Functional characterization of plasma membrane-localized organic acid transporter (CsALMT1) involved in aluminum tolerance in <Emphasis Type="Italic">Camelina sativa</Emphasis> L.

Aluminum (Al) toxicity is a major limiting factor that inhibits root elongation and decreases crop production in acidic soils. The symptoms of inhibited root growth include a reduced uptake of nutrients because the roots become stubby and brittle. The release of organic anions from roots can protect a plant from Al toxicity. The mechanism relies on the efflux of organic anions, such as malate or citrate, which protect roots by chelating the Al³⁺. In this study, homologs of TaALMT1, a Camelina gene that encodes an aluminum-activated malate transporter, were investigated. The expression of this gene was induced by Al in the root, but not in the shoots. Using green fluorescent protein (GFP) fusion constructs and Western-blot analysis, we observed that CsALMT1 was localized in the plasma membrane. Also, to determine the degree to which Al tolerance was affected by malate secretion in Camelina root, we generated CsALMT1 overexpressing plants. CsALMT1 overexpressing transgenic plants showed a higher root elongation rate than the wild-type plant. Damaged cell staining analysis by hematoxylin under 25 µM Al treatment for 2, 4, and 6 h showed a pattern of less damage in CsALMT1 transgenic plants than in wild-type plant, especially in the root elongation zone. Furthermore, the rate of increase of secretion of organic acid in overexpressed plants after Al treatment was higher than that in the wild-type plant. In addition, in the Al-specific dye morin staining on root protoplast under 50 µM Al treatment, less Al accumulation was observed in the CsALMT1 transgenic plants than in the wild-type plant. The Al contents in the roots of the transgenic plants were at a lower level than those in the wild-type plant. These results show that the overexpression of CsALMT1 improves Al tolerance by increasing the release of malate from the root to the soil and, thereby, detoxifies the Al³⁺.