CYTOSOLIC LOCALIZATION IN TOMATO MESOPHYLL-CELLS OF A NOVEL GLUTAMINE-SYNTHETASE INDUCED IN RESPONSE TO BACTERIAL-INFECTION OR PHOSPHINOTHRICIN TREATMENT

In tomato (Lycopersicon esculentum Mill.) leaves, the predominant glut amine synthetase (GS; EC 6.3.1.2) is chloroplastic (GS2; 45 kDa) where as the cytosolic isoform (GS1; 39 kDa) is represented as a minor enzym e. Following either infection by Pseudomonas syringae pv. tomato (Pst) or treatment with phosphinothricin (PPT), a GS inhibitor, GS1 accumul ated in the leaves. In contrast to healthy control leaves, where GS1 w as restricted to the veins, in infected and PPT-treated leaves the GS1 polypeptide was also detected in the leaf blade; moreover, it was mor e abundant than GS2. Different immunological approaches were therefore used to investigate whether or not the GS1 polypeptide expressed in P st-infected and PPT-treated tomato leaves was distributed among differ ent tissues and subcellular compartments in the same way as the consti tutive GS1 expressed in healthy leaves. By tissue-printing analysis, a similar GS immunostaining was observed in epidermis, mesophyll and ph loem of leaflet midrib cross-sections of control, infected and PPT-tre ated leaves. Immunocytochemical localization revealed that GS protein was present in the chloroplast of mesophyll cells and the cytoplasm of phloem cells in healthy leaves; however, in Pst-infected or PPT-treat ed leaves, a strong labelling was observed in the cytoplasm of mesophy ll cells. Two-dimensional analysis of GS polypeptides showed that, in addition to the constitutive GS1, a GS1 polypeptide different in charg e was present in tomato leaflets after microbial infection or herbicid e treatment. All these results indicate that a novel cytosolic GS is i nduced in mesophyll cells of Pst-infected or PPT-treated leaves. A pos sible role for this new cytosolic GS in the remobilization of leaf nit rogen during infection is proposed.