The hunt for plant nitric oxide synthase (NOS): Is one really needed?

Nitric oxide (NO) production is associated with many physiological situations in plants, and NO is a key signaling molecule throughout the lifespan of a plant. The complexity of the underlying signaling events are just starting to be unraveled. The basis for nitric oxide signaling, the production of the signaling molecule itself, is far from understood in plants. While in animals, three homologous NO synthases (NOS) isoforms have been identified, yet in higher plants no corresponding enzymes are known so far. More than half a dozen NO productive reactions have been observed in plants but only few of them have been thoroughly investigated. It remains to be elucidated how these parts act together to form the sophisticated NO signaling network observed in plants.

► NO is a key signaling molecule over the entire lifespan of a plant and regulates many physiological processes.
► In animals, three NOS isoforms have been identified while in higher plants no corresponding enzymes are known so far.
► In higher plants NO signaling might be based on fundamentally different mechanisms.
► At least seven NO productive reactions have been observed in plants.
► It remains to be elucidated how these NO producers interact to form the sophisticated NO signaling network in plants.