Indexed on: 27 Jun '08Published on: 27 Jun '08Published in: Biologia Plantarum
The biosynthesis of L-tryptophan (L-trp) from anthranilic acid-14C (AA-14C) in. undamaged organs of the seedlings of kohlrabi and pea, with high L-trp content and ma ze plants, with low L-trp content was compared. As for maize the experiments were carried oiut 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 (i.e. free and bound L-trp, and secondary metabolites) and to glycosyl esters of AA (i.e. to simple glucosyl ester in pea and kohlrabi and more complex glycosides in maize). In maize seedlings L-trp-14C is synthesized relatively less. (40% in the 1st and 2nd leaf and 33% in the 3rd leaf of the total radioactivity of the incorporated AA-14C is transferred into the L-trp-14C pool after 24 h) than in kohlrabi (52% in the hypocotyl and 85% in the cotyledons) and in pea (58% in the 1st and the 2nd internode and 85% in the 3rd and the 4th internode). Thede novo formation of L-trp-14C is stoped earlier in maize (after 5 h) than in kohlrabi (after 15 h). The level of free L-trp-14C is relatively low ill maize (15% and 13% of the total radioactivity of the incorporated AA-14C is converted to free L-trp-14C and remains in this form after 24 h) in comparison with kohlrabi (31% and 60%) and pea (30% and 49%). 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 (11% and 10% of the activity of the incorporated AA-14C) is comparable with that in kohlrabi (11% and 17%), and it is maximum in pea (29% and 36%). 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 pea, 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.