Is glycine betaine a non-compatible solute in higher plants that do not accumulate it?

Rape (Brassica napus L. var. oleifera cv. Samourai) leaf discs treated in vitro in the presence of glycine betaine (GB) exhibited very high accumulation of GB, suggesting the operation of a specific uptake system. When further subjected to osmotic upshocks by transfer to PEG 6000 media, the typical osmo-induced proline response of the discs was strongly inhibited. The level of this inhibition was quantitatively related to the amount of GB loaded in the tissues. In contrast, the soluble sugar content increased in GB-treated discs. Surprisingly, viability tests (i.e. membrane stability and 2,3,5-triphenyltetrazolium chloride reduction) indicated a destabilizing effect of GB in these tissues. This is at variance with the relative compatibility of sucrose and proline. In addition, the protein content was lower in GB-treated discs. This could be related to an inhibitory effect on protein synthesis, as demonstrated by radiolabelling of polypeptides with [S-35] amino acids. This effect was particularly pronounced on Rubisco large subunit synthesis and was still apparent under non-stress conditions. The GB treatment was also followed by the induction or up-regulation of a set of polypeptides, not seen under stress conditions, while the synthesis of osmo-induced polypeptides was not affected by GB. These novel effects of GB lead us to discuss the reasons for its incompatibility in leaf tissues of a non-GB-accumulating species.