Different induction patterns of glutamate metabolizing enzymes in ripening fruits of the tomato mutant green flesh

Abstract

The stay‐green mutant of tomato green flesh (gf) is deficient in the chloroplast degradation machinery and displays an altered ripening profile. The ripening process of tomato (Lycopersicon esculentum Mill.) fruit is characterized by a substantial induction in glutamate dehydrogenase (GDH, EC 1.4.1.3) and a decline in glutamine synthetase (GS, EC 6.3.1.2) levels, associated with an increase in the relative glutamate content. In this study the level of free amino acids and enzymes associated with glutamate metabolism were followed in mature gf fruits at green and ripe stages. From these fruits, total RNA, protein extracts and intact plastids were isolated. Specific activities of GDH, GS and NADH‐ or ferredoxin‐dependent glutamate synthase (NADH‐GOGAT, EC 1.4.1.14; Fd‐GOGAT, EC 1.4.7.1) were measured and immunoblot analyses were performed. The total free amino acid contents of gf mature fruits decreased during ripening, contrasting with that observed in several cultivated varieties of tomatoes. The relative glutamate content, however, increased markedly in ripe gf fruits, as in all the tomato cultivars tested so far. Although GDH activity and polypeptides were found in green gf fruits, GDH transcripts were not detected in these fruits. In ripe gf fruits GDH was clearly present. NADH‐ and Fd‐GOGAT activities, on the other hand, were barely measurable in gf fruits whereas Fd‐GOGAT protein was detected in green gf fruits and, to a lower extent in ripe gf fruits. Levels of GS protein and transcripts were correlated in all fruits tested. Collectively, the expression of the enzymes involved in the primary glutamate metabolism seems to be differently regulated during the ripening process.

Biodiversity in a Tomato Germplasm for Free Amino Acid and Pigment Content of Ripening Fruits

“…Other important modification in amino acid metabolism during tomato ripening transition was the increase in asparagine levels in the wild tester LA722, also shown by RIL4. In previous reports [2][3][4]6], asparagine decreased from mature green to red ripe in different tomato genotypes (normal and mutant for ripening cultivars, the wild variety S. lycopersicum var. cerasiforme) as well as glutamine and GABA.…”

“…Glutamate was proposed as a precursor of chlorophyll [18], hence the increase in its percent composition towards the end of ripening could be due to the cessation of chlorophyll biosynthesis. On the other hand, another role was claimed for glutamate [17], given that even a mutant defective in chlorophyll degradation also shown the increase at ripe stage [3]. As an "umami taste" is provoked by glutamate, the higher glutamate relative molar content in all tomato germplasm analyzed ( [19,20] in addition to the already mentioned authors) could have an attracting to mammalian predators function [21].…”

“…Pericarp tissue (0.5 -1 g fresh weight) was extracted with 5 ml chloro form/methanol (1.5/3.5 v/v) and the amino acids relative composition was determined in the methanolic phase by derivatization with ninhydrin or o-phthaldialdehyde using an amino acids analyzer following [4]. Pigments were determined according to [3].…”

“…This transition includes morphological, biochemical and physiological changes that lead to acquisition of appropriate color, texture, flavor, among other traits determining fruit quality. Some of these changes are variations in free amino acids and pigment composition [2,3]. Glutamate percent content noticeably increased between mature green and red ripe stages, simultaneously to a reduction in glutamine and GABA levels in tomato varieties [4].…”

Postharvest chilling induces oxidative stress response in the dwarf tomato cultivar Micro‐Tom

“…The contents of chlorophyll a, b; lycopene and b-carotene were calculated from these values (Bortolotti et al 2003).…”