A promoter-swap strategy between the AtALMT and AtMATE genes increased Arabidopsis aluminum resistance and improved carbon-use efficiency for aluminum resistance.

The primary mechanism of Arabidopsis aluminum (Al) resistance is based on root Al exclusion, resulting from Al-activated root exudation of the Al(3+) -chelating organic acids, malate and citrate. Root malate exudation is the major contributor to Arabidopsis Al resistance, and is conferred by expression of AtALMT1, which encodes the root malate transporter. Root citrate exudation plays a smaller but still significant role in Arabidopsis Al resistance, and is conferred by expression of AtMATE, which encodes the root citrate transporter. In this study, we demonstrate that levels of Al-activated root organic acid exudation are closely correlated with expression of the organic acid transporter genes AtALMT1 and AtMATE. We also found that the AtALMT1 promoter confers a significantly higher level of gene expression than the AtMATE promoter. Analysis of AtALMT1 and AtMATE tissue- and cell-specific expression based on stable expression of promoter-reporter gene constructs showed that the two genes are expressed in complementary root regions: AtALMT1 is expressed in the root apices, while AtMATE is expressed in the mature portions of the roots. As citrate is a much more effective chelator of Al(3+) than malate, we used a promoter-swap strategy to test whether root tip-localized expression of the AtMATE coding region driven by the stronger AtALMT1 promoter (AtALMT1(P)::AtMATE) resulted in increased Arabidopsis Al resistance. Our results indicate that expression of AtALMT1(P)::AtMATE not only significantly increased Al resistance of the transgenic plants, but also enhanced carbon-use efficiency for Al resistance.