Metabolic process of raffinose family oligosacharrides during cold stress and recovery in cucumber leaves

Raffinose family oligosacharrides (RFOs) accumulate under stress conditions in many plants and have been suggested to act as stress protectants. To elucidate the metabolic process of RFOs under cold stress, levels of RFOs and relative carbohydrates, the expression and activities of main metabolic enzymes and their subcellular compartments were investigated during low temperature treatment and recovery period in cucumber leaves. Cold stress induced the accumulation of stachyose in vacuoles, galactinol in vacuoles and cytosols, and sucrose and raffinose in vacuoles, cytosols and chloroplasts. After cold stress removal, levels of these sugars decreased gradually in respective compartments. Among 4 galactinol synthase genes ( CsGS ), CsGS1 was not affected by the cold stress, while other three CsGSs were up-regulated by the low temperature. RNA levels of acid-α-galactosidase (GAL) 3, alkaline-α-galactosidase (AGA) 2 and 3 , and the activities of GAL and AGA were up-regulated after cold stress removal. The GAL3 protein and GAL activity were exclusively located in the vacuole, whereas the protein of AGA2 and AGA 3 were found in the cytosol and chloroplast respectively. The results indicate that RFOs accumulated during the cold stress in different subcellular compartments in cucumber leaves could be catabolized in situ by different galactosidases after stress removal.