TISSUE-CULTURE ISOLATION OF A 2ND MUTANT LOCUS FOR INCREASED THREONINE ACCUMULATION IN MAIZE

Regenerable maize (Zea mays L.) tissue cultures were selected for ability to grow in the presence of inhibitory (1.0-1.5 m M) concentrations of L-lysine plus L-threonine. Testcross kernels from one regenerated plant (LT20) segregated for wild-type and high free threonine concentration in a 1:1 ratio consistent with a single dominant gene for high free threonine. Free threonine concentrations (nmol/mg dry weight) increased an average of 29-fold in bulked F2 kernel samples from heterozygous mutant plants, and the total (free plus protein-bound) threonine concentration increased 68%. Increases in protein-bound methionine, lysine and glycine concentrations were also noted, suggesting a possible effect of the mutation on protein concentration and composition. Allelism tests with a previously selected mutant line, Ltr*19, showed that two unlinked, codominant genes conditioned the high free threonine phenotype. Based on a separate study of aspartate kinase feedback inhibition characteristics in the two mutant lines, we propose that the mutant alleles [gene and allele designations are according to guidelines for maize genetic nomenclature (Burnham et al. 1975)] be designated Ask-LT19 and Ask2-LT20 for the Ltr*19 and LT20 mutants, respectively. © 1990 Springer-Verlag.