Microcystin has been suggested to induce dualistic effects in animals and plants by playing a role in apoptosis and cellular proliferation processes (Gehringer, 2004). Several molecular markers have been referred to be involved in the regulation of these cellular processes (Campos and Vasconcelos, 2010; Gehringer, 2004). To our knowledge the dualistic effects have not been reported for the cyanotoxin CYN, nevertheless few molecular and biochemical data is available to rigorously assess this characteristic. Our work suggests that CYN alone or in action with other cyanobacterial metabolites may have a role in these processes in plants by regulating the oxidative stress defense mechanisms.