title: Proline betaine accumulation and metabolism in alfalfa plants under sodium chloride stress. Exploring its compartmentalization in nodules.
abstract: 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.
authors.affiliation: 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.
authors.forename: Jean-Charles
authors.initials: JC
authors.lastname: Trinchant
authors.forename: Alexandre
authors.initials: A
authors.lastname: Boscari
authors.forename: Guillaume
authors.initials: G
authors.lastname: Spennato
authors.forename: Ghislaine
authors.initials: G
authors.lastname: Van de Sype
authors.forename: Daniel
authors.initials: D
authors.lastname: Le Rudulier
mesh_terms: D007700:Kinetics; D000455:Medicago sativa / Q000187:drug effects / Q000254:growth & development / Q000378:metabolism*; D009994:Osmolar Concentration; D011392:Proline / Q000031:analogs & derivatives* / Q000378:metabolism*; D012965:Sodium Chloride / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D012965:Sodium Chloride; D011392:Proline; C003342:stachydrine