|Samac, Deborah - Debby|
Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/14/2001
Publication Date: 12/1/2001
Citation: Tesfaye, M., Temple, S.J., Allan, D.L., Vance, C.P., Samac, D.A. 2001. Overexpression of malate dehydrogenease in transgenic alfalfa enhances organic acid synthesis and confers tolerance to aluminum. Plant Physiology. 127:1836-1844. Interpretive Summary: A major limitation to food production world-wide is the development of acid soils. Currently, between 30-40% of the world's soils that can be farmed have an acidic pH. Under such low pH conditions, aluminum (Al) in soil becomes soluble and toxic to plant roots. Uptake of Al severely inhibits root growth and function. Plants suffering from Al toxicity have reduced capacity for water and nutrient uptake, and significantly reduced yields. In acid soils, elements needed for good plant nutrition such as phosphorus are often in forms that plants cannot use. Some plants that are more tolerant of Al produce increased amounts of organic acids when stressed by Al. The organic acids presumably bind to Al, reducing uptake of Al by plant roots. Thus, plants that secrete increased amounts of organic acids from roots are more Al tolerant. We have shown that alfalfa plants that over- express a unique form of the enzyme malate dehydrogenase (MDH) make increased amounts of organic acids and show enhanced Al tolerance. In acid soil, these lines accumulated higher amounts of phosphorus and out-yielded control plants and an alfalfa variety selected for Al tolerance. Over- expression of another enzyme involved in organic acid synthesis, phosphoenolpyruvate carboxylase, did not result in increased root exudation of organic acids or increased aluminum tolerance. This is the first study to show that over-expression of MDH improves tolerance to Al and increases phosphorus uptake. This approach could be used in other plants to enhance Al tolerance and crop yields.
Technical Abstract: Aluminum (Al) is a severe impediment to crop production in acid soil. Lime application to raise soil pH does not remedy subsoil acidity, furthermore liming may not always be practical. Addition of organic acids to plant nutrient solutions alleviates phytotoxic Al affects, presumably by chelating Al and rendering it less toxic. Thus, one approach to cope with soil acidity and Al toxicity is to develop plants with improved organic acid synthesis. We report that over-expression of a nodule-enhanced form of malate dehydrogenase (neMDH) in transgenic alfalfa (Medicago sativa L.) led to a 12-fold higher root exudation as well as 5-fold higher root concentration of oxalate, citrate, succinate, malate and acetate than in untransformed control plants. Over-expression of phosphoenolpyruvate carboxylase (PEPC) enzyme activity did not result in increased root exudation of organic acids. The degree of Al tolerance by transformed plants in hydroponic solutions and in naturally acid soil coincided with their patterns of organic acid exudation.