Figure 2.

Model of the response of plants to biotrophic and necrotrophic plant pathogens (adapted from Glazebrook (2005): displayed are the interactions of Arabidopsis with the biotrophs Peronospora parasitica and Erysiphe ssp. and with the necrotrophs Alternaria brassicicola and Botrytis cinerea; SA, salicylic acid, JA, jasmonic acid, ET, ethylene; broken line arrows indicate to a possible interaction but which was not found in the chosen experiments while the solid line arrows indicate to the observed plant-pathogen-response). The defense reaction of Arabidopsis against biotrophic pathogens start with gene-for-gene recognition of the pathogen followed by rapid activation of defense and the production of reactive oxygen species (ROS), the so-called “oxidative burst,” which is by self a signal for defense activation. ROS production is connected with the hypersensitive response (HR), also called “programmed cell death,” which limits the access of biotrophs that feed on living tissue to water and nutrients. HR is associated with the activation of the salicylic acid (SA) dependent signaling pathway that is connected with systemic acquired resistance (SAR) and the expression of pathogenesis-related proteins. For necrotrophic pathogens a different defense line takes place as they feed on dead plant tissue and host cell death is not predicted to limit their growth. Defense against necrotrophic pathogens is mainly mediated by JA and ET controlled defense as well as production of phytoalexins such as camalexin. The broken line arrows indicate that also mixed defense lines are possible for other biotrophic or nectrotrophic pathogens.