Variations of the osmotic environment within the rhizosphere may affect all steps of plant-rhizobia interaction, from the root colonization to nodule development and function. Betaines, mainly glycine betaine (GB) and proline betaine (PB) are common osmoprotectant in Gram-negative bacteria. In S. meliloti accumulation of GB and PB restores turgor pressure and growth in response to salt stress. Different GB and PB uptake systems have been detected in S. meliloti, but up to now only one, Hut, an ABC transporter, has been characterized (Boncompagni et al. 2000).
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