The glycine betaine inhibitory effect on the osmoinduced proline response of rape leaf discs

Rape leaf discs submitted in vitro to osmotic upshocks accumulated very high amounts of proline. PEG 6000 and sucrose were found to be strong inducers of the proline response while mannitol and NaCl were low inducers. Whatever the osmoticum, proline accumulation was significantly lowered when the stress media were added with glycine betaine at concentrations ranging from 5 to 50 mM. The antagonistic effect of glycine betaine was investigated in response to different concentrations as well as in terms of timing of its supply before the onset of the treatment or during the time course of the osmoinduced proline response. When 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 on glycine betaine uptake and accumulation via a possible osmotic effect because its endogenous level was close to that of proline accumulated in discs incubated in stress media devoid of glycine betaine. The efficacy of glycine betaine in reversing the effect of osmotic shock on the proline response has been confirmed in leaf discs loaded with glycine betaine prior to their incubation under wilting conditions. Possible targets for the glycine betaine effect were then investigated. It appears that Delta(1)-pyrroline 5-carboxylate reductase activity and the 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 it was found that triethylammonium chloride mimicked the GB inhibitory role on the osmoinduced proline response.