Betaxanthin formation and free amino acids in hairy roots of Beta vulgaris var. lutea depending on nutrient medium and glutamate or glutamine feeding

Feeding of amino acids to hairy roots of the yellow beet (Beta vulgaris var. lutea) usually results in the formation of the respective betaxanthins. One exception is (S)-glutamate whose feeding leads to an increase in the betaxanthin vulgaxanthin I (glutamine as amino-acid moiety) instead of vulgaxanthin II (glutamate as amino-acid moiety). To elucidate this phenomenon, hairy roots were cultivated in modified standard medium and (S)-glutamate was fed. Under most nutrient conditions tested, glutamine and vulgaxanthin I in the tissue dominated over glutamate and vulgaxanthin II. Glutamate, opposed to glutamine, was readily metabolized so that its concentration was lower than that of glutamine. Maximum concentrations of glutamate were reached when the activity of glutamine synthetase was low. Even then, however, vulgaxanthin II stayed on a low level. In contrast, the level of vulgaxanthin I increased with increasing concentrations of glutamine in the tissue. Also 4-aminobutyric acid (GABA) was a major amino acid in the hairy roots. Its concentration reached maximum levels when (S)-glutamate, a GABA precursor, was fed, or when sucrose, the C source of the roots, was replaced by glucose. The respective GABA-betaxanthin, however, was hardly detectable. When both (S)-glutamate and glucose were supplied, the GABA concentration exceeded that of all other amino acids. Only then the GABA-betaxanthin could be characterized in small amounts. Interestingly, the level of the main betaxanthin, miraxanthin V, consisting of betalamic acid and dopamine, was most markedly reduced by a replacement of sucrose with glucose. We conclude that the reaction of betalamic acid with glutamate and GABA was considerably lower than with glutamine and dopamine, irrespective of the concentration of the amino acid in the tissue. Possible reasons will be discussed, also with respect to the occurrence of species-specific patterns of betaxanthins.