Content area
Full Text
Analysis of the Raffinose Family Oligosaccharide Pathway in Pea Seeds with Contrasting Carbohydrate Composition1
Raffinose family oligosaccharides (RFOs) are synthesized by a set of galactosyltransferases, which sequentially add galactose units from galactinol to sucrose. The accumulation of RFOs was studied in maturing seeds of two pea (Pisum sativum) lines with contrasting RFO composition. Seeds of the line SD1accumulated stachyose as the predominant RFO, whereas verbascose, the next higher homolog of stachyose, was almost absent. In seeds of the line RRRbRb, a high level of verbascose was accumulated alongside with stachyose. The increase in verbascose in developing RRRbRb seeds was associated with galactinol-dependent verbascose synthase activity. In addition, a galactinol-independent enzyme activity was detected, which catalyzed transfer of a galactose residue from one stachyose molecule to another. The two enzyme activities synthesizing verbascose showed an optimum at pH 7.0. Both activities were almost undetectable in SDl. Maximum activity of stachyose synthase was about 4-fold higher in RRRbRb compared with SD1, whereas the activities of galactinol synthase and raffinose synthase were only about 1.5-fold higher in RRRbRb. The levels of galactinol synthase and stachyose synthase activity were reflected by steady-state levels of corresponding mRNAs. We suggest that the accumulation of verbascose in RRRbRb was controlled by a coordinated up-regulation of the last steps of verbascose biosynthesis.
Many higher plants accumulate raffinose family oligosaccharides (RFOs) during seed maturation. These carbohydrates consist of Gal units linked to Suc via a-(16) glycosidic linkages. RFOs have been proposed to act as protective agents during desiccation and storage of seeds in the dry state (for review, see Horbowicz and Obendorf, 1994; Obendorf, 1997), although there is no evidence for a causal relationship between their accumulation and the acquisition of desiccation tolerance (Black et al., 1999; Bentsink et al., 2000; Buitink et al., 2000). Although RFOs have long been regarded as antinutritional factors in human nutrition, recent data support a beneficial role of RFOs as so-called prebiotics, by specifically stimulating growth of remedial gut bacteria (Voragen, 1998; Aranda et al., 2000).
A set of galactosyltransferases is involved in the biosynthesis of RFOs (for review, see Peterbauer and Richter, 2001). Galactinol synthase (EC 2.4.1.123) catalyzes the synthesis of galactinol (O-a-D-galactopyranosyl-[1-l]-L-myo-inositol) from UDP-DGal and myo-inositol (Liu et al., 1998; Sprenger and Keller,...