Ammonium requirement for radiation-induced accumulation of polyamines in suspension-cultured grape cells.

The effects of radiation-mediated free radical production on polyamine metabolism were investigated in grape cells (Vitis vinifera L. cv. Gamay) using a cell suspension culture. Putrescine (Put) synthesis was triggered in irradiated cells (0.5 kGy) only when ammonium was present in the culture medium. Under these conditions, Put accumulated to a continuously high level. As also described for other kinds of stress, the level of spermidine was slightly enhanced and that of spermine unchanged. The role of ammonium was assessed by studying non-irradiated cell cultures. In the presence of ammonium, a transient increase of both arginine decarboxylase (ADC) activity and of Put synthesis was observed during the lag phase of growth. This Put enhancement was inhibited by difluoromethyl arginine and not by difluoromethyl ornithine, showing that increased Put synthesis occurs via the ADC pathway. When ammonium was withheld from the culture medium, ADC activity was still triggered though transient Put accumulation was completely suppressed. These results emphasize the importance of ammonium availability in cultured cells as a limiting factor for Put production. Polyamine synthesis, therefore, cannot be stimulated by gamma irradiation in the absence of an ammonium supply. These results support the hypothesis that Put synthesis is a detoxification process of the ammonium produced as a result of nitrogen recycling within stressed plant cells. [ABSTRACT FROM AUTHOR]