Nitric oxide and reactive oxygen species are required for systemic acquired resistance in plants.

Systemic acquired resistance (SAR) is a form of broad-spectrum disease resistance that is induced in response to primary infection and that protects uninfected portions of the plant against secondary infections by related or unrelated pathogens. SAR is associated with an increase in chemical signals that operate in a collective manner to confer protection against secondary infections. These include, the phytohormone salicylic acid (SA), glycerol-3-phosphate (G3P), azelaic acid (AzA) and more recently identified signals nitric oxide (NO) and reactive oxygen species (ROS). NO, ROS, AzA and G3P function in the same branch of the SAR pathway, and in parallel to the SA-regulated branch. NO and ROS function upstream of AzA/G3P and different reactive oxygen species functions in an additive manner to mediate chemical cleavage of the C9 double bond on C18 unsaturated fatty acids to generate AzA. The parallel and additive functioning of various chemical signals provides important new insights in the overlapping pathways leading to SAR.