Elicitor induced defence responses in medicago sativa.

•  The effects are reported here of an elicitor prepared from liquid cultures of an isolate of Verticillium albo-atrum, which is nonpathogenic to Medicago sativa, on accumulation of H2 O2 , medicarpin, deposition of phenolic polymer and phenylalanine ammonia-lyase (PAL) activity in cultured cells of Medicago sativa L. cv. Kabul (lucerne). •  PAL activity and phytoalexins were assayed spectrophotometically and by HPLC, respectively. The scopoletin fluorescence-quenching and thioglycolic acid methods were used to measure H2 O2 and phenolic polymer deposition, respectively. •  Studies with inhibitors suggested that an NAD(P)H oxidase and a peroxidase were involved in the elicitor stimulated accumulation of H2 O2 and that an increase in cytosolic Ca2+ , but not H2 O2 , was part of a signalling pathway leading to the induction of defence responses. •  Both the influx of Ca2+ and release of Ca2+ from intracellular stores forms part of the signalling pathway leading from perception of elicitor to induction of defence responses. Although H2 O2 is not part of the pathway, evidence is presented that O2 - is part of the signal transduction chain.