Effector delivery structures of Gram-negative bacterium, oomycete, fungus, and nematode in plant cell. (A) Type III secretion system in Gram-negative bacterium injects effectors into the host cell. (B) The haustorium in biotrophic and hemibiotrophic filamentous pathogens is believed to be the site of effector release into the host cell. (C) Gland secretions, which include effectors, are injected into the plant cell via the stylet of the nematode. Effectors (E) thus delivered, can either suppress host defenses and/or trigger host cell defenses, which include programmed cell death (PCD) upon recognition by resistance (R) proteins. Recognition of effectors by R proteins may occur directly (observed with some fungal effectors) or indirectly as a result of interaction of the effectors with other host protein(s) (observed with a number of bacterial effectors). Potential subcellular locations of effectors such as the nucleus and chloroplasts are also shown.