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Proc Natl Acad Sci U S A. May 31, 2005; 102(22): 8054–8059.
Published online May 23, 2005. doi:  10.1073/pnas.0501456102
PMCID: PMC1142375
Plant Biology

A central role for S-nitrosothiols in plant disease resistance

Abstract

Animal S-nitrosoglutathione reductase (GSNOR) governs the extent of cellular S-nitrosylation, a key redox-based posttranslational modification. Mutations in AtGSNOR1, an Arabidopsis thaliana GSNOR, modulate the extent of cellular S-nitrosothiol (SNO) formation in this model plant species. Loss of AtGSNOR1 function increased SNO levels, disabling plant defense responses conferred by distinct resistance (R) gene subclasses. Furthermore, in the absence of AtGSNOR1, both basal and nonhost disease resistance are also compromised. Conversely, increased AtGSNOR1 activity reduced SNO formation, enhancing protection against ordinarily virulent microbial pathogens. Here we demonstrate that AtGSNOR1 positively regulates the signaling network controlled by the plant immune system activator, salicylic acid. This contrasts with the function of this enzyme in mice during endotoxic shock, where GSNOR antagonizes inflammatory responses. Our data imply SNO formation and turnover regulate multiple modes of plant disease resistance.

Keywords: S-nitrosylation, salicylic acid, nitric oxide

Plants have evolved a complex series of integrated defense systems in response to microbial colonization. Nonhost disease resistance (NHR) provides the primary bulwark against the vast majority of potential parasites. Characteristically, protection is provided by a series of preformed physical and chemical barriers or by the activation of defense reactions in a nonspecific fashion (1, 2). For successful colonization, a pathogen must first evolve to suppress, avoid, or tolerate these primary defenses by acquiring matching pathogenicity factors. To combat infection by such organisms, plants rely on a plethora of resistance (R) gene products, which recognize corresponding avirulence (avr) proteins in the pathogen, triggering a battery of protective responses (3). This so-called gene-for-gene resistance has long been exploited by plant breeders to provide effective disease control. Superimposed upon this mechanism is an additional defense network that serves to limit the growth of virulent pathogens that manage to evade R protein recognition. Termed basal resistance, this last line of defense provides an additional barrier limiting the extent of infection (4, 5). Collectively, these systems provide a highly effective shield against pathogen colonization; consequently, disease is a rare outcome among plant–microbe interactions.

A conspicuous feature underpinning these multiple modes of disease resistance is the production of nitric oxide (NO) (6, 7). This key redox signaling molecule also has pivotal roles in plant development (8) and is known to function in the immune, nervous, and vascular systems of animals (9). The plant enzyme responsible for pathogen-inducible NO production has recently been uncovered (10, 11) and is distinct from the archetypal NO synthases (NOS) found in mammals (12). Despite the importance of NO signaling in eukaryotes, the mechanisms responsible for the transduction and turnover of NO bioactivity are not well understood. In this context, the S-nitrosylation of cysteine thiols is emerging as an important theme (13). The formation of protein S-nitrosothiols (SNOs) may serve to both stabilize and diversify NO-related signals. Furthermore, the functions of an increasing number of key regulatory proteins in animals are thought to be modulated via this prototypic redox-based posttranslational modification (14).

Until recently, protein S-nitrosylation was thought to be controlled principally through the regulation of NO biosynthesis (14). Emerging evidence, however, indicates that SNO turnover may provide an alternative regulator y mechanism. S-nitrosylation of the antioxidant tripeptide glutathione forms S-nitrosoglutathione (GSNO), which is thought to function as a mobile reservoir of NO bioactivity (9). An enzyme has now been uncovered that metabolizes this molecule (15). Whereas this GSNO reductase (GSNOR) activity is highly specific for GSNO, it has been shown to control intracellular levels of both GSNO and SNO proteins in yeast and mice (15). Recently, a related plant protein has also been shown to exhibit GSNOR activity (16, 17).

Here we show that mutations in AtGSNOR1, an Arabidopsis thaliana GSNOR, influence cellular SNO levels both under basal conditions and during attempted microbial ingress. In the absence of AtGSNOR1 function R gene-mediated defense, basal resistance and NHR are all compromised. Our findings infer that AtGSNOR1 positively regulates the salicylic acid (SA) signaling network. Collectively, these data reveal that SNO formation and turnover control multiple modes of plant disease resistance.

Materials and Methods

Plant Material and Cultivation. Arabidopsis accession Col-0 and mutants derived from it were grown under 16 h of light at 22°C and 8 h of darkness at 18°C. atgsnor1-1 and atgsnor1-2 were identified by screening Syngenta's (Research Triangle Park, NC) SAIL (formerly GARLIC) T-DNA insertion collection (18), for insertions in AtGSNOR1 (At5g43940). atgsnor1-3 was identified from the GABI-Kats T-DNA insertion collection (19). The full length AtGSNOR1 sequence was used to query both the SAIL and GABI-Kat databases of Arabidopsis sequences flanking T-DNA left borders. Plants homozygous for the T-DNA insertion were identified by PCR (18). The atgsnor1-3R line contains a wild-type clone of AtGSNOR1 that was generated by using the following primers: 5′-ATAGCGGCCGCTACTAGAGTACAACCTCTT-3′ and 5′-ATAGCGGCCGCTAATGGGCTTGCGATATC-3′. After sequence verification, this sequence was cloned into the NotI site of pART27 (20) and introduced by floral-dip transformation (21). atgsnor1-3R was a representative line that contained a single copy of the AtGSNOR1 transgene.

RNA Blot Hybridization. Total RNA was isolated by using an RNA isolation kit (Qiagen, Crawley, U.K.). RNA blot hybridization was carried out as described (22) by using the following cDNA probes: AtGSNOR1 and the SA-dependent genes PR1 and PR2 (23).

Enzyme Assays. GSNOR activity was measured at 25°C by monitoring the decomposition of NADH at 340 nm, as described (7). Activity was determined by incubating 100 μg of protein in 300 μl of assay mix that contained 20 mM Tris·HCl (pH 8.0), 0.2 mM NADH, and 0.5 mM EDTA. The reaction was initiated by the addition of GSNO to the mix at a final concentration of 400 μM.

NOS activity was determined by grinding frozen leaf tissue with NOS extraction buffer in liquid nitrogen as described in ref. 8. The resulting extract was used to measure NOS activity by using a NOS assay kit (8).

Laser-Scanning Confocal Microscopy and Histochemical Staining. Arabidopsis leaves infected with Blumeria graminis f.sp. tritici (Bgt) were cleared and stained with Alexa Fluor 488 as described by Yun et al. (24). Laser-scanning confocal microscopy was performed by using a Bio-Rad Radiance 2100 system mounted on a Nikon Eclipse TE300 inverted microscope, as described (24).

To visualize Hyaloperonospora parasitica structures, infected leaves were stained with lactophenol trypan blue and cleared with chloral hydrate, as described (5).

Pathogen Inoculations. Pseudomonas syringae pv. tomato (Pst) strain DC3000 was grown, maintained, and inoculated as described (25). Brief ly, bacterial suspensions of virulent PstDC3000 or avirulent PstDC3000 expressing either avrB or avrRps4 in 10 mM MgCl2 were infiltrated into the abaxial side of the leaf with a 1-ml syringe. All Pst strains were inoculated at 105 colony-forming units (cfu)·ml–1. After 5 days, disease symptoms were recorded by using a Nikon digital camera, and leaf disks were harvested for analysis of bacterial growth (25). Pseudomonas syringae pv. phaseolicola (Psp)(NPS3121) was cultured as described (26), and leaves were inoculated as above at 105 cfu·ml–1. Bgt and H. parasitica Noco2 were maintained and inoculated as reported (5, 24).

Determination of SA, SA-β-glucoside (SAG), and SNO Levels. SA and SAG concentrations were determined as described (27). For the measurement of SNO levels, proteins were extracted in two volumes of 50 mM KH2PO4 and 1 mM PMSF. Protein extracts were filtered through a MicroBioSpin-6 column (Bio-Rad). SNO levels in the total lysate and in a fraction filtered through a 5-kDa cutoff ultrafiltration membrane (low-mass SNO) were measured by photolysis chemiluminescence and normalized for protein content as detailed (15).

Results

GSNOR1 Controls Cellular SNO Levels. An Arabidopsis gene, which we name AtGSNOR1, encodes a product with sequence similarity to proteins that possess GSNOR activity (15). Homozygous lines with distinct T-DNA insertions were isolated in the Arabidopsis ecotype Columbia (Col-0). Two lines, designated atg-snor1-1 and atgsnor1-2, accumulated increased levels of AtG-SNOR1 transcripts compared with wild-type Col-0 plants, whereas no transcripts were detected in a third line, atgsnor1-3 (Fig. 1A). Leaf extracts from these Arabidopsis lines were assayed for the presence of GSNOR activity. atgsnor1-1, atgsnor1-2, and atgsnor1-3 lines exhibited 189%, 165%, and 21%, respectively, of the GSNOR activity detected in wild-type plants (Fig. 1B). GSNOR activity was restored, however, in atgsnor1-3 plants rescued with a wild-type AtGSNOR1 transgene, designated line atgsnor1-3R. Together, these data indicate that atgsnor1-1 and atgsnor1-2 increase AtGSNOR1 activity, whereas atgsnor1-3 is a loss-of-function mutation in AtGSNOR1.

Fig. 1.
AtGSNOR1 regulates intracellular SNO levels. (A) AtGSNOR1 transcript accumulation determined by Northern blot in the stated Arabidopsis genotypes. (B) GSNOR activity in leaf extracts from wild-type (WT) and mutant Arabidopsis lines. Enzyme activity assays ...

The bacterial pathogen Pst DC3000 is ordinarily virulent on Col-0 plants and causes symptoms resembling bacterial speck disease of tomato (25). In Col-0 plants, the product of the bacterial avirulence gene avrB is recognized by a single dominant R gene, RPM1 (28, 29). Thus, PstDC3000 expressing avrB elicit a gene-for-gene resistance response in this Arabidopsis accession, leading to a reduction in bacterial growth. To investigate the impact of mutations in AtGSNOR1 on SNO metabolism, we determined cellular SNO levels in PstDC3000(avrB) challenged and unchallenged plants. In the absence of pathogen, SNO levels in atgsnor1-1, atgsnor1-2, and atgsnor1-3 plants were 62%, 72%, and 198% of those determined in wild-type plants, respectively (Fig. 1C). Increased GSNOR activity in atgsnor1-1 and atg-snor1-2 plants therefore reduced basal SNO levels, whereas a decrease in this enzyme activity in atgsnor1-3 plants produced increased levels of these molecules. Cellular SNO levels increased in atgsnor1-1, atgsnor1-2, and atgsnor1-3 plants to 20%, 23%, and 220%, respectively, of those determined for wild-type plants, after challenge with avirulent PstDC3000(avrB) (Fig. 1C). Attempted pathogen infection therefore triggers a pronounced accumulation of SNOs in host cells, which was most conspicuous in atgsnor1-3 plants. This defect in SNO catabolism was rescued in the atgsnor1-3R line.

The observed perturbations in total and low molecular weight (Mr) SNOs in atgsnor1-1 and atgsnor1-3 plants are unlikely to be a consequence of changes in inducible NOS (iNOS) activity in these lines, because NO production after PstDC3000(avrB) challenge was similar to that determined for wild-type plants (Fig. 6, which is published as supporting information on the PNAS web site). Mutations in AtGSNOR1 therefore provide a genetic means to distinguish in planta the bioactivity of SNOs from NO or other reactive nitrogen intermediates (RNI). We also determined whether increased SNO formation induced by PstDC3000(avrB) was iNOS-dependent. Inhibition of iNOS activity in wild-type plants by NG-nitro-l-Arg-methyl ester during attempted PstDC3000(avrB) infection reduced SNO formation to 17% of that recorded for wild-type plants in the absence of this inhibitor (Fig. 7, which is published as supporting information on the PNAS web site).

GSNOR1 Is Required for R Gene-Mediated Resistance. We next investigated the impact of mutations in AtGSNOR1 upon the establishment of R gene-mediated disease resistance. The major class of Arabidopsis R genes encodes nucleotide-binding site and leucine-rich repeat (NBS-LRR) proteins (3), which resemble Nod proteins involved in animal innate immunity (30). These Nod-like proteins are subdivided into two subclasses, which have distinct signaling requirements, depending upon the possession of either a coiled-coil or a domain with similarity to the Drosophila Toll and mammalian interleukin-1 receptors (TIR), in their N termini (31). We determined whether AtGSNOR1 was required for R gene function by measuring growth of the avirulent pathogen PstDC3000(avrB) within inoculated leaves.

Wild-type plants expressed gene-for-gene resistance against PstDC3000(avrB) due to the presence of the CC-NBS-LRR gene RPM1 (28, 29). Although the response of the atgsnor1-1 line was indistinguishable from wild-type plants, RPM1-mediated protection was abolished in the atgsnor1-3 mutant line (Fig. 2 A and B). To determine whether AtGSNOR1 function is required for disease resistance established by an alternative R gene subclass, plants were inoculated with PstDC3000 expressing the avrRps4 gene. This gene product is recognized in the Arabidopsis Col-0 accession by the TIR-NBS-LRR protein, RPS4 (32). Both wild-type and atgsnor1-1 plants expressed resistance against this bacterial strain (Fig. 2 C and D). Conversely, conspicuous disease symptoms developed on atgsnor1-3 plants, implying RPS4 function was compromised. Resistances established by either of these R genes were rescued in the atgsnor1-3R line.

Fig. 2.
R gene-mediated resistance requires ATGSNOR1 function. (A and C) Leaves from stated Arabidopsis plant lines 5 days postinoculation with either PstDC3000(avrB) (A) or PstDC3000(avrRps4) (C). (B and D) Growth of either PstDC3000(avrB)(B) or PstDC3000(avrRps4 ...

GSNOR1 Controls Basal Disease Resistance. R gene-mediated disease resistance is superimposed upon a basal defense system that serves to limit the growth of virulent pathogens within susceptible plant hosts (4, 5). To explore whether GSNOR1 activity is also required for the development of this resistance mechanism, we challenged plants with the virulent pathogen PstDC3000. Severe disease symptoms, reflecting increased pathogen growth, developed on the atgsnor1-3 line (Fig. 3 A and B). Compared with wild-type and atgsnor1-3R plants, the atgsnor1-1 mutant line was strikingly resistant against PstDC3000 (Fig. 3 A and B).

Fig. 3.
ATGSNOR1 controls basal disease resistance. (A and B) Growth of virulent PstDC3000 containing an empty vector (A) and images of the corresponding inoculated leaves from the stated Arabidopsis lines (B), 5 days postinoculation (dpi). (C and D) Quantification ...

The Noco2 race of the H. parasitica causes downy mildew on some accessions of Arabidopsis (5). To determine whether AtGSNOR1 is also a requirement for the development of basal resistance against this pathogen, plants were inoculated with the Noco2 race of H. parasitica, which is virulent on wild-type Col-0 plants. Substantially increased growth of H. parasitica Noco2 was observed on the atgsnor1-3 line compared with either wild-type or atgsnor1-3R plants, resulting in the production of more abundant hyphae, conida, and oospores (Fig. 3 C and D). atgsnor1-1 plants conversely failed to support significant growth of H. parasitica Noco2.

GSNOR1 Is Required for NHR. Arabidopsis expresses NHR against the wheat powdery mildew pathogen Bgt (24). Spores (conidia) produced from this fungus germinate on the surface of wheat leaves, their host plant, rapidly producing infection structures (appressoria) to aid host cell penetration. This is followed by the development of digitate bilateral haustoria, which function as conduits for the transfer of host nutrients within penetrated wheat epidermal cells. To examine whether AtGSNOR1 also plays a role in NHR, Arabidopsis plants were inoculated with Bgt conidia. In both wild-type, atgsnor1-1, and atgsnor1-3R plants, Bgt growth was typically strongly suppressed (Fig. 4A), only ≈1.0% of infection attempts resulted in the development of an abnormal unilateral haustorium (Fig. 4B). In contrast, atgsnor1-3 plants supported a marked increase in the formation of haustoria, with 3.4% of infection attempts producing these fungal structures (Fig. 4A). Furthermore, 1.5% of these infection attempts resulted in the development of bilateral haustoria, similar to those that develop in wheat, a host plant for Bgt (Fig. 3C). Many of these bilateral haustoria also supported the growth of secondary hyphae. Neither bilateral haustoria nor secondary hyphae were ever observed on wild-type, atgsnor1-3R, or atgsnor1-1 plants.

Fig. 4.
NHR is significantly reduced in atgsnor1-3 plants. (A) Growth of Bgt in the stated plant lines scored as the percentage of Bgt conidia that develop to form a haustorium. (B and C) Three-dimensional projections of confocal optical sections showing an abnormal ...

Arabidopsis also expresses NHR against the bean halo blight pathogen Psp(NPS3121) (26). NHR against Psp(NPS3121) was also compromised in atgsnor1-3 plants (Fig. 4D). In contrast, the atgsnor1-1 and atgsnor1-3R lines exhibited a similar level of resistance to wild-type plants.

GSNOR1 Is a Positive Regulator of the SA Signaling Network. Plants that constitutively express subsets of defense genes are often resistant to pathogens (33). We therefore asked whether either basal resistance in atgsnor1-1 plants or disease susceptibility in the atgsnor1-3 mutant line were associated with alterations in defense gene expression. Accumulation of PR1 mRNA, which marks the expression of SA-dependent genes (23), was substantial in wild-type plants but was reduced and delayed in the atgsnor1-3 line in response to virulent PstDC3000 (Fig. 5A). In contrast, PR1 was expressed with accelerated kinetics in atg-snor1-1 plants. Corresponding profiles of PR1 gene expression were also obtained after inoculation of these plant lines with either avirulent PstDC3000(avrB) (Fig. 5B) or Bgt (Fig. 5C).

Fig. 5.
AtGSNOR1 positively regulates the SA-signaling network. (A–C) Northern blot analysis of PR1 transcript accumulation in the stated Arabidopsis lines at the listed times postinoculation of virulent PstDC3000 (A), PstDC3000(avrB)(B), and Bgt (C). ...

To determine whether the suppression of SA-dependent genes occurs either upstream or downstream of SA accumulation in atgsnor1-3 plants, we first tested whether exogenous application of this signaling molecule could induce the buildup of PR1 mRNA. A substantial increase in PR1 transcripts were detected from 6 h after SA application in both wild-type and atgsnor1-1 plants (Fig. 5D). However, PR1 mRNA transcript accumulation was substantially reduced in the atgsnor1-3 mutant line. Thus, atgsnor1-3 compromises SA-induced gene expression.

We also measured the concentration of SA and its sugar conjugate, SAG (34), in PstDC3000(avrB) challenged and unchallenged plants. Both the basal and pathogen-induced levels of SA in the atgsnor1-3 mutant line were only 16% and 30%, respectively, of those present in wild-type plants (Fig. 5E). The concentrations of SA in unchallenged and challenged atgsnor1-1 plants, however, were not significantly different from those found in wild-type. Furthermore, the relative levels of SAG in these plants paralleled those exemplified by SA (Fig. 5F). These trends were also recapitulated in atgsnor1-3 and atgsnor1-1 plants after inoculation with either PstDC3000 or Bgt. However, as SA and SAG accumulated to significantly lower levels in these interactions, combined SA and SAG (total SA) levels were determined (Fig. 5 G and H).

Discussion

The transduction and turnover of NO bioactivity remain poorly understood, especially within the context of plant biology. In this study, we demonstrate that SNOs play a central role in the regulation of multiple modes of disease resistance. NO bioactivity is therefore not exclusively controlled at the level of NO biosynthesis during the plant defense response. An additional regulatory system, which depends upon AtGSNOR1 activity, is also operational. Although GSNO is thought to be the only substrate SNO for this enzyme, loss of AtGSNOR1 function resulted in greater increases in SNO proteins than low Mr SNOs (including GSNO) alone, as has recently been observed in mice and yeast (15). Hence, key SNO proteins may be in equilibrium with GSNO in pathogen challenged and unchallenged plant cells, and this equilibrium may favor SNO proteins. Furthermore, increases in SNOs occurred in the absence of increased NO production, suggesting SNOs can transduce NO bioactivity in plants after attempted pathogen infection. Importantly, our data indicate that NO and SNOs may undertake distinct functional roles during the establishment of plant disease resistance. Thus, whereas a reduction in NO accumulation leads to pathogen susceptibility (6, 11), a decrease in the concentration of SNOs promotes protection against microbial infection.

Our findings indicate that AtGSNOR1 is required for the expression of defense responses established by both subclasses of NBS-LRR genes. Therefore, AtGSNOR1 is likely to operate either coincident with or downstream of the point at which these distinct recognition systems function. It is noteworthy that a PstDC3000 strain carrying an empty vector grew to a significantly higher level in atgsnor1-3 plants than one expressing either avrB or avrRps4. Thus, R gene-mediated resistance is likely to be reduced rather than abolished in the atgsnor1-3 line. Nevertheless, these data highlight AtGSNOR1 as an important and widely utilized component of resistance protein signaling networks.

In the absence of AtGSNOR1, basal resistance against either a bacterial or an oomycete pathogen was strongly reduced compared with that expressed in wild-type plants. Conversely, the efficacy of this defense system was substantially enhanced in atgsnor1-1 and atgsnor1-2 plants, which exhibit increased GSNOR1 activity. The sum of these data indicates that AtGSNOR1 function is also required for the establishment of basal disease resistance against diverse microbial pathogens. The genetic or chemical manipulation of GSNOR1 activity may therefore provide new opportunities for disease control.

In addition, the atgsnor1-3 line exhibited enhanced susceptibility to Bgt and Psp(NPS3121). Neither atgsnor1-1 nor atgsnor1-2 plants, however, showed increased NHR, as was the case for R gene-mediated protection. Therefore, unlike basal resistance, these defense systems may already be operating at levels of efficiency that are not further enhanced by increases in GSNOR1 activity. We infer that AtGSNOR1 is required for NHR expressed against either a fungal or bacterial pathogen, suggesting this gene product may constitute a key regulator of NHR. Collectively, our pathology data are consistent with the notion that AtGSNOR1 is required for multiple modes of plant disease resistance.

Experiments designed to uncover the molecular mechanism(s) underpinning AtGSNOR1 function revealed that this gene product may impact the SA signaling network (35). Disease susceptibility in atgsnor1-3 plants was associated with reduced and delayed expression of SA-dependent genes. Conversely, increased basal disease resistance in atgsnor1-1 plants correlated with the accelerated expression of this subset of defense genes. Together, these data suggest that AtGSNOR1 mediates SA gene expression in response to microbial pathogens. Surprisingly, the accumulation of PR gene transcripts in atgsnor1-3 plants after the exogenous application of SA was strikingly reduced compared with that of wild-type and atgsnor1-1 plants, indicating that AtGSNOR1 functions either coincident with or downstream of SA accumulation. This was unexpected, because numerous genetic screens undertaken in a number of independent laboratories have identified only alleles of a single nonredundant gene, npr1/nim1/sai1 (4, 3638), whose product is known to operate downstream of SA accumulation. Presumably this is because atgsnor1-3 strongly reduces, rather than abolishes, SA-induced PR gene expression. Also noteworthy was the discovery that SA production was strikingly reduced in atgsnor1-3 plants in response to virulent, avirulent, and nonhost pathogens. Therefore, defects in the control of SA biosynthesis may be a general feature of atgsnor1-3 plants in response to attempted microbial ingress. Collectively, these data support the assumption that AtGSNOR1 controls the SA signaling network by positively regulating at least two distinct nodes, one positioned upstream of SA accumulation and the other located either coincident with or downstream of this immune system activator. In this context, a number of plant proteins have recently been identified that are S-nitrosylated in vitro (39). Alternatively, AtGSNOR1 may act only downstream of SA. Because EDS1 and PAD4 are required for SA biosynthesis after infection and are themselves induced by SA (40, 41), if AtGSNOR1 is involved in SA-induced expression of EDS1/PAD4, then a mutation in AtGSNOR1 could affect both SA signaling and SA biosynthesis.

Recent data indicate that GSNOR acts as a negative regulator of inflammatory responses, providing protection against nitrosative stress and tissue injury in a mouse model of endotoxic shock (42). In contrast, our findings highlight a role for GSNOR1 as a pivotal positive regulator of immune function in plants that operates early in the infection process to suppress pathogen growth.

Supplementary Material

Supporting Figures:

Acknowledgments

We thank Roger Innes (Indiana University, Bloomington) and Walter Gassmann (University of Missouri, Columbia) for the kind gift of PstDC3000 strains expressing avrB and avrRps4, respectively. We thank Jane Parker (Max Planck Institute, Cologne, Germany) for providing H. parasitica Noco2. Psp(NPS3121) was kindly provided by Jian-Min Zhou (Kanasas State University, Manhattan). We acknowledge Syngenta for providing Bgt isolate RW14 and PR1 and PR2. Arabidopsis T-DNA insertion mutants were obtained from the GABI-Kat (Bernd Weisshaar, Max Planck Institute for Plant Breeding Research, Cologne, Germany) and SAIL (Syngeneta) populations. A.F. was supported by a Biotechnology and Biological Sciences Research Council CASE studentship.

Notes

Author contributions: G.J.L. designed research; A.F., E.K., B.-W.Y., and Y.W. performed research; A.F., E.K., B.-W.Y., Y.W., J.A.P., and G.J.L. analyzed data; and G.J.L. wrote the paper.

This paper was submitted directly (Track II) to the PNAS office.

Abbreviations: NHR, nonhost disease resistance; R, resistance gene; avr, avirulence; SNO, S-nitrosothiols; GSNO, S-nitrosoglutathione; GSNOR, GSNO reductase; AtGSNOR1, A. thaliana GSNOR; SA, salicylic acid; NOS, NO synthase; SAG, SA-β-glucoside; Pst, Pseudomonas syringae pv. Tomato; Bgt, Blumeria graminis f.sp. tritici; Psp, Pseudomonas syringae pv. phaseolicola.

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