Inheritance and expression of lysine plus threonine resistance selected in maize tissue culture

Selection for resistance to growth inhibition by lysine plus threonine in equimolar concentrations (LT) in tissue cultures of maize yielded a stable resistant line, LT19. Genetic analysis of progeny of plants regenerated from LT19 showed that LT resistance was inherited as a single dominant nuclear gene, temporarily designated Ltr(*)-19. Tissue cultures initiated from resistant embryos required 5-10 times higher levels of LT to inhibit growth than did cultures from LT-sensitive embryos. LT resistance in Ltr(*)-19 was expressed as much reduced inhibition of root and shoot growth in the presence of LT. The free pool of threonine was increased 6 times in cultures initiated from immature embryos of LT-resistant plants, and 75-100 times in kernels homozygous for Ltr(*)-19, as compared to cultures and kernels from LT-sensitive embryos and plants, respectively. Overproduction of free threonine increased the total threonine content in homozygous Ltr(*)-19 kernels by 33-59%. The results demonstrate that LT resistance selected with tissue culture methods is heritable and is expressed in cultures, seedlings, and kernels. Furthermore, they demonstrate a method to obtain amino acid-overproducer mutants in maize, which have the potential to increase substantially specific amino acids in kernels. The capability to increase specifically the nutritionally limiting amino acid(s) could have important nutritional implications for the grain of cereals and other crops.