Sucrose storage in cell suspension cultures of Saccharum sp. (sugarcane) is regulated by a cycle of synthesis and degradation

We have investigated the regulation of sucrose storage in cell-suspension cultures of sugarcane. When grown in batch culture, sucrose accumulation commences after about 5 d when the nitrogen supply is exhausted. Sucrose storage is also induced by decreasing the nitrogen supply to cells growing in a chemostat. The measured activity of sucrose-phosphate synthase is high enough to account for the rate of sucrose accumulation, provided precautions are taken to avoid the hydrolysis UDP during the assay. The cells contained high sucrose-synthase activity but pulsing experiments with [14C]glucose and unlabelled fructose indicated that this enzyme did not contribute substantially to the synthesis of sucrose because the glucosyl and fructosyl moieties of sucrose were equally labelled. Several lines of evidence demonstrate the presence of a cycle in which sucrose is synthesized and degraded simultaneously; sucrose-phosphate-synthase activity doubles during the phase when the cells are actively storing sucrose but activity is also high after storage has ceased, or when the sucrose is being remobilised; pulse experiments with [14C]fructose also showed that sucrose synthesis occurs not only during the storage phase, but also after storage has stopped and during the rapid mobilisation of sucrose; the cells contain high activities of sucrose synthase and alkaline invertase and these are both at a maximum when sucrose storage is occurring; even during the storage phase. [14C]fructose pulses lead to labelling of free glucose which is evidence for rapid synthesis and degradation of sucrose. It is proposed that the rate and extent of sucrose storage is regulated by this cycle of synthesis and degradation. Measurements of enzyme activities and metabolite levels are presented, and it is discussed which factors could contribute to the regulation of these two opposing fluxes and, hence, the rate of net sucrose storage and mobilisation.