Víctor M. Loyola-Vargas

Regulation of Glutamine Synthetase/Glutamate Synthase Cycle in Maize Tissues, Effect of the Nitrogen Source

Summary Glutamine synthetase (EC. 6.3.1.2) and NADH- and ferredoxin-dependent glutamate synthase (EC. 1.4.1.14) activities were measured in leaves, roots and callus tissues of maize exposed to different nitrogen sources. NADH-dependent glutamate synthase activity was detected in roots where the enzyme level was increased by ammonium citrate, NH 4 Cl plus nitrate, glutamate and urea, and in callus tissue where the level was increased by nitrate. No NADH-dependent glutamate synthase could be detected in leaves, which showed a high ferre-doxin-dependent glutamate synthase activity. The level of this latter enzyme was reduced by all nitrogen sources used except for ammonium citrate and glutamate. The results suggest that NADH-dependent glutamate synthase is regulated in different ways in the tissues tested. Glutamine synthetase was strongly regulated in the leaf but not in the root. Glutamine synthetase activity was reduced by NH4CI and increased by nitrate, ammonium citrate, glutamate, glutamine and urea. The results suggest that in the root the GS/GOGAT cycle is used as such, but that GS also synthesized glutamine, which the tissue exports for the synthesis of glutamate.

Growth promoting activity of certain penicillins on cultivated cells of Bouvardia ternifolia

Abstract A growth promoting effect similar to that of 2,4-D is induced in Bouvardia ternifolia calli and cell suspension cultures by some beta lactam antibiotics but not by others. It was found that this effect was exerted only by those penicillins that contain an auxin-like moiety associated with the 6-aminopenicillanic acid, such as carbenicillin or penicillin G. Phenylacetic acid and 6-aminopenicillanic acid were inactive on their own.

Nitrogen Metabolism in Canavalia ensiformis (L.) DC.: II. Changing Activities of Nitrogen-Assimilating Enzymes during Growth

Summary The ontogeny of glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 1.4.1.14), asparagine synthetase (EC 6.4.5.4) and glutamate dehydrogenase (EC 1.4.1.2) has been followed in cotyledons, roots, stems and leaves of young plants of Canavalia ensiformis . The four enzymes were found in all regions. Glutamine synthetase activity increased in the leaves with time and remained low in the cotyledons. Initially glutamate synthase activity in the cotyledons was NADH-dependent but later became Fd-dependent. Asparagine synthetase specific activity was highest in the roots until day 5 after which activity was similar in all regions. While glutamate dehydrogenase specific activity decreased with time in the leaves it increased in the cotyledons and roots. We suggest that in the early stages of seedling growth asparagine synthetase and glutamate dehydrogenase both play a fundamental role in the reassimilation of ammonium released from canavanine. When the plant acquires full photosynthetic capacity the glutamine synthetase/glutamate synthase pathway becomes the principal pathway of reassimilation in the aerial parts of the plant.

Effect of Nitrate, Ammonium and Glutamine on Nitrogen Assimilation Enzymes during Callus Growth of Maize

Summary The activities of the enzymes glutamine synthetase, glutamate synthase and glutamate dehydrogenase were measured during the cell cycle in maize tissue cultures grown in different nitrogen sources: 20 mM nitrate, 2 mM ammonium, 5 mM glutamine and a mixture of 40 mM nitrate plus 20 mM ammonium.Similar growth curves were obtained with all the nitrogen sources employed, except for ammonium.The callus grown on nitrate presented two GS activity peaks.While with the mixture of nitrate and ammonium only one peak was present and in the presence of glutamine no definitive peaks of activity were observed.All nitrogen sources showed three GOGA T activity peaks, with the exception of nitrate where only two were observed.GDH-NADH displayed three peaks in cultures grown on nitrate, ammonium and glutamine.In the presence of the mixture only one peak was observed.

Determination of l-canavanine and l-canaline in plant tissues by high-performance liquid chromatography

Dosage simultane de ces deux acides amines en presence dautres acides amines. Applications au haricot de Madagascar

In vitro Effects of PEG and Proline on Ammonium Assimilation Enzymes of Maize

Summary In order to determine which could be the pathway of ammonium assimilation during stress, the effects of polyethylene glycol and proline in glutamine synthetase and glutamate dehydrogenase activities were studied employing the in vitro system of Paleg et al. (Plant Physiol. 75: 974-978, 1984). A differential action of polyethylene glycol on the enzymes extracted from leaves and roots was observed. The results suggest that in the roots ammonium assimilation could be accomplished by glutamate dehydrogenase since it proved to be more resistant to osmotic stress, and the affinity of the stressed enzyme for ammonium was increased 100 % by proline.

Amine oxidase activity in Canavalia ensiformis during early growth: a histochemical study

Summary The distribution of amine oxidase has been studied in light-grown Canavalia ensiformis (L.) DC. plants during germination and early growth, using a histochemical staining technique. Different regions of the plant showed characteristic staining profiles which varied according to the age of the plant and the substrate used. Putrescine- and cadaverine-induced staining occurred more frequently than spermidine-induced staining. Spermine-induced staining was rarely seen. Except for roots and the embryo, amine oxidase activity was detected in all regions of the plant although in some parts, such as the cotyledons, its presence was limited and transient. However, high activity was found in young internodes and petioles. In the internodes and petioles of 13-d-old plants staining was inhibited by the carbonyl-group reagents aminoguanidine and hydroxyethylhydrazine and the metal chelators 8-hydroxyquinoline and diethyl-dithiocarbamate, while p-chloromercuribenzoic acid inhibited spermine-induced staining only. Staining profiles in the roots, cotyledons and embryos were not affected by the presence of 2,4-dichlorophenol. The results suggest that staining is a reliable indicator of amine oxidase activity, and indicate the presence of a diamine oxidase similar to that found in other legumes.

Nitrogen Metabolism in Canavalia ensiformis L. DC.: I. Arginase and Urease Ontogeny

Summary The ontogeny of arginase and urease has been followed in cotyledons, roots, stems and leaves of young Canavalia ensiformis plants. Both enzymes were found in all organs. Arginase activities were highest in stems and roots, whereas urease activity was highest in the cotyledons. Urea and ammonia pools were measured in all organs studied and, in general, decreased with time, except in leaves where a marked increase in ammonia was observed. The data are discussed in relation to the mobilization and distribution of canavanine from cotyledons to other regions during germination and subsequent seedling growth.

Effect of the Auxin/Cytokinin Ratio on the Enzymes of Nitrogen Metabolism in Canavalia ensiformis L. Tissue Cultures

Abstract Glutamate dehydrogenase In the presence of indoleacetic acid or naphthaleneacetic acid this ratio decreased as the 6-BAP concentration increased. The activity of the three enzymes showed a very clear dose-response effect with respect to the BAP/auxin ratio, it being a function of the auxin present.