Nitric Oxide-Mediated Chemical Signaling during Systemic Acquired Resistance

Reactive nitrogen (nitric oxide, NO) and oxygen species are important free radicals that play a vital role in a number of physiological responses, including plant defence. More recently they have been shown to play an important role in systemic acquired resistance (SAR), which is induced in response to primary infection and confers broad-spectrum disease resistance to secondary infections. NO functions upstream of reactive oxygen species (ROS) and other SAR signals (azelaic acid (AzA) and glycerol-3-phosphate (G3P)) and is regulated by galactolipid digalactosyl diacylglycerol as well as fatty acid (oleic acid). Oleic acid and other C18 unsaturated fatty acids containing a double bond at carbon 9 also serve as a precursor to AzA, which functions upstream of G3P. The NO-ROS branch of SAR pathway functions in parallel to salicylic acid, and these branches likely undergo crosstalk to regulate the optimal induction of signals. This review summarizes the recent advances in the role of free radicals in SAR and discusses its relationship to other SAR inducers.