Early Signalling Events in the Apoplastic Oxidative Burst in Suspension Cultured French Bean Cells Involve cAMP and Ca2

• Modulators of cAMP, calcium and G proteins were used to treat bean (Phaseolus vulgaris) cells before addition of an elicitor from Colletotrichum lindemuthianum in order to elucidate the early steps of signal transduction leading to the production of the apoplastic oxidative burst. • Hydrogen peroxide production by elicited bean cells was monitored with luminolor xylenol-orange-based assays. • Pretreatment with forskolin, dibutyryl cAMP or the Ca2+ ionophore A23187 enhanced the production of reactive oxygen species (ROS). The Ca2+ channel blocker, verapamil, and the calmodulin antagonist W7 led to a decreased oxidative burst and cancelled the dibutyryl cAMP effect. The production of ROS was increased by cholera toxin (CTX), an activator of G proteins. • Thus, an increase of cytosolic calcium ([ Ca2+] cyt) mediated through an increased level of cAMP is required for ROS production. The data support a role for G proteins and cAMP in extracellular alkalinization and Ca2+ influx, possibly in the provision of a reductant, which with the extracellular peroxidase, are required for the apoplastic oxidative burst.