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

Abstract

<jats:p> <jats:italic>β</jats:italic>‐Alanine betaine is an osmoprotective compound accumulated by most members of the plant family Plumbaginaceae. Leaf and root tissues of <jats:italic>Limonium latifolium</jats:italic> known to accumulate <jats:italic>β</jats:italic>‐alanine betaine readily convert supplied <jats:italic>β</jats:italic>‐alanine to <jats:italic>β</jats:italic>‐alanine betaine. To identify the intermediates and the enzymes involved in <jats:italic>β</jats:italic>‐alanine betaine synthesis, radiotracer experiments using [ C] formate were employed. These studies demonstrate that <jats:italic>β</jats:italic>‐alanine betaine is synthesized from <jats:italic>β</jats:italic>‐alanine via <jats:italic>N</jats:italic>‐methyl and <jats:italic>N,N‐</jats:italic>dimethyl <jats:italic>β</jats:italic>‐alanines. A rapid and sensitive radiometric assay was developed to measure <jats:italic>N</jats:italic>‐methyltransferase (NMT) activities by using [methyl‐<jats:sup>14</jats:sup>C] or [methyl‐<jats:sup>3</jats:sup>H] <jats:italic>S</jats:italic>‐adenosyl‐ <jats:sc>l</jats:sc>‐methionine (AdoMet) as the methyl donor. Leaf extracts from <jats:italic>β</jats:italic>‐alanine betaine accumulators –<jats:italic>Armeria maritima</jats:italic>, <jats:italic>L. latifolium</jats:italic> and <jats:italic>L. ramosissimum</jats:italic>– had detectable NMT activities while none were found in <jats:italic>L. perezii</jats:italic>, a species that does not accumulate <jats:italic>β</jats:italic>‐alanine betaine. The NMT activities were further characterized from the leaves of <jats:italic>L. latifolium</jats:italic>. The activities had a pH optimum of 8.0, were soluble and inhibited by <jats:italic>S</jats:italic>‐adenosyl‐ <jats:sc>l</jats:sc>‐homocysteine. Extractable activities were similar from plants grown under control and salinity stress conditions. Radiolabeling with [ C] <jats:sc>l</jats:sc>‐aspartic acid indicated that, unlike in bacteria, decarboxylation of <jats:sc>l</jats:sc>‐aspartic acid is not the source of <jats:italic>β</jats:italic>‐alanine in the Plumbaginaceae.