A ROLE FOR CYTOSOLIC GLUTAMINE-SYNTHETASE IN THE REMOBILIZATION OF LEAF NITROGEN DURING WATER-STRESS IN TOMATO

Tomato (Lycopersicon esculentum L.) responded to a prolonged period of water stress with stomatal closure followed by premature flowering an d the subsequent production of small fruits containing fertile seeds. Water stress was correlated with a net loss of protein from tomato lea ves and the concomitant accumulation of free amino acids, reflecting t he remobilization of leaf nitrogen to meet the N-requirement for the r apid development of reproductive organs. We show by northern blot anal ysis of the transcript pools, and by immunoblot analysis of the protei n levels that water stress stimulates tomato cytosolic glutamine synth etase (EC 6.1.3.2; GS-I) gene expression, while plastidic glutamine sy nthetase (GS-2) gene expression remains unchanged during drought. Thes e results suggest a role of GS-I in the generation of glutamine for th e transport of the nitrogen that is remobilized in tomato leaves in re sponse to chronic water stress. The remobilization of leaf N during wa ter stress appears to be, at least in part, initiated by a specific do wn-regulation of the leaf transcript pool corresponding to the small s ubunit of ribulose-1,5-bisphosphate carboxylase/oxygenase.