5-Fluorouracil resistance in carrot cell cultures : Its use in studying the interaction of the pyrimidine and arginine pathways.

Physiological studies of 5-fluorouracil (5-FU)-resistant cell line of wild carrot (Daucus carota L.), F5, showed that this variant is also resistant to 5-fluorouridine, but is as sensitive to 6-azauracil as the 5-FU-sensitive parent line, WOO1C. High levels of exogenous uracil, uridine, and thymine are slightly toxic to F5, but not to WOO1C. 5-FU sensitivity in WOO1C cannot be reversed by bases and nucleosides; bases like uracil and thymine even increase 5-FU toxicity. No substantial differences were found in the uptake, incorporation and degradation of WOO1C and F5. Carrot cultures seem to take up 5-FU by rapid diffusion, the kinetics being characteristic of non-saturable uptake, with infinite Km and zero Vmax. The rapid uptake of 5-FU and extensive degradation of bases and nucleosides are probably responsible for the inability of uracil and uridine to reverse the growth inhibition caused by 5-FU in carrot cells while, as shown earlier, phaseolotoxin ((N-phosphosulfamyl)ornithinylalanylhomoarginine), an inhibitor of the arginine biosynthetic enzyme, ornithine transcarbamylase was capable of reducing 5-FU toxicity. F5 callus contained less histidine and arginine than WOO1C. 5-FU increased the endogenous levels of arginine, histidine and aspartate in both lines. The aspartate transcarbamylase of F5 appears to be normal; it is as sensitive to uridine-monophosphate inhibition as that of WOO1C. The 5-FU resistance of F5 was stable in undifferentiated cells, but only 8 out of 50 calli reinitiated from the regenerated plantlets remained resistant to 5-FU. F5 is an aneuploid culture. Five 5-FU-sensitive reinitiated calli that were examined were all diploid whereas of the eight 5-FU-resistant reinitiated calli two became diploid and six remained as aneuploid.