A. Perezgarcia et al., LIGHT-DEPENDENT CHANGES OF TOMATO GLUTAMINE-SYNTHETASE IN RESPONSE TOPSEUDOMONAS-SYRINGAE INFECTION OR PHOSPHINOTHRICIN TREATMENT, Physiologia Plantarum, 102(3), 1998, pp. 377-384
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.