Non-allelism of two threonine accumulating mutants --David R. Duncan and Jack M. Widholm The addition of both lysine and threonine to maize tissue culture medium inhibits callus growth through the feedback inhibition of the aspartate biosynthetic pathway. Ultimately, the tissue is starved for methionine, and the addition of methionine to the culture medium can reverse the growth inhibition. Through their inhibition of growth, the combination of lysine and threonine has been used as a selection agent in regenerable maize callus cultures for the recovery of mutants that accumulate threonine (K. A. Hibberd and C. E. Green, Proc. Natl. Acad. Sci. 79:559-563, 1982; and S. Miao, D. R. Duncan and J. M. Widholm, Plant Cell Tissue Organ Cult. 14:3-14, 1988).

It was the goal of this work to determine if the mutant gene selected by Hibberd and Green (LT19) was allelic to that of the mutant selected in our lab. Homozygous LT19 seed, supplied by Burle Gengenbach, was crossed with our homozygous LT-R3. Randomly selected F1 plants were selfed and their progeny were screened for the mutant phenotype by assaying the growth of excised, 1 cm long, root tips on H medium (D. R. Duncan, M. E. Williams, B. E. Zehr and J. M. Widholm, Planta 165:322-331, 1985) containing 3 mM lysine and 3 mM threonine.

Of 198 F2 segregants tested, 17 expressed the wildtype phenotype of no root growth on the assay medium. A chi square analysis of these data showed a close fit (p > .10) to the 15:1 ratio of two independent, dominant genes. No difference in response was seen for reciprocal crosses of the homozygous parental material.

These data indicate that LT19 and LT-R3 are non-allelic genes for the same phenotype. At this point we do not know if these genes code for different enzymes, isozymes of the same enzyme or perhaps different subunits of the same enzyme. We have also not studied the allelism of LT-R3 and LT20, another threonine accumulating mutant (D. A. Frisch and B. G. Gengenbach).

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