls below 5.0. The Al-activated release from the roots of simple organic acids such as malate and citrate confers resistance to Al in a many plant species including maize ( Zea mays L.). The Al-dependent release of citrate from resistant lines of maize is controlled by ZmMATE1 which encodes a multidrug and toxic compound extrusion (MATE) transporter protein. ZmMATE1 resides on chromosome 6 and its expression in roots is induced up to four-fold by Al treatment. ZmMATE6 is another member of this family in maize whose expression is also increased by Al treatment. We investigated the function of this gene in more detail to determine whether it also contributes to Al resistance. Results: : Quantitative RT-PCR measurements found that in the absence of Al ZmMATE6 was expressed in the roots and leaves of Al-resistant and sensitive in-bred lines, and that expression levels in the root tips were lower than in the rest of the root tissue. Treatment with Al induced ZmMATE6 expression in all tissues but several other divalent or trivalent cations tested had no effect on expression. This expression pattern and the induction by Al treatment was confirmed in Arabidopsis lines transformed with β-glucuronidase where the ZmMATE6 promoter was used to drive expression. While subcellular localization of the ZmMATE6 protein could not be determined with confidence transgenic Arabidopsis lines expressing ZmMATE6 displayed a greater Al-activated release of citrate from the roots and were significantly more resistant to Al toxicity than controls. This was associated with reduced accumulation of Al in the root tissues. Conclusions: : Our results demonstrated that ZmMATE6 expression is induced by Al and functions as a citrate transporter. While some findings are consistent with ZmMATE6 contributing to Al resistance in maize, further research is required to confirm this result.