Abstract
Abiotic stress induces differential expression of genes responsible for the synthesis of raffinose family of oligosaccharides (RFOs) in plants. RFOs are described as the most widespread D-galactose containing oligosaccharides in higher plants. Biosynthesis of RFOs begin with the activity of galactinol synthase (GolS; EC 2.4.1.123), a GT8 family glycosyltransferase that galactosylates myo-inositol to produce galactinol. Raffinose and the subsequent higher molecular weight RFOs (Stachyose, Verbascose, and Ajugose) are synthesized from sucrose by the subsequent addition of activated galactose moieties donated by Galactinol. Interestingly, GolS, the key enzyme of this pathway is functional only in the flowering plants. It is thus assumed that RFO synthesis is a specialized metabolic event in higher plants; although it is not known whether lower plant groups synthesize any galactinol or RFOs. In higher plants, several functional importance of RFOs have been reported, e.g., RFOs protect the embryo from maturation associated desiccation, are predominant transport carbohydrates in some plant families, act as signaling molecule following pathogen attack and wounding and accumulate in vegetative tissues in response to a range of abiotic stresses. However, the loss-of-function mutants reported so far fail to show any perturbation in those biological functions. The role of RFOs in biotic and abiotic stress is therefore still in debate and their specificity and related components remains to be demonstrated. The present review discusses the biology and stress-linked regulation of this less studied extension of inositol metabolic pathway.
Highlights
Raffinose family of oligosaccharides (RFOs) are α-1, 6-galactosyl extensions of sucrose (Suc)
galactinol synthase (GolS) serves as a crosslink between central inositol (Ino) metabolism and RFO biosynthesis, and controls entry of Ino into the process
There are two RFO pools in its leaves: a storage pool associated with leaf mesophyll and a transport pool associated with the phloemloading sites (Bachmann et al, 1994) where Raf and especially Sta are produced and loaded in the phloem, according to the polymer trapping model (Turgeon, 1991)
Summary
Raffinose family of oligosaccharides (RFOs) are α-1, 6-galactosyl extensions of sucrose (Suc). This group of oligosaccharides is found in plants and is known to serve as desiccation protectant in seeds, as transport sugar in phloem sap and as storage sugars. The galactosyl group of RFOs is donated by galactinol (Gol; 1-O-α- -galactopyranosyl- -myo-inositol). Synthesis of Gol is a key and absolute requirement for entering into the pathway of RFO biosynthesis. The key enzyme galactinol synthase (GolS, EC 2.4.1.123) is the primary checkpoint in RFO flux, which synthesizes Gol in plants using UDP-Galactose (UDP-Gal) and L-myo-inositol. Within the RFO biosynthetic pathway, the other two major enzymes are (a) raffinose synthase (RafS, EC 2.4.1.82) that transfers a galactosyl moiety from Gol to Suc and synthesizes Raffinose (Raf)
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