|Samac, Deborah - Debby|
Submitted to: North American Alfalfa Improvement Conference
Publication Type: Abstract Only
Publication Acceptance Date: 7/17/2000
Publication Date: N/A
Technical Abstract: Aluminum (Al) toxicity is a major factor limiting crop production in acid soils, which make up about 40% of the world's arable land. Aluminum inhibits root growth and uptake of water and nutrients. The roots of some acid soil tolerant crop varieties secrete organic acids that chelate Al and prevent uptake of Al by the root apex. In an effort to increase organic acid secretion in alfalfa, we produced alfalfa using nodule enhanced forms of malate dehydrogenase (neMDH) and phosphoenolpyruvate carboxylase (nePEPC)cDNAs under the control of the CaMV 35S promoter. We identified lines with higher MDH and PEPC enzyme activity, enzyme-protein and mRNA transcript levels. Plants overexpressing neMDH showed considerably higher overall organic acid synthesis and exudation. Roots of these transgenic lines showed up to 6.5-fold higher amounts of oxalate, citrate, succinate and malate than control plants. Tolerance to Al by selected lines was tested using root elongation over a 24-hour period. Relative root growth o selected lines in a 50 uM AlCl3 hydroponic solution, also containing 0.5 mM CaCl2, pH 4.3, was 3.4-fold greater than relative root growth of wild type plants. When plants were grown in pots filled with a highly weathered soil (pHH2O = 4.0, AlKCl = 71 ppm), shoot and root dry weight of transgenic plants increased up to 1.5-fold over controls. An alfalfa germplasm with increased acid tolerance, GA-AT, was developed previously by recurrent selection. Root organic acid profiles of both GA-AT and control alfalfa plants were similar, suggesting that the mechanism of Al tolerance in GA-AT may not involve enhanced organic acid release. F1 progeny of crosses between GA-AT and selected transgenic lines are being evaluated for Al tolerance and organic acid synthesis.