AMMONIUM REQUIREMENT FOR RADIATION-INDUCED ACCUMULATION OF POLYAMINESIN SUSPENSION-CULTURED GRAPE CELLS

The effects of radiation-mediated free radical production on polyamine metabolism were investigated in grape cells (Vitis vinifera L. cv. Ga may) using a cell suspension culture. Putrescine (Put) synthesis was t riggered in irradiated cells (0.5 kGy) only when ammonium was present in the culture medium. Under these conditions, Put accumulated to a co ntinuously high level. As also described for other kinds of stress, th e level of spermidine was slightly enhanced and that of spermine uncha nged. The role of ammonium was assessed by studying non-irradiated cel l cultures. In the presence of ammonium, a transient increase of both arginine decarboxylase (ADC) activity and of Put synthesis was observe d during the lag phase of growth. This Put enhancement was inhibited b y difluoromethyl arginine and not by difluoromethyl ornithine, showing that increased Put synthesis occurs via the ADC pathway. When ammoniu m was withheld from the culture medium, ADC activity was still trigger ed 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, th erefore, cannot be stimulated by gamma irradiation in the absence of a n ammonium supply. These results support the hypothesis that Put synth esis is a detoxification process of the ammonium produced as a result of nitrogen recycling within stressed plant cells.