Evidence that cyclin AMP is involved in the hypersensitive response of Medicago sativa to a fungal elicitor

In response to challenge with a glycoprotein elicitor from the phytopathogenic fungus, Verticillium alboatrum, synthesis of the phytoalexin medicarpin is induced in Medicago sativa. Treatment of seedlings with the cell-permeating cyclic AMP analogue, dibutyryl cyclic AMP, resulted in the stimulation of phenylalanine ammonia lyase activity and induction of medicarpin synthesis. Challenge of M. sativa cell suspension cultures with fungal elicitor resulted in a significant, transient increase in intracellular cyclic AMP content, together with a pulse of adenylyl cyclase activity, both within a few minutes of the elicitor challenge, and a subsequent increase in cyclic AMP phosphodiesterase activity. Incubation of a membrane fraction from the M. sativa cells with the fungal elicitor resulted in a dose-dependent stimulation of the adenylyl cyclase activity. These observations are discussed in the context of the signal transduction mechanism with the M. sativa cellular defence system.