Osmoprotectant beta-alanine betaine synthesis in the Plumbaginaceae: S-adenosyl-L-methionine dependent N-methylation of beta-alanine to its betaine is via N-methyl and N,N-dimethyl beta-alanines

beta-Alanine betaine is an osmoprotective compound accumulated by most memb ers of the plant family Plumbaginaceae. Leaf and root tissues of Limonium l atifolium known to accumulate beta-alanine betaine readily convert supplied beta-alanine to beta-alanine betaine. To identify the intermediates and th e enzymes involved in beta-alanine betaine synthesis, radiotracer experimen ts using [C-14] formate were employed. These studies demonstrate that beta- alanine betaine is synthesized from beta-alanine via N-methyl and N,N-dimet hyl beta-alanines. A rapid and sensitive radiometric assay was developed to measure N-methyltransferase (NMT) activities by using [methyl-C-14] or [me thyl-H-3] S-adenosyl-L-methionine (AdoMet) as the methyl donor, Leaf extrac ts from beta-alanine betaine accumulators - Armeria maritima, L. latifolium and L. ramosissimum - had detectable NMT activities while none were found in L. perezii, a species that does not accumulate beta-alanine betaine. The MMT activities were further characterized from the leaves of L. latifolium . The activities had a pH optimum of 8.0, were soluble and inhibited by S-a denosyl-L-homocysteine. Extractable activities were similar from plants gro wn under control and salinity stress conditions. Radiolabeling with [C-14] L-aspartic acid indicated that, unlike in bacteria, decarboxylation of L-as partic acid is not the source of beta-alanine in the Plumbaginaceae.