AMINO ACID METABOLISM IN RIPENING FRUITS OF LYCOPERSICON ESCULENTUM

ESTELA M. VALLE; SANTIAGO BORTOLOTTI; TELMA E. SCARPECI; SILVANA B. BOGGIO

Reims, Francia

Simposio; 6º International Symposium on Inorganic Nitrogen Assimilation; 2001

INRA-Versalles

The relative molar content of free amino acids in tomato (Lycopersicon esculentum Mill.) fruits changed at different developing stages (1).  The ripening of tomato is characterized by a dramatic increase in the glutamate content and the tasty cv. Cherry had the highest relative glutamate molar content (52 %) of all the tested tomato fruit cultivars.  The events triggering this process and the metabolic pathways involved are still poorly understood.  Measurements of nitrogen-assimilating enzyme activities in pericarp extracts of mature green and red fruits showed a decrease in glutamine synthetase (GS), alanine aminotransferase and glutamate decarboxylase during fruit ripening and an increase in glutamate dehydrogenase (GDH) and aspartate aminotransferase activities during this transition (2).  Ferredoxin-glutamate synthase was barely detected in these fruits.  Western blot analysis revealed that while GS was principally present in green fruit extracts, GDH was almost exclusively observed in the extracts of red fruits, suggesting an opposite pattern of induction between GS and GDH.  The activity and protein variations of GS and GDH levels during fruit ripening were also observed in their RNAm levels.  The ripening process involves the chloroplasts differentiation into chromoplasts and is accompanied by the coordinated regulation of complex gene expression patterns.  To further study this reciprocal pattern of induction between GS and GDH, experiments were performed in tomato green flesh (gf), a stay-green mutant, which is deficient in the chloroplast degradation machinery.  The enzymatic activity, protein and mRNA of GS were present at similar levels in green as well as in ripe gf fruits indicating a correlation in GS expression with the presence of chloroplasts.  GDH protein and transcripts, on the other hand, were observed in green and to some extend in ripe fruits.  These results suggest that the expression of GDH and GS are likely controlled by different mechanisms in tomato fruits. References: 1. Valle et al. (1998) Plant Cell Physiol 39: 458-461. 2. Boggio et al. (2000) Plant Sci (2000)  159: 125-133