L-tryptophan synthesis from 14C-anthranilic acid in plants with high and low tryptophan content

The biosynthesis of L-tryptophan (L-trp) from anthranilic acid-14C (AA-14C) in undamaged organs of the seedlings of kohlrabi and peas, with high L-trp content and maize plants, with low L-trp content was compared. As for maize the experiments were carried out with normal and opaque-2 phenotypes, both with the seedlings and with the ripening kernels. AA-14C is metabolized in the plants to L-trp pool and to glycosyl esters of AA. In maize seedlings L-trp-14C is synthesized relatively less than in kohlrabi and in pea. The de novo formation of L-trp-14C is stopped earlier in maize than in kohlrabi. The level of free L-trp-14C is relatively low in maize in comparison with kohlrabi and peas. In spite of this the formation of L-trp-14C from AA-14C is sufficient in maize to incorporate L-trp both into the proteins and into a secondary metabolite that is not yet defined. At the period of seedlings the incorporation in maize of L-trp into the proteins is comparable with that in kohlrabi, and it is maximum in pea. Maize, at the stage of germination, thus forms proteins rich in L-trp. The formation of free L-trp is approximately ten times lower in ripening kernels and in the leaves adjacent to the ear and it further decreases in the course of the ripening of the kernels. Although the activity of the biosynthesis of the AA-14C→L-trp-14C pathway is relatively lower in maize than in kohlrabi and peas, this pathway is most responsible for the differences in the content of L-trp in these plants. Neither amitrol nor histidine affected the biosynthesis of L-trp in kohlrabi; the interaction of the biosynthetic pathways of L-trp and histidine known in microorganisms is thus not important in a higher plant. (author)