GLYCINE BETAINE BIOSYNTHESIS AND CATABOLISM IN BACTEROIDS OF RHIZOBIUM-MELILOTI - EFFECT OF SALT STRESS

In medium of low or high osmolarity, bacteroids isolated from Medicago sativa nodules induced by Rhizobium meliloti 102F34 rapidly catabolized [C-14]choline. Trimethylamine was never detected and glycine betaine was the predominant product within the ethanol-soluble fraction of bacteroids subjected to salt stress (0.4 M-NaCl). Both choline oxidase and glycine betaine aldehyde dehydrogenase activities were characterized; the apparent K(m) values for choline and glycine betaine aldehyde were 2.0 and 0.4 mM, respectively. A 3 h incubation of the bacteroids in medium of high osmolarity, supplemented or not with choline, did not significantly modify the specific activity of the two enzymes. Similarly, salinization of the host plants for 2 weeks with 0.15 M-NaCl had only a slight effect on both enzymic activities. Thus, the choline-glycine betaine biosynthetic pathway was not modulated by the external osmotic pressure. Glycine betaine itself was actively degraded by bacteroids suspended in low-osmolarity medium, but the demethylation process producing sarcosine and glycine was extremely slow in bacteroids subjected to salt stress. Thus, high concentration of glycine betaine can be maintained in salt-stressed bacteroids.