Osmotic stress-induced changes of sucrose metabolism in cultured sweet potato cells

The intra- and extracellular sugar contents, the activities of sucrose-meta bolizing enzymes, and the metabolism of [U-C-14] glucose in a pulse-chase e xperiment were compared between the normal and osmotically stressed (by 0.6 M sorbitol) sweet potato (Ipomoea batatas) suspension cells. The stress en hanced the levels of sucrose and sucrose phosphate synthase (SPS) activity. Northern blot analysis also showed that prolonged osmotic stress enhanced the SPS gene expression at the transcriptional level. Stressed cells also h ad higher activities of sucrose cleaving enzymes, such as alkaline invertas e and sucrose synthase. The C-14-sucrose isolated from normal and stressed cells had C-14-fructose and C-14-glucose ratios of 0.68 and 1, respectively . These data suggest the continual cycling of degradation and synthesis of sucrose in both types of cells. Among the enzymes used in constructing such futile cycling, besides invertase and SPS, sucrose synthase (SS) should be involved in normal cells, but not in stressed ones. It is apparent that th e osmotic stress caused a significant change in the pattern of sucrose meta bolism.