Introduction

Betaines are quaternary ammonium compounds (QACs) which contain a carboxylic acid group. They may generally be regarded as fully N-methylated amino or imino acids. While the most commonly encountered betaine (glycinebetaine) is structurally, although not biosynthetically, based on the simplest amino acid, a wide range of aliphatic and aromatic betaines have been described (Fig. 1). These include imino and aromatic amino acid betaines, and compounds such as trigonelline (nicotinic acid betaine) and homostachydrine (pipecolic acid betaine). Related compounds such as choline-O-sulphate and the tertiary sulphonium analogue of β-alaninebetaine, 3-dimethylsulphoniopropionate (DMSP), are also considered in this review since they may have similar physiological roles in some plants.

Although the distribution of betaines in plants is imperfectly known, partly because many taxa have not been thoroughly investigated, it is clear that some families normally contain significant amounts (> 10 mol m-3 plant water) of betaines. In lower plants betaines and related compounds may be incorporated into lipids (Sato, 1992). Recent developments in mass and NMR spectroscopy have greatly facilitated the qualitative and quantitative analysis of betaines. Molecular biology has provided useful tools for the study of regulation of key enzymes of betaine biosynthesis. In vitro studies have clarified the role of betaines as compatible solutes in plants subjected to a variety of stresses. Developments in these areas since the review by Wyn Jones and Storey (1981) will be discussed in detail here, and have also been reviewed by Rhodes and Hanson (1993).