Selection procedures to isolate mutants resistant to toxic combinations of amino acids or amino analogues have been developed, using Arabidopsis thaliana as a model plant. Selection based on growth inhibition by lysine plus threonine, S-2-aminoethyl-L-cysteine, or on lysine alone was applied to the offspring of mutagenized seeds of Arabidopsis and has led to the isolation of a broad variety of mutants. Some of these mutants were characterized biochemically and genetically. The mutant RLT 11 was resistant to lysine plus threonine but also to various lysine and threonine analogues. No lysine or threonine accumulation could be detected. The resistance could be associated with an alteration in the uptake system of basic amino acids and could be ascribed to a recessive, single nuclear gene. RLT 40 and 4 were also resistant to toxic concentrations of lysine plus threonine. The threonine overproduction of these mutants appeared to be related to a partial loss of feedback control of the lysine-sensitive aspartate kinase isoenzyme. The decreased sensitivity to lysine inhibition of RLT 40 was due to a single dominant nuclear mutation, conferring a resistance to lysine plus threonine. RL 4 selected on toxic concentrations of lysine alone, showed a slightly increased level of free lysine. Aspartate kinase of this mutant seemed to be less sensitive to threonine inhibition compared with the wild type enzyme. RL 4 transmitted its resistance as a single recessive gene trait.
