Alfalfa cell cultures treated with a fungal elicitor accumulate flavone metabolites rather than isoflavones in the presence of the methylation inhibitor tubericidin

Alfalfa cell suspension cultures accumulating isoflavonoid phytoalexins in response to exposure to a fungal elicitor were treated with cycloleucine, sinefungin or tubericidin; these three compounds disrupt transmethylation reactions in cells directly, or by altering the accumulation of S-adenosyl-L-methionine or S-adenosyl-L-homocysteine, respectively. As determined by measuring the reduction of the dye 2,3,5-triphenyltetrazolium, all compounds were equally toxic to the cells, but tubericidin was the most effective compound at inhibiting the accumulation of the phytoalexin medicarpin and the incorporation of radioactivity from L-[methyl-H-3]-methionine into phenolic metabolites. The specificity of the inhibition by tubericidin was confirmed by showing that the compound was an effective inhibitor of S-adenosyl-L-homocysteine hydrolase both in vivo and in vitro. In the presence of tubericidin, elicitor-treated alfalfa cell cultures turned yellow and this was associated with a reduction in the synthesis of isoflavonoid phytoalexins and the concomitant accumulation of the retrochalcone licodione, together with lesser amounts of 7,4'-dihydroxyflavone and 7,4'-dihydroxyflavanone. These results suggest that the methylation inhibitor tubericidin selectively inhibits the accumulation of isoflavones, with the flavone and licodione accumulating as alternative phytoalexins. Copyright (C) 1996 Elsevier Science Ltd