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

β ‐Alanine betaine is an osmoprotective compound accumulated by most members of the plant family Plumbaginaceae. Leaf and root tissues of Limonium latifolium known to accumulate β ‐alanine betaine readily convert supplied β ‐alanine to β ‐alanine betaine. To identify the intermediates and the enzymes involved in β ‐alanine betaine synthesis, radiotracer experiments using [C] formate were employed. These studies demonstrate that β ‐alanine betaine is synthesized from β ‐alanine via N ‐methyl and N,N‐ dimethyl β ‐alanines. A rapid and sensitive radiometric assay was developed to measure N ‐methyltransferase (NMT) activities by using [methyl‐ 14 C] or [methyl‐ 3 H] S ‐adenosyl‐ l ‐methionine (AdoMet) as the methyl donor. Leaf extracts from β ‐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 β ‐alanine betaine. The NMT 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 ‐adenosyl‐ l ‐homocysteine. Extractable activities were similar from plants grown under control and salinity stress conditions. Radiolabeling with [C] l ‐aspartic acid indicated that, unlike in bacteria, decarboxylation of l ‐aspartic acid is not the source of β ‐alanine in the Plumbaginaceae.