Plant Science 159 (2000) 125–133 Changes in amino acid composition and nitrogen metabolizing enzymes in ripening fruits of Lycopersicon esculentum Mill Silvana B. Boggio a , Javier F. Palatnik a , Hans W. Heldt b , Estela M. Valle a, * a Diisio ´n Biologı ´a Molecular, Instituto de Biologı ´a Molecular y Celular de Rosario (IBR-CONICET), Facultad de Ciencias Bioquı ´micas y Farmace ´uticas, UNR, Suipacha 531, 2000 Rosario, Argentina b Albrecht – on Haller – Institut fu ¨r Pflanzenwissenschaften der Uniersita ¨t Go ¨ttingen, Abteilung fu ¨r Biochemie der Pflanze, Untere Karspu ¨le 2, 37073 Go ¨ttingen, Germany Received 25 March 2000; received in revised form 12 June 2000; accepted 12 July 2000 Abstract The free amino acid content of tomato (Lycopersicon esculentum Mill.) fruits from cultivars Platense, Vollendung and Cherry were determined during ripening. It was found that glutamate markedly increased in red fruits of the three cultivars under study. At this stage, the cv Cherry had the highest relative glutamate molar content (52%) of all the analyzed tomato fruit cultivars. Measurements of nitrogen-assimilating enzyme activities of these fruits showed a decrease in glutamine synthetase (GS, EC 6.3.1.2) during fruit ripening and a concomitant increase in NADH-glutamate dehydrogenase (GDH, EC 1.4.1.3) and aspartate aminotransferase (EC 2.6.1.1) activities. Western blot analysis of protein extracts revealed that while GS was principally present in green fruit extracts, GDH was almost exclusively observed in the extracts of red fruits. These results suggest a reciprocal pattern of induction between GS and GDH during tomato fruit ripening. © 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Amino acid; Glutamate dehydrogenase; Glutamine synthetase; Lycopersicon esculentum; Fruit ripening www.elsevier.com/locate/plantsci 1. Introduction The growth of tomato fruits goes through dif- ferent phases. The early phase of fruit develop- ment is characterized by high metabolic activity and a rapid cell division of the tissue, whereas at a later developmental phase the cells expand [1]. Fruit ripening begins when seeds are completely formed and the fruit reaches its final size [1,2]. This ripening process involves a series of coordi- nated events including changes at the physiological and biochemical levels [3]. During the initial phases of tomato ripening, chloroplasts differenti- ate into chromoplasts. This plastid transition is accompanied by the expression of specific genes involved in chromoplast formation and the subse- quent synthesis of enzymes correlated with ripen- ing [4,5]. As sink organs, fruits are dependent on the translocation of sucrose, amino acids, and organic acids to the developing fruit cells. The rate of import of these photoassimilates from the leaves is governed by the metabolic activity of the fruit [6]. In the case of the tomato, green fruit cells contain most of the photosynthetically active chloroplasts that give the developing fruit its green appearance, and play a significant role in carbon dioxide scav- enging [7]. Nevertheless, the activity per mg of protein of ribulose-1,5-bisphosphate carboxylase in the tomato leaf was shown to be three times higher than in the green fruit pericarp [4]. In tomato fruits, carbohydrate metabolism has been principally studied so far [7–11], while there is less information regarding the metabolism of nitrogen Abbreiations: GABA, -aminobutyric acid; GAD, glutamate de- carboxylase; GDH, glutamate dehydrogenase; GOGAT, glutamate synthase; GS, glutamine synthetase. * Corresponding author Tel.: +54-341-4350661/4350596; fax: + 54-341-4390465. E-mail address: evalle@arnet.com.ar (E.M. Valle). 0168-9452/00/$ - see front matter © 2000 Elsevier Science Ireland Ltd. All rights reserved. PII:S0168-9452(00)00342-3