Isolation and Initial Characterization of Constitutive Nitrate Reductase-Deficient Mutants NR328 and NR345 of Soybean (Glycine max)

Two nitrate reductase deficient mutants of soybean (Glycine max [L.] Merr. cv Bragg) were isolated from approximately 10,000 M(2) seedlings, using a direct enzymic assay in microtiter plates. Stable inheritance of NR345 and NR328 phenotypes has been demonstrated through to the M(5) generation. Both mutants were affected in constitutive nitrate reductase activity. Assayable activities of cNR in nitrate-free grown seedlings was about 3 to 4% of the control for NR345 and 14 to 16% of the control for NR328. Both mutants expressed inducible NR during early plant development and were sensitive to nitrate and urea inhibition of nodulation. These new mutants will allow an extension of the characterization of nitrate reductases and their function in soybean. Preliminary evidence indicates that NR345 is similar to the previously isolated mutant nr(1), while NR328 is different.