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Overexpression of BetS, a Sinorhizobium meliloti High-Affinity Betaine Transporter, in Bacteroids from Medicago sativa Nodules Sustains Nitrogen Fixation During Early Salt Stress Adaptation

August 2006 , Volume 19 , Number  8
Pages  896 - 903

Alexandre Boscari , Ghislaine Van de Sype , Daniel Le Rudulier , and Karine Mandon

Unité Interactions Plantes-Microorganismes et Santé Végétale, UMR 6192 CNRS- 1064 INRA-Université de Nice-Sophia Antipolis, Centre de Recherche INRA Agrobiotech, 400 Route des Chappes, BP 167, 06903 Sophia Antipolis cédex, France

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Accepted 20 March 2006.

Sinorhizobium meliloti possesses several betaine transporters to cope with salt stress, and BetS represents a crucial high-affinity glycine and proline betaine uptake system involved in the rapid acquisition of betaines by cells subjected to osmotic upshock. Using a transcriptional lacZ (β-galactosidase) fusion, we showed that betS is expressed during the establishment of the symbiosis and in mature nitrogen-fixing nodules. However, neither Nod nor Fix phenotypes were impaired in a betS mutant. BetS is functional in isolated bacteroids, and its activity is strongly activated by high osmolarity. In bacteroids from a betS mutant, glycine betaine and proline betaine uptake was reduced by 85 to 65%, indicating that BetS is a major component of the overall betaine uptake activity in bacteroids in response to osmotic stress. Upon betS overexpression (strain UNA349) in free-living cells, glycine betaine transport was 2.3-fold higher than in the wild-type strain. Interestingly, the accumulation of proline betaine, the endogenous betaine synthesized by alfalfa plants, was 41% higher in UNA349 bacteroids from alfalfa plants subjected to 1 week of salinization (0.3 M NaCl) than in wild-type bacteroids. In parallel, a much better maintenance of nitrogen fixation activity was observed in 7-day-salinized plants nodulated with the over-expressing strain than in wild-type nodulated plants. Taken altogether, these results are consistent with the major role of BetS as an emergency system involved in the rapid uptake of betaines in isolated and in planta osmotically stressed bacteroids of S. meliloti.

© 2006 The American Phytopathological Society