In order to determine why the activated methyl cycle is up-regulated in plants 
undergoing defence responses to fungal pathogens we have monitored the 
utilisation of methyl groups derived from methionine in cell-suspension cultures 
of alfalfa (Medicago sativa L.) treated for various times with fungal elicitor, 
by carrying out a parallel labelling study with [(35)S]methionine and 
[methyl-(3)H]methionine. The distribution of the two radiolabels among the 
medium, soluble cellular components and cell wall was then determined. In the 
absence of elicitor the utilisation of the two radiolabels was similar. However, 
in the presence of the elicitor the total incorporation of radioactivity from 
[methyl-(3)H]methionine into metabolites was far greater than from 
[(35)S]methionine, indicating that the methyl label had been utilised in 
methylation reactions. Elicitor treatment resulted in up to a sixfold increase 
in the use of (3)H-methyl groups in the methylation of hydrophobic metabolites. 
In the period 0-24 h after elicitor treatment, increased methylation was 
directed largely into the synthesis of the isoflavonoid phytoalexin medicarpin 
and related metabolites. Newly synthesized phytoalexins were exported into the 
medium, while a significant proportion of the medicarpin accumulating in the 
cell in the early stages of elicitation was derived from the hydrolysis of its 
respective conjugate. Elicitor treatment also modified the incorporation of 
(3)H-methyl groups into the cell wall. Between 0 and 24 h after elicitor 
treatment the methylation of pectin in the cell wall declined. After 24 h, 
pectin methylation recovered and was associated with an increase in the 
methylation of other wall-bound polysaccharide components. Since no other major 
metabolic sink for the increased methylation was determined we conclude that the 
increased activity of the activated methyl cycle during defence interactions in 
alfalfa is required to support phytoalexin synthesis and cell wall 
modifications.