NaCl effects on proline metabolism in rice (Oryza sativa) seedlings

Salt-stress effects on osmotic adjustment, ion and proline concentrations a s well as proline metabolizing enzyme activities were studied in two rice ( Oryza sativa L.) cultivars differing in salinity resistance: I Kong Pao (IK P; salt-sensitive) and Nona Bokra (salt-resistant), The salt-sensitive cult ivar exposed to 50 and 100 mM NaCl in nutritive solution for 3 and 10 days accumulated higher levels of sodium land proline than the salt-resistant cu ltivar and displayed lower levels of osmotic adjustment. Proline accumulati on was not related to proteolysis and could not be explained by stress-indu ced modifications in Delta(1)-pyrroline-5-carboxylate reductase (P5CR; EC 1 .5.1.2) or proline dehydrogenase (PDN; EC 1.5.1.2) activities recorded in v itro. The extracted ornithine delta-aminotransferase (OAT;: EC 2.6.1.13) ac tivity was increased by salt stress in the salt-sensitive cultivar only. In both genotypes, salt stress induced an increase in the aminating activity of root glutamate dehydrogenase (GDH; EC 1.4.1.2) while deaminating activit y was reduced in the leaves of the salt-sensitive cultivar, The total extra cted glutamine synthetase activity (CS; EC 6.3.1.2) was reduced in response to salinity but NaCl had contrasting effects on GS1 and GS2 isoforms in sa lt-sensitive IKP, Salinity increased the activity of ferredoxin-dependent g lutamate synthase (Fd-GOGAT; EC 1.4.7.1) extracted from leaves of both geno types and increased the activity of NADH-dependent glutamate synthase (NADH -GOGAT; EC 1.4.1.14) in the salt-sensitive cultivar, It is suggested that p roline accumulation is a symptom of salt-stress injury in rice and that its accumulation in salt-sensitive plants results from an increase in OAT acti vity and an increase in the endogenous pool of its precursor glutamate. The physiological significance of the recorded changes are analyzed in relatio n to the functions of these enzymes in plant metabolism.