Nitric oxide (NO) can mediate cellular effects through the second messenger cGMP or by direct interaction with target proteins. The mechanisms of direct interaction of NO with target proteins includes S-nitrosylation of thiol-containing proteins and tyrosine nitration, as well as direct interaction of NO with metal-containing proteins. NO signaling is essential for pathogen defense pathways in plants. Navarre et al. have discovered that tobacco aconitase (an iron-sulfur-containing enzyme) is inhibited in vitro by NO via a direct interaction. Animal aconitases play an important role in regulating cellular metabolism in response to oxidative stress and in regulating iron homeostasis by controlling translation of iron-responsive genes. It remains to be determined whether the plant aconitases have similar functions.