THE GLYCINE BETAINE INHIBITORY EFFECT ON THE OSMOINDUCED PROLINE RESPONSE OF RAPE LEAF DISCS

Rape leaf discs submitted in vitro to osmotic upshocks accumulated ver y high amounts of proline. PEG 6000 and sucrose were found to be stron g inducers of the proline response while mannitol and NaCl were low in ducers. Whatever the osmoticum, proline accumulation was significantly lowered when the stress media were added with glycine betaine at conc entrations ranging from 5 to 50 mM. The antagonistic effect of glycine betaine was investigated in response to different concentrations as w ell as in terms of timing of its supply before the onset of the treatm ent or during the time course of the osmoinduced proline response. Whe n supplied with the osmoticum during the whole treatment, the deduced apparent I-50's for glycine betaine ranged from 15-25 mM for mannitol, sucrose and NaCl to 250 mM for PEG 6000, The observed effect relied o n glycine betaine uptake and accumulation via a possible osmotic effec t because its endogenous level was close to that of proline accumulate d in discs incubated in stress media devoid of glycine betaine. The ef ficacy of glycine betaine in reversing the effect of osmotic shock on the proline response has been confirmed in leaf discs loaded with glyc ine betaine prior to their incubation under wilting conditions. Possib le targets for the glycine betaine effect were then investigated. It a ppears that Delta(1)-pyrroline 5-carboxylate reductase activity and th e capacity of leaf discs to mobilize proline after releasing of stress conditions are not modulated by glycine betaine. It is suggested that glycine betaine might exert some effect at the membrane level since i t was found that triethylammonium chloride mimicked the GB inhibitory role on the osmoinduced proline response.