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 and the subsequent production of small fruits containing fertile seeds. Water stress was correlated with a net loss of protein from tomato leaves and the concomitant accumulation of free amino acids, reflecting the remobilization of leaf nitrogen to meet the N-requirement for the rapid development of reproductive organs. We show by northern blot analysis of the transcript pools, and by immunoblot analysis of the protein levels that water stress stimulates tomato cytosolic glutamine synthetase (EC 6.1.3.2; GS-I) gene expression, while plastidic glutamine synthetase (GS-2) gene expression remains unchanged during drought. These results suggest a role of GS-I in the generation of glutamine for the transport of the nitrogen that is remobilized in tomato leaves in response to chronic water stress. The remobilization of leaf N during water stress appears to be, at least in part, initiated by a specific down-regulation of the leaf transcript pool corresponding to the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase.