REVIEW ARTICLE Betaine aldehyde dehydrogenase in plants T. L. Fitzgerald, D. L. E. Waters & R. J. Henry Grain Foods CRC, Centre for Plant Conservation Genetics, Southern Cross University, Lismore, NSW, Australia INTRODUCTION Betaine aldehyde dehydrogenase (BADH) enzymes are classified as substrate-specific oxidoreductases (EC 1.2.1.8) and belong to family 10 of the large superfamily of aldehyde dehydrogenases (Sophos & Vasiliou 2003). BADHs catalyse the oxidation of betaine aldehyde (BA) to betaine. Trimethylglycine was the first betaine to be characterised and is by far the most extensively studied; this compound is now referred to as ‘glycine betaine’ (GB) to distinguish it from other betaines. In plants, the study of betaine has almost exclusively focused on GB. GB is known to be particularly effective in conferring protection against abiotic stresses such as salt, water defi- cit, heat and chilling (Le Rudulier et al. 1984). In general terms, a plant BADH refers to an enzyme that converts BA to GB, using an oxidising co-factor (Fig. 1). The reaction can be either NAD + or NADP + dependent, however, plant BADHs show higher activity using NAD + (Weigel et al. 1986; Nakamura et al. 1997). Choline monooxgenase (CMO) and BADH work together in the pathway to GB production in plants, with CMO initially converting choline, a derivative of proline, to BA (Fig. 1). Many plants possess more than one putative BADH-encod- ing gene homologue, most commonly two have been reported, and these are generally referred to as BADH1 and BADH2 (for the purposes of this review they are annotated BADH1 and BADH2 to distinguish the gene from the enzyme). However, not all plants accumulate GB, and it has been suggested that this is due to the lack of a func- tional CMO (Nuccio et al. 1998). Initial enzyme activity studies and molecular cloning of a plant BADH were per- formed in spinach (Pan et al. 1981; Weretilnyk & Hanson 1990). Subsequently, numerous putative BADHs have been isolated based on the homology of these genes to the spin- ach BADH gene. This has led to the classification of many putative BADH-encoding genes without substrate specific- ity and enzyme activity data for the enzyme that they encode. This review provides an analysis of developments in the field of plant genes and enzymes that have been clas- sified as a betaine aldehyde dehydrogenase or putative beta- ine aldehyde dehydrogenase. HIGH BADH HOMOLOGY AMADHS A number of studies performed in the last decade have shown that some genes with high BADH homology encode enzymes that possess affinity for a range of Keywords Abiotic stress tolerance; aminoaldehydes; antibiotic-free selection; genetic engineering; glycine betaine; rice fragrance; salt stress. Correspondence R. J. Henry, Grain Foods CRC, Centre for Plant Conservation Genetics, Southern Cross University, PO Box 157, Lismore, NSW, Australia. E-mail: robert.henry@scu.edu.au Editor W. Ernst Received: 7 August 2008; Accepted: 16 September 2008 doi:10.1111/j.1438-8677.2008.00161.x ABSTRACT Plant betaine aldehyde dehydrogenases (BADHs) have been the target of substantial research, especially during the last 20 years. Initial characterisa- tion of BADH as an enzyme involved in the production of glycine betaine (GB) has led to detailed studies of the role of BADH in the response of plants to abiotic stress in vivo, and the potential for transgenic expression of BADH to improve abiotic stress tolerance. These studies have, in turn, yielded significant information regarding BADH and GB function. Recent research has identified the potential for BADH as an antibiotic-free marker for selection of transgenic plants, and a major role for BADH in 2-acetyl-1- pyrroline-based fragrance associated with jasmine and basmati style aro- matic rice varieties. Plant Biology ISSN 1435-8603 Plant Biology 11 (2009) 119–130 ª 2008 German Botanical Society and The Royal Botanical Society of the Netherlands 119