Nitric oxide and salicylic acid signaling in plant defense

Salicylic acid (SA) plays a critical signaling role in the activation of pl ant defense responses after pathogen attack. We have identified several pot ential components of the SA signaling pathway, including (i) the H2O2-scave nging enzymes catalase and ascorbate peroxidase. (ii) a high affinity SA-bi nding protein (SABP2), (iii) a SA-inducible protein kinase (SIPK), (iv) NPR 1, an ankyrin repeat-containing protein that exhibits limited homology to I kappa B alpha and is required for SA signaling, and (v) members of the TGA /OBF family of bZIP transcription factors. These bZIP factors physically in teract with NPR1 and bind the SA-responsive element in promoters of several defense genes, such as the pathogenesis-related 1 gene (PR-1). Recent stud ies have demonstrated that nitric oxide (NO) is another signal that activat es defense responses after pathogen attack. NO has been shown to play a cri tical role in the activation of innate immune and inflammatory responses in animals. Increases in NO synthase (NOS)-like activity occurred in resistan t but not susceptible tobacco after infection with tobacco mosaic virus. He re we demonstrate that this increase in activity participates in PR-1 gene induction. Two signaling molecules, cGMP and cyclic ADP ribose (cADPR), whi ch function downstream of NO in animals, also appear to mediate plant defen se gene activation (e.g., PR-1). Additionally, NO may activate PR-1 express ion via an NO-dependent, cADPR-independent pathway. Several targets of NO i n animals, including guanylate cyclase, aconitase, and mitogen-activated pr otein kinases (e.g., SIPK), are also modulated by NO in plants. Thus, at le ast portions of NO signaling pathways appear to be shared between plants an d animals.