Molecular cloning, characteristics and low temperature response of raffinose synthase gene in Cucumis sativus L

Raffinose synthase (RS, EC2.4.1.82) is one of the key enzymes that channels sucrose into the raffinose family oligosaccharides (RFOs) biosynthetic pathway. However, the gene encoding RS is poorly characterized in cucumber (Cucumis sativus L.), which is a typical RFOs-translocating plant species. Here we isolated the gene encoding RS (CsRS) from the leaves of cucumber plants. The complete cDNA of CsRS consisted of 2552 nucleotides with an open reading frame encoding a polypeptide of 784 amino acid residues. Reverse transcription-polymerase chain reaction and RNA hybridization analysis revealed that expression of CsRS was the highest in leaves followed by roots, fruits, and stems. The RS activity was up-regulated and the raffinose content was high in the leaves of transgenic tobacco with over-expression of CsRS, while both the RS activity and the raffinose content decreased in the transgenic cucumber plants with anti-sense expression of CsRS. The expression of CsRS could be induced by low temperature and exogenous phytohormone abscisic acid (ABA). In cucumber growing under low temperature stress, CsRS expression, RS activity and raffinose content increased gradually in the leaves, the fruits, the stems and the roots. The most notable increase was observed in the leaves. Similarly, the expression of CsRS was induced in cucumber leaves and fruits with 200 μM and 150 μM ABA treatments, respectively.