Raffinose Family Oligosaccharides (RFOs) are water-soluble but non-reducing and non-structural derivatives of sucrose to which galactosyl units are added. RFOs are currently emerging as crucial molecules during stress response in plants, because of their membrane-stabilizing, antioxidant and, perhaps, predicted signal functions. RFOs are reported to be synthesized and stored in seeds as an essential source of rapidly metabolizable carbon for early germination events, to protect the embryo from maturation associated desiccation, to participate in several cellular functions, and to accumulate in vegetative tissues in response to a range of abiotic stresses. The role of RFOs in abiotic stress is due to any reported perturbations in those biological functions in loss-of-function mutants still in debate and their specificity and related components remain to be experimentally demonstrated. Biochemical data of plant raffinose and stachyose synthases are insufficiently reported, due to obvious expression problems in Escherichia coli. This project focuses on the functional expression of six putative raffinose synthase genes (AtRS1-6) in E. coli, especially AtRS4 and AtRS5, annotated in the genome of A. thaliana. Little is known about these putative raffinose synthases and their biochemical characteristics and their contribution to the RFO physiology in Arabidopsis.