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Review
. 2024 Mar 19;2(2):13.
doi: 10.1007/s44307-024-00022-y.

Significance of Raffinose Family Oligosaccharides (RFOs) metabolism in plants

Huan Liu #  1 , Fan Wang #  1 , Baohui Liu  1 , Fanjiang Kong  2 , Chao Fang  3
Affiliations
Review

Significance of Raffinose Family Oligosaccharides (RFOs) metabolism in plants

Huan Liu et al. Adv Biotechnol (Singap). .

Abstract

Raffinose Family Oligosaccharides (RFOs) are a kind of polysaccharide containing D-galactose, and they widely exist in higher plants. Synthesis of RFOs begins with galactinol synthase (GolS; EC 2.4.1.123) to convert myo-inositol into galactinol. The subsequent formation of raffinose and stachyose are catalyzed by raffinose synthase (RS; EC 2.4.1.82) and stachyose synthase (STS; EC 2.4.1.67) using sucrose and galactinol as substrate, respectively. The hydrolysis of RFOs is finished by α-galactosidase (α-Gal; EC 3.2.1.22) to produce sucrose and galactose. Importance of RFOs metabolism have been summarized, e.g. In RFOs translocating plants, the phloem loading and unloading of RFOs are widely reported in mediating the plant development process. Interference function of RFOs synthesis or hydrolysis enzymes caused growth defect. In addition, the metabolism of RFOs involved in the biotic or abiotic stresses was discussed in this review. Overall, this literature summarizes our current understanding of RFOs metabolism and points out knowledge gaps that need to be filled in future.

Keywords: Galactinol synthase; RFOs; Raffinose synthase; Stachyose synthase; Stress; α-Galactosidase.

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Conflict of interest statement

Declarations. Ethics approval and consent to participate: All analyses were performed under laboratory conditions. Consent for publication: The authors declare that they agreed with the content and that all gave explicit consent to submit and that they obtained consent from the responsible authorities at the institute where the work has been carried out, before the work is submitted. Competing interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Fig. 1
Fig. 1
Proposed model of RFOs synthesis. The RFOs pathway is represented with key genes, namely, GolS, RS and STS. GolS: galactinol synthase; RS, raffinose synthase; STS, stachyose synthase. The structure of sugars was downloaded from NCBI (https://pubchem.ncbi.nlm.nih.gov/)
Fig. 2
Fig. 2
The structure of cucumber leaf veins and phloem loading. a Representation of cucumber vein orders in leaves. b The anatomic structure of vascular bundle in the fifth vein. MC, mesophyll cell; IC, intermediary cell; CC, companion cell; SE, seive element; Tr, tracheids cell; PPC, phloem parenchyma cell
Fig. 3
Fig. 3
The phloem unloading pathway of cucumber. a The schematic diagram of cucumber plants when altered CsAGA2 expression. b The feedback regulation of sink organs on source leaves around CsAGA2. c Model cucumber fruit phloem unloading. Sugar (mostly stachyose and raffinose with a small amount of sucrose) synthesized in source leaves is transported to and unloaded in sink organs, as fruit. Some of the stachyose and raffinose are hydrolyzed to sucrose by α-galactosidase (α-Gal) in the release phloem from the peduncle to gynophore and fruit MVB, and then may be broken down into fructose and UDP-glucose by SUS or into fructose and glucose by INV in companion cells. CsSWEET7a can further export hexoses to the apoplasmic space. Cell wall acid invertase (CWINV) would most likely break down sucrose into fructose and glucose in the apoplasm. The apoplasmic hexoses could be transported into phloem parenchyma cells (PPCs) by hexose transporters (HTs). There also may exist sucrose unloading pathway mediated by other sucrose exporters and importers. Abbreviation: MVB, main vascular bundle; SE, sieve element; CC, companion cell; PPC, phloem parenchyma cell
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