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
Di6isio´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 – 6on Haller – Institut fu¨r Pflanzenwissenschaften der Uni6ersita¨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.comlocateplantsci
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
Abbre6iations
:
GABA, g-aminobutyric acid; GAD, glutamate de- carboxylase; GDH, glutamate dehydrogenase; GOGAT, glutamate
synthase; GS, glutamine synthetase. Corresponding author Tel.: + 54-341-43506614350596; fax: +
54-341-4390465. E-mail address
:
evallearnet.com.ar E.M. Valle. 0168-945200 - see front matter © 2000 Elsevier Science Ireland Ltd. All rights reserved.
PII: S 0 1 6 8 - 9 4 5 2 0 0 0 0 3 4 2 - 3
compounds such as amino acids. In developing fruits, about 70 of the total amino acid content
found in the pericarp belongs to the glutamate family [12]. g-Aminobutyric acid GABA was the
predominant N-form almost 60 of the total amino acid molar content at the earlier growing
stages of the tomato fruit and glutamine ca. 30 in mature green fruits [12].
There are few reports about the enzymes in- volved in amino acid biosynthesis in the fruit.
Glutamine synthetase GS, which catalyzes the synthesis of glutamine from glutamate, ATP and
ammonium, was detected in green and red toma- toes [13] and in avocado fruit [14]. NADH-gluta-
mate
synthase GOGAT;
EC 1.4.1.14
and ferredoxin-GOGAT EC 1.4.7.1, both enzymes
catalyzing glutamate synthesis, were observed at low activities in the pericarp of green tomato fruits
[15] and in intact fruit chloroplasts [16]. Glutamate dehydrogenase GDH, which catalyzes the amina-
tion of 2-oxoglutarate synthetic reaction and the deamination of glutamate catabolic reaction,
showed an increase in the enzyme protein content during ripening of avocado fruits [14]. Other amino
acid metabolizing enzymes or transcripts for them expressed at different levels during tomato fruit
ripening were reported: aspartate aminotrans- ferase, which was exclusively found in red fruits
[17]; a putative glutamate decarboxylase GAD; EC 4.1.1.15 [18] and arginine decarboxylase EC
4.1.1.19, whose transcripts appeared to peak at the breaker stage [19], and a histidine decarboxy-
lase EC 4.1.1.22 mRNA, which accumulated dur- ing early fruit ripening and then declined [20].
The present study was carried out to contribute to the understanding of the metabolism of amino
acids during the ripening of tomato fruits. For this purpose the molar contents of free amino acids
were quantified in three different tomato cultivars Platense, Cherry and Vollendung at green, yellow
and red ripening stages of the fruits. Concomi- tantly, the activities of nitrogen-metabolizing en-
zyme and proteins of GS, GOGAT and GDH were investigated.
2. Materials and methods