Figure 1.

Sequence of events associated with the establishment of SAR. Upon primary infection of a plant leaf with a necrotizing pathogen,¹ a yet unknown systemic signal(s) is distributed systemically throughout the plant.² The signal causes systemic accumulation of salicylic acid (SA).³ SA causes direct activation of SAR genes,⁴ some of which encode enzymes with antimicrobial activity. SA also conveys the tissue to the primed state⁵ which is characterized by an enhanced capacity to activate defense responses upon secondary pathogen attack. The faster and/or stronger activation of defense responses at the sites of secondary infection results in a decrease in disease symptoms,⁶ reflecting the SAR state.⁷