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.