Tissues resistant to lethal levels of equimolar L-lysine plus L-threonine (LT), 5-methyl-DL-tryptophan (5MT, a tryptophan analog), or S-2-aminoethyl-L-cysteine (AEC, a lysine analog) were selected from maize callus capable of plant regeneration (H99 and W77-R3019 genotypes).Resistance to LT resulted from resistant calli having a 19 times greater level of free threonine than wild type tissues. The resistance was expressed in roots of whole plants; threonine levels were two to nine times greater in leaves and kernels of resistant plants than in wild type plants. Slightly greater levels of isoleucine, lysine and methionine were also noted, particularly in the kernel. Genetic studies with individual resistant plants did not always produce inheritance ratios typical of simple Mendelian inheritance, but by the third generation after plant regeneration a trend towards homozygosity was apparent and the data suggests that LT resistance is inherited as a single dominant nuclear gene.Resistance to 5MT resulted from resistant calli having a 133 to 161 times greater level of free tryptophan than wild type tissues. Also, phenylalanine was 22 to 30 times as great and histidine, tyrosine and valine were about two times as great as in wild type tissues. Resistance was expressed in roots of whole plants, and tryptophan levels were at least 2000 times greater in resistant than in wild type plants. Phenylalanine was also 32 times greater. All regenerant plants resistant to 5MT were both male and female sterile.Resistance to AEC was caused by decreased AEC uptake by the callus tissue and was not due to increased levels of free lysine. Plants were not regenerated from this callus.
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