Nitric oxide in plant immunity.

Abstract

Once touted as a toxin and then as a powerful effector of cardiac, brain, lung, genitourinary, gastrointestinal, and immune function—but ultimately exposed by the relatively unremarkable phenotypes of transgenic mice deficient in each of the three major NO synthase (NOS) isoforms—NO and molecules derived from it are now revealing more subtle, but highly influential, roles in signaling. So it is that more than one decade since nitric oxide biosynthesis was discovered in animals, scientists are only beginning to unravel the major function of this complex system (Fig. 1). Ironically, the very reactions of NO with redox centers in proteins and membranes, that were originally identified with injurious and polluting effects of the molecule, are now being established as molecular components of signal transduction pathways controlling smooth muscle tone, cell proliferation and adhesion, platelet activation, force production in heart and skeletal muscle, respiration, neurotransmission, hormone secretion, ion channel activity, apoptosis, transcriptional mechanisms, and host responses to infection (1–5). That NO has been widely adapted to serve a signaling role in biology is underscored by the distribution of NOSs throughout the animal kingdom (6) and in some fungi and bacteria (7–9).