Interactions between polyamine and ethylene metabolisms in a hormone autonomous sugarbeet callus

In a fully habituated non-organogenic sugarbeet callus (HNO) overproducing polyamines and underproducing ethylene (in comparison with its normal hormone-dependent counterpart), the question raised about a possible competition between these two metabolites for their common precursor, S-adenosyl-methionine (SAM). The experimental strategy consisted in determining the effects of exogenous polyamines and inhibitors of polyamine biosynthetic pathway on growth, polyamine accumulation and ethylene production. Exogenous putrescine or spermidine decreased polyamine contents and ethylene production. Inhibitors of the diamine putrescine biosynthesis, DFMO and DFMA, induced a reduction of both polyamine content and ethylene production with an increase of HNO callus growth. However, when a mixture of the two inhibitors was used, an increase of ethylene production was observed without any effect on growth. The inhibitors of spermidine synthase (CHA) and of SAM decarboxylase (MGBG) also decreased polyamine content and ethylene production with different effects on growth according to the concentrations used. The combination of the two inhibitors (CHA + MGBG) increased ethylene production of the HNO callus. The effect of growth regulators (auxin and cytokinin) on growth and ethylene production of HNO callus is also discussed. These results suggest that polyamines affect directly the ethylene biosynthesis. In the absence of an exogenous hormonal control, the lower ethylene metabolism of HNO callus could not be explained by a competition with polyamines for their common precursor.