Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: August 1, 1999
Publication Date: N/A
Technical Abstract: For the past twenty five years we have used several genetic approaches to enhance biological nitrogen fixation(BNF) and nitrate nitrogen (N) uptake in alfalfa (Medicago sativa L). Nodule specific forms of several of the enzymes involved in the assimilation of N from BNF into amides in alfalfa have been identified, sequenced, and their functions have been isolated to either the cytosol or specific organelles within infected root nodule cells. Alfalfa populations divergently selected for nodule enzyme activities, nodule mass, herbage dry weight or increased lateral or fibrous roots in nil N environments in the greenhouse demonstrated changes in BNF and herbage yield when evaluated in the N greenhouse, but no changes in yield were detected in field trials. Experiments using ineffectively nodulated alfalfa have shown that (i) some nodule enzymes are expressed in effectively and ineffectively nodulated plants and while others are only expressed in effectively nodulated, N2 fixing plants,(ii) normal alfalfa continues BNF under high rates of inorganic N fertilization, (iii) ineffectively nodulated alfalfa is 30 to 40 % more efficient at extracting nitrate-N from the subsoil compared to normal alfalfa, (iv) BNF inhibits absorption of inorganic N, (v) increased nitrate-N concentrations within an alfalfa plant decreases BNF. Current efforts to modify BNF and/or nitrate-N uptake in alfalfa include field selection for changes in root morphology and root elongation rate, divergent selection for nitrate-N uptake using bromide uptake as an analog under field conditions, insertion of antisense constructs and/or overexpression of nodule specific enzymes, and identifying genes involved in BNF and nitrate-Uptake through a genomics project sequencing Medicago truncatula L.