Glutamine synthetase isoforms in Lycopersicon esculentum fruits and roots

SCARPECI, TELMA E; DELGADO, LUCIANA; ORELLANO, ELENA G; VALLE, ESTELA M; BOGGIO, SILVANA B

USA

Congreso; Plant Biology, ASPB; 2000

ASPB

Glutamine synthetase (GS) is involved in the primary assimilation of ammonia in plants. The enzyme is an octamer and presents two isoforms, one located in the cytosol (GS1) and the other in the chloroplast (GS2). While GS2 is enconded by a single copy gene, GS1 is encoded by a small gene family. The molecular mass of the subunit of each isoenzyme was used as a criterium of identification for GS1 (37-43 kDa) or GS2 (43-45 kDa). Recently, Mäck [1] presented data showing that both GS1 and GS2 are composed by 38 and 41 kDa subunits, indicating that the size of the subunit cannot be taken as synonym for the intracellular localization of a GS isoenzyme. In tomato, GS2 was detected in cotyledons [2] and their subunit composition was influenced by light, while in the leaves the expression of GS1 was enhanced by senescence [3]. Little is known about the GS isoform regulation in the fruit, root and other tissues. In order to identify if both isoforms were present in the fruit, chloroplasts and chromoplast of green and red fruits were subjected to western blot analysis. Results indicated that GS was principally present in green fruits, GS2 showing two bands and GS1 one band, comigrating with the fast-migrating band of GS2 and the single GS band detected in roots. Activity of GS in native polyacrylamide gels was visualized as a single stained band in protein extracts of leaves, green fruits and roots. It is likely that GS regulation involves a heteromeric association of subunits to modulate its activity in different tissues. 1.Mäck,G: Planta 205: 113-120 (1998). 2.Migge,A, et al.: Plant Mol.Biol. 37: 689-700 (1998). 3.Pérez-Rodríguez,J, Valpuesta,V: Physiol.Plant. 97: 576-582 (1996).