Hordatines, dimerised hydroxycinnamoylagmatine conjugates of barley (Hordeum vulgare L.): an appraisal of the biosynthesis, chemistry, identification and bioactivities

Hydroxycinnamoylamides are specialised metabolites widely distributed in the plant kingdom. These are phenolic moieties covalently linked to mono- or polyamines through amide bonds. Their oxidative coupling (dimerisation) leads to neolignanamides, a group of compounds showing high chemical, structural and functional diversity. Typical to barley, dehydro dimers of hydroxycinnamoylagmatines, hordatines are primarily found in germinated seeds and at the seedling stage. The first step in the biosynthesis of hordatines is catalysed by acyl-coenzyme A-dependent N-hydroxycinnamoyltransferases, and lead to the formation of hydroxycinnamoylagmatines (HCAgms). The oxidative homo- or hetero-dimerisation of the latter results in different hordatines (A, B, C or D). Hordatines can also undergo various types of conjugation and form hydroxylated, methylated or glycosylated derivatives. Although the research on the bioactivities of the hordatines is still nascent, the in planta antifungal properties have long been recognised. While hordatines are naturally and uniquely synthesised in barley plants, these molecules or lead compounds derived therefrom, also exhibit medicinal and pharmaceutical uses important for human health, stimulating research into the utilisation of biotechnology in alternative production hosts and to enhance agricultural yields and value-added production. This review summarises the older and recent knowledge about hordatines and derivatives and may serve as a springboard for future research on this intriguing class of secondary plant metabolites.