Veith, R; Komor, E: Regulation of growth, sucrose storage and ion content in sugarcane cells, measured with suspension cells in continuous-culture grown under nitrogen, phosphorus of carbon limitation, Journal of Plant Physiology, 142, 414-424 (1993)
A continuous culture of heterotrophic sugarcane (Saccharum sp.) suspension cells was established on a chemically defined medium. This allowed culture growth at constant, defined nutrient conditions including very low concentrations, where a particular nutrient is growth-limiting. Cells grown under nitrogen, phosphorus or carbon limitation were analyzed for cell components such as sucrose, amino acids, cations, anions and enzymes, which are involved in sucrose metabolism. Thereby, the influence of the particular nutrient on sucrose storage and solute composition of the cells was identified.High levels of sucrose storage were only reached under nitrogen starvation, i.e. at very low levels of amino acids, not under phosphorus or carbon starvation, indicating a direct regulation of sucrose metabolism by amino acids. The starch content was not correlated to sucrose storage. The amino acid content of cells, which was very high under optimal growth conditions, became low under carbon as well as under nitrogen limitation, whereas under phosphorus limitation a strong shift into amino acids deriving from pyruvate was observed. Phosphate and sulfate accumulation were inversely correlated in the sense that the decrease in cellular phosphate (e.g. under P- or N-limitation) led to an increase in sulfate. Highest phosphate levels were observed under carbon limitation. Accumulation of sulfate had the additional effect that it was accompanied by an increase in cellular sodium.The rate of net uptake of carbon was roughly correlated with the growth rate. The same was true for the respiration rate, except under nitrogen limitation, which strongly suppressed respiration. The ratio of carbon assimilation to carbon dissimilation therefore shifted significantly in favor of assimilation under nitrogen limitation, whereas carbon and phosphorus limitation had only a small effect on this ratio. Sucrose synthesis usually constituted only a very small portion of the assimilated carbon (1-6%), except in the case of nitrogen limitation where it amounted to 37%. Net sucrose storage is the result of a rapid cycle of sucrose synthesis and sucrose hydrolysis. The rates of the individual enzymes involved in sucrose metabolism were more related to the growth rate than to the rate of sucrose storage. Invertase activity neither correlated with growth nor with storage, but it responded immediately to changes of the growth rate.The metabolic background for the effect of particular nutrients, especially for the regulatory.role of amino acids in sucrose storage, is discussed.

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