In animals, nitric oxide (NO) is produced by a family of enzymes named nitric oxide synthases (NOSs). Although no NOS-like gene has been found in the Arabidopsis thaliana genome, biochemical studies have suggested that a NOS-like protein is likely to be activated in plants resisting pathogens. This protein has been recently identified as a variant P protein of glycine decarboxylase. This discovery means that studies of nitric oxide signalling functions in plants are now entering a new phase.