Brassica oleracea MATE Encodes a Citrate Transporter and Enhances Aluminum Tolerance in Arabidopsis thaliana

The secretion of organic acid anions from roots is an important mechanism for plant aluminum (Al) tolerance. Here we report cloning and characterizing BoMATE (KF031944), a multidrug and toxic compound extrusion (MATE) family gene from cabbage (Brassica oleracea). The expression of BoMATE was more abundant in roots than in shoots, and it was highly induced by Al treatment. The 14 C-citrate efflux experiments in oocytes demonstrated that BoMATE is a citrate transporter. Electrophysiological analysis and SIET analysis of Xenopus oocytes expressing BoMATE indicated BoMATE is activated by Al. Transient expression of BoMATE in onion epidermal cells demonstrated that it localized to the plasma membrane. Compared with the wild-type Arabidopsis, the transgenic lines constitutively overexpressing BoMATE enhanced Al tolerance and increased citrate secretion. In addition, Arabidopsis transgenic lines had a lower K+ efflux and higher H+ efflux, in the presence of Al, than control wild type in the distal elongation zone (DEZ). This is the first direct evidence that MATE protein is involved in the K+ and H+ flux in response to Al treatment. Taken together, our results show that BoMATE is an Al-induced citrate transporter and enhances aluminum tolerance in Arabidopsis thaliana.