Abstract
The diversity of biosynthesis of pyridine nucleotides and related secondary metabolites in plants was investigated using more than 40 species. In contrast to animals, the de novo pathway of NAD synthesis in plants is initiated from aspartate, and nicotinamide produced by the catabolism of NAD and was converted to nicotinic acid, and then reutilised the synthesis of NAD. In contrast to other organisms, the seven-member pyridine nucleotide cycle is operative in plants. In addition, bypasses catalysed by newly discovered nicotinic acid riboside kinase and/or nicotinamide deaminase appear to be functional in some plant species. All plants investigated produced trigonelline (TG) or nicotinic acid glucoside (NaG) as secondary metabolites derived from nicotinic acid. TG is formed in a wide variety of plant species. Large quantity of TG is accumulated in seeds of coffee and Leguminosae plants. NaG formation was found in limited species including Brassicaceae plants, such as Arabidopsis and canola. Organ specific formation of TG and NaG was also found in some species, for example, TG is formed in potato leaves, but NaG is formed in tubers. Possible roles of TG and NaG will be discussed.
