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Published on July 2, 2004; 10.1104/pp.103.037556

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Received December 11, 2003
Returned for revision March 10, 2004
Accepted April 1, 2004

Proline Betaine Accumulation and Metabolism in Alfalfa Plants under Sodium Chloride Stress. Exploring Its Compartmentalization in Nodules

Jean-Charles Trinchant , Alexandre Boscari , Guillaume Spennato , Ghislaine Van de Sype , and Daniel Le Rudulier *

Laboratoire de Biologie Végétale et Microbiologie, Centre National de la Recherche Scientifique, Formation de Recherche en Evolution 2294, Université de Nice-Sophia Antipolis, Faculté des Sciences, Parc Valrose, F-06108 Nice cedex 2, France

* Corresponding author; email: leruduli{at}unice.fr.

The osmoprotectant Pro betaine is the main betaine identified in alfalfa (Medicago sativa). We have investigated the long-term responses of nodulated alfalfa plants to salt stress, with a particular interest for Pro betaine accumulation, compartmentalization, and metabolism. Exposure of 3-week-old nodulated alfalfa plants to 0.2 M NaCl for 4 weeks was followed by a 10-, 4-, and 8-fold increase in Pro betaine in shoots, roots, and nodules, respectively. Isotope-labeling studies in alfalfa shoots indicate that [14C]Pro betaine was synthesized from L-[14C]Pro. [14C]Pro betaine was efficiently catabolized through sequential demethylations via N-methylPro and Pro. Salt stress had a minor effect on Pro betaine biosynthesis, whereas it strongly reduced Pro betaine turnover. Analysis of Pro betaine and Pro compartmentalization within nodules revealed that 4 weeks of salinization of the host plants induced a strong increase in cytosol and bacteroids. The estimated Pro betaine and Pro concentrations in salt-stressed bacteroids reached 7.4 and 11.8 mM, respectively, compared to only 0.8 mM in control bacteroids. Na+ content in nodule compartments was also enhanced under salinization, leading to a concentration of 14.7 mM in bacteroids. [14C]Pro betaine and [14C]Pro were taken up by purified symbiosomes and free bacteroids. There was no indication of saturable carrier(s), and the rate of uptake was moderately enhanced by salinization. Ultrastructural analysis showed a large peribacteroid space in salt-stressed nodules, suggesting an increased turgor pressure inside the symbiosomes, which might partially be due to an elevated concentration in Pro, Pro betaine, and Na+ in this compartment.




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