Biochemistry and physiology of raffinose family oligosaccharides and galactosyl cyclitols in seeds

Abstract

Raffinose family oligosaccharides (RFOs) are of almost ubiquitous occurrence in plant seeds. They accumulate during seed development and disappear rapidly during germination. The biosynthesis of raffinose, the first member of the series, proceeds by addition of a galactosyl unit to sucrose. Galactinol, a galactosyl derivative of myo-inositol, acts as a galactosyl donor. It is synthesized from UDP-D-galactose and myo-inositol. Stachyose, verbascose and ajugose, the next higher RFOs, are either synthesized by galactinol-dependent galactosyltransferases or by transfer of galactosyl units between two RFO molecules. In seeds, the metabolism of methylated inositols, such as D-ononitol and D-pinitol, is linked with the RFO pathway. In contrast to myoinositol, these cyclitols are galactosylated by transfer of galactosyl residues from galactinol and not from UDP-Dgalactose. However, the resulting galactosyl cyclitols can replace galactinol as galactosyl donors for the biosynthesis of stachyose. These recently discovered branches of the RFO pathway are active in seeds of a range of crop species, especially in legumes. We focus here on the biochemistry and molecular biology of the enzymes of RFO and galactosyl cyclitol biosynthesis. The metabolic control and hormonal regulation of the pathway during seed development and germination is discussed. The controversial role of � -galactosidases, which are believed to hydrolyse RFOs during germination, is reviewed critically.