Nitric oxide and salicylic acid signaling in plant defense

^*Waksman Institute and Department of Molecular Biology and Biochemistry, Rutgers, The State University of New Jersey, 190 Frelinghuysen Road, Piscataway, NJ 08854-8020; and ^‡‡Plant Science Department/AgBiotech Center, Rutgers, The State University of New Jersey, 59 Dudley Road, New Brunswick, NJ, 08901

Abstract

Salicylic acid (SA) plays a critical signaling role in the activation of plant defense responses after pathogen attack. We have identified several potential components of the SA signaling pathway, including (i) the H₂O₂-scavenging enzymes catalase and ascorbate peroxidase, (ii) a high affinity SA-binding protein (SABP2), (iii) a SA-inducible protein kinase (SIPK), (iv) NPR1, an ankyrin repeat-containing protein that exhibits limited homology to IκBα and is required for SA signaling, and (v) members of the TGA/OBF family of bZIP transcription factors. These bZIP factors physically interact with NPR1 and bind the SA-responsive element in promoters of several defense genes, such as the pathogenesis-related 1 gene (PR-1). Recent studies have demonstrated that nitric oxide (NO) is another signal that activates defense responses after pathogen attack. NO has been shown to play a critical role in the activation of innate immune and inflammatory responses in animals. Increases in NO synthase (NOS)-like activity occurred in resistant but not susceptible tobacco after infection with tobacco mosaic virus. Here we demonstrate that this increase in activity participates in PR-1 gene induction. Two signaling molecules, cGMP and cyclic ADP ribose (cADPR), which function downstream of NO in animals, also appear to mediate plant defense gene activation (e.g., PR-1). Additionally, NO may activate PR-1 expression via an NO-dependent, cADPR-independent pathway. Several targets of NO in animals, including guanylate cyclase, aconitase, and mitogen-activated protein kinases (e.g., SIPK), are also modulated by NO in plants. Thus, at least portions of NO signaling pathways appear to be shared between plants and animals.

Footnotes

↵ † To whom reprint requests should be addressed. E-mail: klessig{at}mbcl.rutgers.edu.
↵ ‡ Present address: Institute of Biochemical Plant Pathology, GSF–National Research Center for Environment and Health, D-85764 Oberschleissheim, Germany.
↵ § Present address: Natural Environment Research Council Institute of Virology and Environmental Microbiology, University of Oxford, Mansfield Road, Oxford OX1 3SR, U.K.
↵ ¶ Present address: Biochimie, Biologie Cellulaire et Moleculaire des Interactions–Plantes/Microorganismes, Unite Mixte Institut National de la Recherche Agronomique–Universite de Bourgogne, Institut National de la Recherche Agronomique, BV 1540, 17 rue Sully, 21034 Dijon, France.
↵ ‖ Present address: Division of Biology, Kansas State University, 303 Ackert Hall, Manhattan, KS, 66506.
↵ ** Department of Biochemistry, University of Missouri, 117 Schweitzer Hall, Columbia, MO 65211.
↵ †† Department of Biology, Faculty of Sciences, University of Chile, Casilla 653, Santiago, Chile.
This paper was presented at the National Academy of Sciences colloquium “Virulence and Defense in Host–Pathogen Interactions: Common Features Between Plants and Animals,” held December 9–11, 1999, at the Arnold and Mabel Beckman Center in Irvine, CA.
Abbreviations:

ADPRC,

ADP-ribosyl cyclase;

BTH,

benzothiadiazole;

cADPR,

cyclic ADP ribose;

CWD,

cell wall-derived;

GC,

guanylate cyclase;

HR,

hypersensitive response;

INA,

2,6-dichloroisonicotinic acid;

IRP-1,

iron regulatory protein 1;

JA,

jasmonic acid;

MAP,

mitogen-activated protein;

NO,

nitric oxide;

NOS,

NO synthase;

NtACO1,

Nicotiana tabacum aconitase 1;

PAL,

phenylalanine ammonia lyase;

PR,

pathogenesis-related;

ROS,

reactive oxygen species;

SA,

salicylic acid;

SAR,

systemic acquired resistance;

SIPK,

salicylic acid-induced protein kinase;

TMV,

tobacco mosaic virus;

WIPK,

wounding-induced protein kinase