LIGHT-DEPENDENT CHANGES OF TOMATO GLUTAMINE-SYNTHETASE IN RESPONSE TOPSEUDOMONAS-SYRINGAE INFECTION OR PHOSPHINOTHRICIN TREATMENT

Glutamine synthetase (GS, EC 6.3.1.2), a key enzyme in nitrogen assimi lation, was investigated in tomato (Lycopersicon esculentum Mill. cv. Hellfrucht Fruhstamm) leaves infected with Pseudomonas syringae pv. to mato (Pst) or treated with the herbicide phosphinothricin (PPT), an ir reversible inhibitor of GS. GS activity decreased markedly when Pst in fection occurred in illuminated leaves, bur only a slight decrease in relation to control leaves was observed under non-photosynthetic condi tions. In leaves treated with PPT, a rapid inhibition of GS activity w as observed under ail experimental conditions. When bacterial infectio n or herbicide treatment was carried out in the light, cytosolic GS (G S1) appeared as the predominant GS polypeptide; however, under non-pho tosynthetic conditions GS2 remained the most abundant molecular CTS sp ecies as occurs in non-stressed plants. These results suggest a close correlation between the photosynthetic process and changes in the rela tive proportions of GS polypeptides during infection or herbicide trea tment. Ammonium has been described as an inducer of GS genes, but as a mmonium accumulated during all treatments, other light-dependent facto rs could be involved in GS regulation of stressed leaves.