Nuclear localization of NPR1 is required for activation of PR gene expression

Isolation of Arabidopsis nuclei and measurement of gene transcription rates using nuclear run-on assays

Isolation of transcriptionally active nuclei from plant tissues is a fundamental first step in many plant molecular biology protocols. Enriched nuclear fractions may be used in "run-on" assays to measure the rate of transcription for any given gene, adding additional resolution to assays of steady-state transcript accumulation such as RNA-gel blots, RT-PCR or microarrays. The protocols presented here streamline, adapt and optimize existing methods for use in Arabidopsis thaliana. Plant materials are ground in hexylene glycol-based buffers and highly enriched nuclear fractions are obtained using Percoll density gradients. Standard and small-scale protocols are presented, along with a tested method for nuclear run-on assays. The entire process may be completed within 3 days. This capability complements the immense body of steady-state transcript measurements and indirectly identifies instances where message turnover may have a critical and/or primary role in regulating gene expression levels.

SAD2, an importin -like protein, is required for UV-B response in Arabidopsis by mediating MYB4 nuclear trafficking

We report that the Arabidopsis thaliana mutant sensitive to ABA and drought2 (sad2), which harbors a T-DNA insertion in an importin beta-like gene, is more tolerant to UV-B radiation than the wild type. Analysis of cyclobutane pyrimidine dimer accumulation revealed that less DNA damage occurred in sad2 than in the wild type during UV-B treatment. No significant growth difference was observed between sad2 and the wild type when treated with the genotoxic drug methyl methanesulfonate, suggesting that SAD2 functions in UV-B protection rather than in DNA damage repair. Whereas the R2R3-type transcription repressor MYB4 has previously been shown to negatively regulate the transcription of cinnamate 4-hydroxylase (C4H) and thus to regulate the synthesis of sinapate esters, expression of both MYB4 and C4H and accumulation of UV-absorbing compounds were significantly higher in sad2 than in the wild type. MYB4 did not localize to the nucleus in the sad2 mutant, suggesting that SAD2 is required for MYB4 nuclear trafficking. SAD2 and MYB4 coimmunoprecipitated, indicating that these proteins localize in the same complex in vivo. MYB4 protein specifically bound to its own promoter in gel shift assays and repressed its own expression, demonstrating that MYB4 protein and mRNA are part of a negative autoregulatory loop. This feedback loop is altered in the sad2 mutant due to the absence of MYB4 protein in the nucleus, leading to the constitutive expression of MYB4 and C4H and resulting in accumulation of UV-absorbing pigments that shield the plant from UV-B radiation.

Tobacco NIMIN2 proteins control PR gene induction through transient repression early in systemic acquired resistance

NPR1 (for Nonexpressor of PR genes; also known as NIM1) is a positive regulator of systemic acquired resistance (SAR) in Arabidopsis, which controls the induction of Pathogenesis-Related (PR) genes by salicylic acid (SA). NPR1 interacts with members of two protein families, TGA transcription factors and NIMIN (for NIM1-interacting) proteins. In Arabidopsis, NIMIN1, NIMIN2 and NIMIN3 constitute a small gene family of structurally related, yet distinct members. To unravel the biological significance of NIMIN interaction with NPR1, we searched a tobacco yeast two-hybrid cDNA library for NPR1- and NIMIN2-binding proteins. One NPR1 cDNA clone and three clones encoding NIMIN proteins were isolated. Although clearly similar to At NPR1, Nt NPR1 does not interact with At NIMIN3. Furthermore, all Nt NIMIN proteins identified are structurally related to At NIMIN2, thus forming a small NIMIN2 subfamily in tobacco. cDNA clones encoding At NIMIN1 or At NIMIN3 homologues were not identified. The function of NIMIN2 proteins was studied by expression of Nt NIMIN2a chimeric genes in tobacco. While constitutive NIMIN2a over-expression delayed PR-1 protein induction, suppression of NIMIN2 transcripts enhanced the accumulation of PR-1 proteins. In both cases, the effects of altered NIMIN2 transcript levels became evident foremost early in SAR. Notably, Nt NIMIN2 gene expression is elevated prior to the induction of the PR-1a gene. Together, the data suggest that, in tobacco, NIMIN2 proteins control PR-1 gene expression, and that NIMIN2-mediated control is exerted through transient PR-1 repression before SAR has fully developed. Furthermore, although sharing conserved domains and functions, tobacco and Arabidopsis NPR1 and NIMIN proteins are clearly distinct.

MDM2 splice variants predominantly localize to the nucleoplasm mediated by a COOH-terminal nuclear localization signal

Of the >40 alternative and aberrant splice variants of MDM2 that have been described to date, the majority has lost both the well-characterized nuclear localization signal (NLS1) and the nuclear export signal (NES) sequence. Because cellular localization of proteins provides insight regarding their potential function, we determined the localization of three different MDM2 splice variants. The splice variants chosen were the common variants MDM2-A and MDM2-B. In addition, MDM2-FB26 was chosen because it is one of the few variants described that contains the complete p53-binding site. All three splice variants predominantly localized to the nucleus. Nuclear localization of MDM2-A and MDM2-B was controlled by a previously uncharacterized nuclear localization signal (NLS2), whereas nucleoplasmic localization of MDM2-FB26 was mediated by NLS1. p53 and full-length MDM2 colocalized with the splice variants in the nucleus. MDM2-A and MDM2-B both contain a COOH-terminal RING finger domain, and interaction with full-length MDM2 through this domain was confirmed. MDM2-FB26 was the only splice variant evaluated that contained a p53-binding domain; however, interaction between MDM2-FB26 and p53 could not be shown. p14(ARF) did not colocalize with the splice variants and was predominantly expressed within the nucleoli. In summary, nuclear localization signals responsible for the nucleoplasmic distribution of MDM2 splice variants have been characterized. Colocalization and interaction of MDM2-A and MDM2-B with full-length MDM2 in the nucleus have important physiologic consequences, for example, deregulation of p53 activity.

WRKY62 Transcription Factor Acts Downstream of Cytosolic NPR1 and Negatively Regulates Jasmonate-Responsive Gene Expression

Cytosolic NPR1 has been shown to be essential for the salicylic acid (SA)-mediated suppression of jasmonic acid (JA)-responsive gene expression. However, factors downstream of NPR1 in the cross-talk between SA and JA signaling are unclear. Here we show that Arabidopsis WRKY62, a member of WRKY group III transcription factors, was induced by methyl jasmonate (MeJA) and SA treatment. The presence of basal SA is required for the MeJA-induced WRKY62 expression, and both chemicals exhibit a synergistic effect on WRKY62 induction. In addition, upon treatment with an extremely low concentration of SA, cytosolic NPR1 controls the MeJA-induced expression of WRKY62. TGA transcription factors, which up-regulate SA-induced expression of WRKY62, are dispensable for the induction of WRKY62 in JA signaling. Genetic dissection of both wrky62 mutants and WRKY62-overexpressing plants indicated that WRKY62 down-regulates JA-responsive LOX2 and VSP2 expression. Our results demonstrate that WRKY62 acts downstream of cytosolic NPR1 and negatively regulates JA-responsive gene expression, suggesting that WRKY62 may be involved in the SA-mediated suppression of JA signaling.

OsWRKY13 mediates rice disease resistance by regulating defense-related genes in salicylate- and jasmonate-dependent signaling

Although 109 WRKY genes have been identified in the rice genome, the functions of most are unknown. Here, we show that OsWRKY13 plays a pivotal role in rice disease resistance. Overexpression of OsWRKY13 can enhance rice resistance to bacterial blight and fungal blast, two of the most devastating diseases of rice worldwide, at both the seedling and adult stages, and shows no influence on the fertility. This overexpression was accompanied by the activation of salicylic acid (SA) synthesis-related genes and SA-responsive genes and the suppression of jasmonic acid (JA) synthesis-related genes and JA-responsive genes. OsWRKY13 bound to the promoters of its own and at least three other genes in SA- and JA-dependent signaling pathways. Its DNA-binding activity was influenced by pathogen infection. These results suggest that OsWRKY13, as an activator of the SA-dependent pathway and a suppressor of JA-dependent pathways, mediates rice resistance by directly or indirectly regulating the expression of a subset of genes acting both upstream and downstream of SA and JA. Furthermore, OsWRKY13 will provide a transgenic tool for engineering wider-spectrum and whole-growth-stage resistance rice in breeding programs.

Genetic interactions of TGA transcription factors in the regulation of pathogenesis-related genes and disease resistance in Arabidopsis

TGA transcription factors are implicated as regulators of pathogenesis-related (PR) genes because of their physical interaction with the known positive regulator, nonexpresser of PR gene1 (NPR1). A triple-knockout mutant tga2-1 tga5-1 tga6-1 was shown previously to be defective in the induction of PR genes and systemic acquired resistance, confirming their role in disease resistance. However, the contributions of individual TGA factors have been difficult to discern because of functional redundancy among these factors, as well as possible dual functions for some single factors. In this study, we characterized six TGA factors by reverse genetics. We show that TGA3 is required for both basal and 2,6-dichloroisonicotinic acid-induced transcription of PR genes. The tga3-1 mutants were found to be defective in basal pathogen resistance, whereas induced resistance was unaffected. TGA1 and TGA4 play partially redundant roles in regulation of basal resistance, having only moderate effects on PR gene expression. Additionally, an activation-tagged mutant of TGA6 was able to increase basal as well as induced expression of PR1, demonstrating a positive role for TGA6 on PR gene expression. In contrast, TGA2 has repressor activity on PR gene expression even though it can act as a positive regulator in the tga5-1 tga6-1 null mutant background. Finally, we examined the genetic interaction between tga2-2 and suppressor of npr1 inducible1 (sni1-1). TGA2's repressor activity overlaps with SNI1 because the tga2-2 sni1-1 double mutant shows a synergistic effect on PR gene expression.

SA-inducible Arabidopsis glutaredoxin interacts with TGA factors and suppresses JA-responsive PDF1.2 transcription

Salicylic acid (SA) is a plant signaling molecule that mediates the induction of defense responses upon attack by a variety of pathogens. Moreover, it antagonizes gene induction by the stress signaling molecule jasmonic acid (JA). Several SA-responsive genes are regulated by basic/leucine zipper-type transcription factors of the TGA family. TGA factors interact with NPR1, a central regulator of many SA-induced defense responses including SA/JA antagonism. In order to identify further regulatory proteins of SA-dependent signaling pathways, a yeast protein interaction screen with tobacco TGA2.2 as bait and an Arabidopsis thaliana cDNA prey library was performed and led to the identification of a member of the glutaredoxin family (GRX480, encoded by At1g28480). Glutaredoxins are candidates for mediating redox regulation of proteins because of their capacity to catalyze disulfide transitions. This agrees with previous findings that the redox state of both TGA1 and NPR1 changes under inducing conditions. Transgenic Arabidopsis plants ectopically expressing GRX480 show near wild-type expression of standard marker genes for SA- and xenobiotic-inducible responses. In contrast, transcription of the JA-dependent defensin gene PDF1.2 was antagonized by transgenic GRX480. This, together with the observation that GRX480 transcription is SA-inducible and requires NPR1, suggests a role of GRX480 in SA/JA cross-talk. Suppression of PDF1.2 by GRX480 depends on the presence of TGA factors, indicating that the GRX480/TGA interaction is effective in planta.

A functional genetic assay for nuclear trafficking in plants

The receptor importin-alpha mediates the nuclear import of functionally diverse cargo proteins that contain arginine/lysine-rich nuclear localization signals (NLSs). Functional homologs of importin-alpha have been characterized in a wide range of species including yeast, human and plants. However, the differential cargo selectivity of plant importin-alpha homologs has not been established. To advance nuclear import studies conducted in plant cells, we have developed a method that allows importin-alpha-dependent nuclear import to be assayed in Nicotiana benthamiana. We employed virus-induced gene silencing (VIGS) to knock down the expression of two importin-alpha homologs, NbImpalpha1 and NbImpalpha2, which we identified from N. benthamiana. Agro-infiltration was then used to transiently express the NLS-containing proteins Arabidopsis thaliana fibrillarin 1 (AtFib1) and the Nuk6, Nuk7 and Nuk12 candidate effector proteins of the oomycete plant pathogen Phytophthora infestans. In this manner, we demonstrate importin-alpha-dependent nuclear import of Nuk6 and Nuk7. In contrast, the nuclear import of Nuk12 and AtFib1 was unaffected in cells of NbImpalpha-silenced plants. These data suggest that P. infestans Nuk6 and Nuk7 proteins are dependent on one or more alpha-importins for nuclear import. Our VIGS-based assay represents a powerful new technique to study mechanisms underlying the transport of proteins from cytoplasm to nucleus in plants.

Functional analysis of rice NPR1-like genes reveals that OsNPR1/NH1 is the rice orthologue conferring disease resistance with enhanced herbivore susceptibility

The key regulator of salicylic acid (SA)-mediated resistance, NPR1, is functionally conserved in diverse plant species, including rice (Oryza sativa L.). Investigation in depth is needed to provide an understanding of NPR1-mediated resistance and a practical strategy for the improvement of disease resistance in the model crop rice. The rice genome contains five NPR1-like genes. In our study, three rice homologous genes, OsNPR1/NH1, OsNPR2/NH2 and OsNPR3, were found to be induced by rice bacterial blight Xanthomonas oryzae pv. oryzae and rice blast Magnaporthe grisea, and the defence molecules benzothiadiazole, methyl jasmonate and ethylene. We confirmed that OsNPR1 is the rice orthologue by complementing the Arabidopsis npr1 mutant. Over-expression of OsNPR1 conferred disease resistance to bacterial blight, but also enhanced herbivore susceptibility in transgenic plants. The OsNPR1-green fluorescent protein (GFP) fusion protein was localized in the cytoplasm and moved into the nucleus after redox change. Mutations in its conserved cysteine residues led to the constitutive localization of OsNPR1(2CA)-GFP in the nucleus and also abolished herbivore hypersensitivity in transgenic rice. Different subcellular localizations of OsNPR1 antagonistically regulated SA- and jasmonic acid (JA)-responsive genes, but not SA and JA levels, indicating that OsNPR1 might mediate antagonistic cross-talk between the SA- and JA-dependent pathways in rice. This study demonstrates that rice has evolved an SA-mediated systemic acquired resistance similar to that in Arabidopsis, and also provides a practical approach for the improvement of disease resistance without the penalty of decreased herbivore resistance in rice.

Arabidopsis WRKY70 is required for full RPP4-mediated disease resistance and basal defense against Hyaloperonospora parasitica

AtWRKY70, encoding a WRKY transcription factor, is co-expressed with a set of Arabidopsis genes that share a pattern of RPP4- and RPP7-dependent late upregulation in response to Hyaloperonospora parasitica infection (LURP) genes. We show that AtWRKY70 is required for both full RPP4-mediated resistance and basal defense against H. parasitica. These two defense pathways are related to each other, because they require PAD4 and salicylic acid (SA). RPP7 function, which is independent from PAD4 and SA, is not affected by insertions in AtWRKY70. Although AtWRKY70 is required for RPP4-resistance, it appears not to contribute significantly to RPP4-triggered cell death. Furthermore, our data indicate that AtWRKY70 functions downstream of defense-associated reactive oxygen intermediates and SA. Constitutive and RPP4-induced transcript levels of two other LURP genes are reduced in AtWRKY70 T-DNA mutants, indicating a direct or indirect role for AtWRKY70 in their regulation. We propose that AtWRKY70 is a component of a basal defense mechanism that is boosted by engagement of either RPP4 or RPP7 and is required for RPP4-mediated resistance.

Cotton GhDREB1 increases plant tolerance to low temperature and is negatively regulated by gibberellic acid

The transcription factors C-repeat binding factors/dehydration-responsive element binding proteins (CBFs/DREBs) control the expression of many stress-inducible genes in Arabidopsis. A cDNA clone, designated GhDREB1, was isolated from cotton (Gossypium hirsutum) by cDNA library screening. Northern blot analysis indicated that mRNA accumulation of GhDREB1 was induced by low temperatures and salt stress, but was not induced by abscisic acid (ABA) or drought stress in cotton seedlings. Transgenic tobacco (Nicotiana tabacum) plants overexpressing GhDREB1 displayed stronger chilling tolerance than wild-type plants. Their leaf chlorophyll fluorescence, net photosynthetic rate and proline concentrations were higher than those of control plants during low-temperature treatment. However, under normal growth conditions, the transgenic tobacco plants exhibited retarded growth and delayed flowering. Interestingly, GhDREB1 transcripts in cotton seedlings were negatively regulated by gibberellic acid (GA(3)) treatment. Analysis of the promoter of the GhDREB1 gene revealed the presence of one low-temperature and four gibberellin-responsive elements. Green fluorescent protein (GFP) signal intensity or beta-glucuronidase (GUS) activity driven by the GhDREB1 promoter was clearly enhanced by low temperature but repressed by GA(3). These results suggest that GhDREB1 functions as a transcription factor and plays an important role in improving cold tolerance, and also affects plant growth and development via GA(3).

Salicylic acid-mediated innate immunity in Arabidopsis is regulated by SIZ1 SUMO E3 ligase

Reversible modifications of target proteins by small ubiquitin-like modifier (SUMO) proteins are involved in many cellular processes in yeast and animals. Yet little is known about the function of sumoylation in plants. Here, we show that the SIZ1 gene, which encodes an Arabidopsis SUMO E3 ligase, regulates innate immunity. Mutant siz1 plants exhibit constitutive systemic-acquired resistance (SAR) characterized by elevated accumulation of salicylic acid (SA), increased expression of pathogenesis-related (PR) genes, and increased resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. Transfer of the NahG gene to siz1 plants results in reversal of these phenotypes back to wild-type. Analyses of the double mutants, npr1 siz1, pad4 siz1 and ndr1 siz1 revealed that SIZ1 controls SA signalling. SIZ1 interacts epistatically with PAD4 to regulate PR expression and disease resistance. Consistent with these observations, siz1 plants exhibited enhanced resistance to Pst DC3000 expressing avrRps4, a bacterial avirulence determinant that responds to the EDS1/PAD4-dependent TIR-NBS-type R gene. In contrast, siz1 plants were not resistant to Pst DC3000 expressing avrRpm1, a bacterial avirulence determinant that responds to the NDR1-dependent CC-NBS-type R gene. Jasmonic acid (JA)-induced PDF1.2 expression and susceptibility to Botrytis cinerea were unaltered in siz1 plants. Taken together, these results demonstrate that SIZ1 is required for SA and PAD4-mediated R gene signalling, which in turn confers innate immunity in Arabidopsis.

A genomic approach to identify regulatory nodes in the transcriptional network of systemic acquired resistance in plants

Many biological processes are controlled by intricate networks of transcriptional regulators. With the development of microarray technology, transcriptional changes can be examined at the whole-genome level. However, such analysis often lacks information on the hierarchical relationship between components of a given system. Systemic acquired resistance (SAR) is an inducible plant defense response involving a cascade of transcriptional events induced by salicylic acid through the transcription cofactor NPR1. To identify additional regulatory nodes in the SAR network, we performed microarray analysis on Arabidopsis plants expressing the NPR1-GR (glucocorticoid receptor) fusion protein. Since nuclear translocation of NPR1-GR requires dexamethasone, we were able to control NPR1-dependent transcription and identify direct transcriptional targets of NPR1. We show that NPR1 directly upregulates the expression of eight WRKY transcription factor genes. This large family of 74 transcription factors has been implicated in various defense responses, but no specific WRKY factor has been placed in the SAR network. Identification of NPR1-regulated WRKY factors allowed us to perform in-depth genetic analysis on a small number of WRKY factors and test well-defined phenotypes of single and double mutants associated with NPR1. Among these WRKY factors we found both positive and negative regulators of SAR. This genomics-directed approach unambiguously positioned five WRKY factors in the complex transcriptional regulatory network of SAR. Our work not only discovered new transcription regulatory components in the signaling network of SAR but also demonstrated that functional studies of large gene families have to take into consideration sequence similarity as well as the expression patterns of the candidates.

Many biological processes are controlled by intricate regulatory networks of gene expression. Identifying the regulatory nodes in these networks and understanding the hierarchical relationship between them are vital to our understanding of biological systems. However, this task is frequently hampered by the intrinsic complexity of these processes. Here, the authors used a controlled transcriptional profiling strategy to a plant immune response called systemic acquired resistance to study the transcriptional events one at a time. Systemic acquired resistance is activated through the induction of thousands of genes by the transcriptional regulator protein NPR1. The authors found that downstream of NPR1 are several regulatory nodes comprised of members from a large family of transcriptional factors. Disrupting these regulatory nodes compromised various functions assigned to NPR1, providing the information needed to construct a gene regulation network.

Negative regulation of defense responses in Arabidopsis by two NPR1 paralogs

NPR1 is required for systemic acquired resistance, and there are five NPR1 paralogs in Arabidopsis. Here we report knockout analysis of two of these, NPR3 and NPR4. npr3 single mutants have elevated basal PR-1 expression and the npr3 npr4 double mutant shows even higher expression. The double mutant plants also display enhanced resistance against virulent bacterial and oomycete pathogens. This enhanced disease resistance is partially dependent on NPR1, can be in part complemented by either wild-type NPR3 or NPR4, and is not associated with an elevated level of salicylic acid. NPR3 and NPR4 interact with TGA2, TGA3, TGA5 and TGA6 in yeast two-hybrid assays. Using bimolecular fluorescence complementation analysis, we show that NPR3 interacts with TGA2 in the nucleus of onion epidermal cells and Arabidopsis mesophyll protoplasts. Combined with our previous finding that basal PR-1 levels are also elevated in the tga2 tga5 tga6 triple mutant, we propose that NPR3 and NPR4 negatively regulate PR gene expression and pathogen resistance through their association with TGA2 and its paralogs.

The coactivator function of Arabidopsis NPR1 requires the core of its BTB/POZ domain and the oxidation of C-terminal cysteines

NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) regulates systemic acquired resistance (SAR) in Arabidopsis thaliana, and current models propose that after treatment with salicylic acid (SA), Cys-82 and Cys-216 of NPR1 are reduced, leading to nuclear import. The interaction of nucleus-localized NPR1 with TGA transcription factors results in the activation of defense genes, including the SAR marker PATHOGENESIS-RELATED-1 (PR-1), and the deployment of SAR. Little is known about how TGA factors or NPR1 regulate transcription or whether a TGA-NPR1 complex forms on DNA. We show that TGA2 and NPR1 are recruited to PR-1 independently of each other and of SA treatment. Consistent with the result that a triple knockout in TGA2/5/6 derepresses PR-1, in vivo plant transcription assays revealed that TGA2 is not an autonomous transcription activator but is a transcriptional repressor in both untreated and SA-treated cells. However, after stimulation with SA, TGA2 is incorporated into a transactivating complex with NPR1, forming an enhanceosome that requires the core of the NPR1 BTB/POZ domain (residues 80 to 91) and the oxidation of NPR1 Cys-521 and Cys-529. These Cys residues are found in a new type of transactivation domain that we term Cys-oxidized. These data further our understanding of the mechanism by which TGA2 and NPR1 activate Arabidopsis PR-1.

Arabidopsis WRKY33 transcription factor is required for resistance to necrotrophic fungal pathogens

Plant WRKY transcription factors are key regulatory components of plant responses to microbial infection. In addition to regulating the expression of defense-related genes, WRKY transcription factors have also been shown to regulate cross-talk between jasmonate- and salicylate-regulated disease response pathways. The two pathways mediate resistance against different types of microbial pathogens, and there are numerous reports of antagonistic interactions between them. Here we show that mutations of the Arabidopsis WRKY33 gene encoding a WRKY transcription factor cause enhanced susceptibility to the necrotrophic fungal pathogens Botrytis cinerea and Alternaria brassicicola concomitant with reduced expression of the jasmonate-regulated plant defensin PDF1.2 gene. Ectopic over-expression of WRKY33, on the other hand, increases resistance to the two necrotrophic fungal pathogens. The wrky33 mutants do not show altered responses to a virulent strain of the bacterial pathogen Pseudomonas syringae, although the ectopic expression of WRKY33 results in enhanced susceptibility to this pathogen. The susceptibility of WRKY33-over-expressing plants to P. syringae is associated with reduced expression of the salicylate-regulated PR-1 gene. The WRKY33 transcript is induced in response to pathogen infection, or treatment with salicylate or the paraquat herbicide that generates activated oxygen species in exposed cells. WRKY33 is localized to the nucleus of plant cells and recognizes DNA molecules containing the TTGACC W-box sequence. Together, these results indicate that pathogen-induced WRKY33 is an important transcription factor that regulates the antagonistic relationship between defense pathways mediating responses to P. syringae and necrotrophic pathogens.

Systemic acquired resistance

Upon infection with necrotizing pathogens many plants develop an enhanced resistance to further pathogen attack also in the uninoculated organs. This type of enhanced resistance is referred to as systemic acquired resistance (SAR). In the SAR state, plants are primed (sensitized) to more quickly and more effectively activate defense responses the second time they encounter pathogen attack. Since SAR depends on the ability to access past experience, acquired disease resistance is a paradigm for the existence of a form of "plant memory". Although the phenomenon has been known since the beginning of the 20th century, major progress in the understanding of SAR was made over the past sixteen years. This review covers the current knowledge of molecular, biochemical and physiological mechanisms that are associated with SAR.

Systemic acquired resistance

Upon infection with necrotizing pathogens many plants develop an enhanced resistance to further pathogen attack also in the uninoculated organs. This type of enhanced resistance is referred to as systemic acquired resistance (SAR). In the SAR state, plants are primed (sensitized) to more quickly and more effectively activate defense responses the second time they encounter pathogen attack. Since SAR depends on the ability to access past experience, acquired disease resistance is a paradigm for the existence of a form of "plant memory". Although the phenomenon has been known since the beginning of the 20th century, major progress in the understanding of SAR was made over the past sixteen years. This review covers the current knowledge of molecular, biochemical and physiological mechanisms that are associated with SAR.

WRKY70 modulates the selection of signaling pathways in plant defense

Cross-talk between signal transduction pathways is a central feature of the tightly regulated plant defense signaling network. The potential synergism or antagonism between defense pathways is determined by recognition of the type of pathogen or pathogen-derived elicitor. Our studies have identified WRKY70 as a node of convergence for integrating salicylic acid (SA)- and jasmonic acid (JA)-mediated signaling events during plant response to bacterial pathogens. Here, we challenged transgenic plants altered in WRKY70 expression as well as WRKY70 knockout mutants of Arabidopsis with the fungal pathogens Alternaria brassicicola and Erysiphe cichoracearum to elucidate the role of WRKY70 in modulating the balance between distinct defense responses. Gain or loss of WRKY70 function causes opposite effects on JA-mediated resistance to A. brassicicola and the SA-mediated resistance to E. cichoracearum. While the up-regulation of WRKY70 caused enhanced resistance to E. cichoracearum, it compromised plant resistance to A. brassicicola. Conversely, down-regulation or insertional inactivation of WRKY70 impaired plant resistance to E. cichoracearum. Over-expression of WRKY70 resulted in the suppression of several JA responses including expression of a subset of JA- and A. brassicicola-responsive genes. We show that this WRKY70-controlled suppression of JA-signaling is partly executed by NPR1. The results indicate that WRKY70 has a pivotal role in determining the balance between SA-dependent and JA-dependent defense pathways.

Genetically engineered resistance to Fusarium head blight in wheat by expression of Arabidopsis NPR1

Fusarium head blight (FHB) is a devastating disease of wheat and barley which causes extensive losses worldwide. Monogenic, gene-for-gene resistance to FHB has not been reported. The best source of resistance to FHB is a complex, quantitative trait derived from the wheat cv. Sumai 3. Here, we show that the Arabidopsis thaliana NPR1 gene (AtNPR1), which regulates the activation of systemic acquired resistance, when expressed in the FHB-susceptible wheat cv. Bobwhite, confers a heritable, type II resistance to FHB caused by Fusarium graminearum. The heightened FHB resistance in the transgenic AtNPRI -expressing wheat is associated with the faster activation of defense response when challenged by the fungus. PR1 expression is induced rapidly to a high level in the fungus-challenged spikes of the AtNPR1-expressing wheat. Furthermore, benzothiadiazole, a functional analog of salicylic acid, induced PR1 expression faster and to a higher level in the AtNPR1-expressing wheat than in the nontransgenic plants. We suggest that FHB resistance in the AtNPR1-expressing wheat is a result of these plants being more responsive to an endogenous activator of plant defense. Our results demonstrate that NPR1 is an effective candidate for controlling FHB.

Fine-Tuning Plant Defence Signalling: Salicylate versus Jasmonate

Plant defences against pathogens and herbivorous insects form a comprehensive network of interacting signal transduction pathways. The signalling molecules salicylic acid (SA) and jasmonic acid (JA) play important roles in this network. SA is involved in signalling processes providing systemic acquired resistance (SAR), protecting the plant from further infection after an initial pathogen attack. SAR is long-lasting and provides broad spectrum resistance to biotrophic pathogens that feed on a living host cell. The regulatory protein NPR1 is a central positive regulator of SAR. SA-activated NPR1 localizes to the nucleus where it interacts with TGA transcription factors to induce the expression of a large set of pathogenesis-related proteins that contribute to the enhanced state of resistance. In a distinct signalling process, JA protects the plant from insect infestation and necrotrophic pathogens that kill the host cell before feeding. JA activates the regulatory protein COI1 that is part of the E3 ubiquitin ligase-containing complex SCFCOI1, which is thought to derepress JA-responsive genes involved in plant defence. Both synergistic and antagonistic interactions have been observed between SA- and JA-dependent defences. NPR1 has emerged as a critical modulator of cross-talk between the SA and JA signal and is thought to aid in fine tuning defence responses specific to the encountered attacker. Here we review SA- and JA-dependent signal transduction and summarize our current understanding of the molecular mechanisms of cross-talk between these defences.

Identification of NPR1-dependent and independent genes early induced by salicylic acid treatment in Arabidopsis

Salicylic acid (SA) plays a crucial role in stress resistance in plants by modifying the expression of a battery of genes. In this paper, we report the identification of a group of early SA-regulated genes of Arabidopsis (activated between 0.5-2.5 h), using the cDNA-amplified fragment length polymorphism technique (cDNA-AFLP). Using 128 different primer combinations, we identified several genes based on their differential expression during SA treatment. Among these, we identified 12 genes up-regulated by SA whose patterns of induction were confirmed by Northern analysis. The identified genes can be grouped into two functional groups: Group 1: genes involved in cell protection (i.e. glycosyltransferases, glutathion S-transferases), and Group 2: genes involved in signal transduction (protein kinases and transcription factors). We also evaluated NPR1 requirement for the induction of the 12 up-regulated genes, and found that only those belonging to Group 2 require this co-activator for their expression. In silico analysis of the promoter sequences of the up-regulated genes, allowed us to identify putative cis-elements over-represented in these genes. Interestingly, as-1-like elements, previously characterized as SA-responsive elements, were specifically over-represented in Group 1 genes. The identification of early SA-regulated genes is an important step towards understanding the complex role of this hormone in plant stress resistance.

Ascorbic acid deficiency activates cell death and disease resistance responses in Arabidopsis

Programmed cell death, developmental senescence, and responses to pathogens are linked through complex genetic controls that are influenced by redox regulation. Here we show that the Arabidopsis (Arabidopsis thaliana) low vitamin C mutants, vtc1 and vtc2, which have between 10% and 25% of wild-type ascorbic acid, exhibit microlesions, express pathogenesis-related (PR) proteins, and have enhanced basal resistance against infections caused by Pseudomonas syringae. The mutants have a delayed senescence phenotype with smaller leaf cells than the wild type at maturity. The vtc leaves have more glutathione than the wild type, with higher ratios of reduced glutathione to glutathione disulfide. Expression of green fluorescence protein (GFP) fused to the nonexpressor of PR protein 1 (GFP-NPR1) was used to detect the presence of NPR1 in the nuclei of transformed plants. Fluorescence was observed in the nuclei of 6- to 8-week-old GFP-NPR1 vtc1 plants, but not in the nuclei of transformed GFP-NPR1 wild-type plants at any developmental stage. The absence of senescence-associated gene 12 (SAG12) mRNA at the time when constitutive cell death and basal resistance were detected confirms that elaboration of innate immune responses in vtc plants does not result from activation of early senescence. Moreover, H2O2-sensitive genes are not induced at the time of systemic acquired resistance execution. These results demonstrate that ascorbic acid abundance modifies the threshold for activation of plant innate defense responses via redox mechanisms that are independent of the natural senescence program.

An importin alpha homolog, MOS6, plays an important role in plant innate immunity

Plant disease resistance is the consequence of an innate defense mechanism mediated by Resistance (R) genes [1]. The conserved structure of one class of R protein is reminiscent of Toll-like receptors (TLRs) and Nucleotide binding oligomerization domain (NOD) proteins-immune-response perception modules in animal cells [2, 3, and 4]. The Arabidopsis snc1 (suppressor of npr1-1, constitutive, 1) mutant contains a mutation in a TIR-NBS-LRR-type of R gene that renders resistance responses constitutively active without interaction with pathogens [5]. Few components of the downstream signaling network activated by snc1 are known. To search for regulators of R-gene-mediated resistance, we screened for genetic suppressors of snc1. Three alleles of the mutant mos6 (modifier of snc1, 6) partially suppressed constitutive-resistance responses and immunity to virulent pathogens in snc1. Furthermore, the mos6-1 single mutant exhibited enhanced disease susceptibility to a virulent oomycete pathogen. MOS6, identified by positional cloning, encodes importin alpha3, one of eight alpha importins in Arabidopsis [6]. alpha importins mediate the import of specific proteins across the nuclear envelope. We previously reported that MOS3, a protein homologous to human nucleoporin 96, is required for constitutive resistance in snc1 [7]. Our data highlight an essential role for nucleo-cytoplasmic trafficking, especially protein import, in plant innate immunity.

Arabidopsis SENESCENCE-ASSOCIATED GENE101 stabilizes and signals within an ENHANCED DISEASE SUSCEPTIBILITY1 complex in plant innate immunity

Plant innate immunity against invasive biotrophic pathogens depends on the intracellular defense regulator ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1). We show here that Arabidopsis thaliana EDS1 interacts in vivo with another protein, SENESCENCE-ASSOCIATED GENE101 (SAG101), discovered through a proteomic approach to identify new EDS1 pathway components. Together with PHYTOALEXIN-DEFICIENT4 (PAD4), a known EDS1 interactor, SAG101 contributes intrinsic and indispensable signaling activity to EDS1-dependent resistance. The combined activities of SAG101 and PAD4 are necessary for programmed cell death triggered by the Toll-Interleukin-1 Receptor type of nucleotide binding/leucine-rich repeat immune receptor in response to avirulent pathogen isolates and in restricting the growth of normally virulent pathogens. We further demonstrate by a combination of cell fractionation, coimmunoprecipitation, and fluorescence resonance energy transfer experiments the existence of an EDS1-SAG101 complex inside the nucleus that is molecularly and spatially distinct from EDS1-PAD4 associations in the nucleus and cytoplasm. By contrast, EDS1 homomeric interactions were detected in the cytoplasm but not inside the nucleus. These data, combined with evidence for coregulation between individual EDS1 complexes, suggest that dynamic interactions of EDS1 and its signaling partners in multiple cell compartments are important for plant defense signal relay.

Rice NRR, a negative regulator of disease resistance, interacts with Arabidopsis NPR1 and rice NH1

Arabidopsis NPR1/NIM1 is a key regulator of systemic acquired resistance (SAR), which confers lasting broad-spectrum resistance. Over-expression of Arabidopsis NPR1 or the NPR1 homolog 1 (NH1) in rice results in enhanced resistance to the pathogen Xanthomonasoryzae pv. oryzae (Xoo), suggesting the presence of a related defense pathway in rice. We investigated this pathway in rice by identifying proteins that interact with NH1. Here we report the isolation and characterization of a rice cDNA encoding a novel protein, named NRR (for negative regulator of resistance). NRR interacts with NPR1 in the NPR1-interacting domain (NI25) consisting of 25 amino acids. NRR also interacts with NH1; however, NI25 was not sufficient for a strong interaction, indicating a difference between the rice and the Arabidopsis proteins. Silencing of NRR in rice had little effect on resistance to Xoo. When constitutively over-expressed in rice, NRR affected basal resistance, age-related resistance and Xa21-mediated resistance, causing enhanced susceptibility to Xoo. This phenotype was correlated with elevated NRR mRNA and protein levels and increased Xoo growth. Over-expression of NRR suppressed the induction of defense-related genes. NRR:GFP (green fluorescent protein) protein was localized to the nucleus, indicating that NRR may act directly to suppress the activation of defense genes. The fact that NRR compromises Xa21-mediated resistance indicates cross-talk or overlap between NH1- and Xa21-mediated pathways.

Induced disease resistance and gene expression in cereals

Disease resistance strategies reduce chemical input into the environment and are therefore powerful approaches to sustainable agriculture. Induced resistance (IR) has emerged as a potential alternative, or a complementary strategy, for crop protection. IR signifies the control of pathogens and pests by prior activation of plant defence pathways. A molecular understanding of IR in cereals, including the most important global crops wheat and rice, has been largely missing. Evidence indicating that central elements of IR pathways are conserved among Di- and Monocotyledoneae has only recently been presented, although their regulation and interaction with other plant pathways may be quite divergent. We present here a synopsis of current molecular knowledge of cereal IR mechanisms.

Alteration of TGA factor activity in rice results in enhanced tolerance to Xanthomonas oryzae pv. oryzae

In dicotyledonous plants broad-spectrum resistance to pathogens is established after the induction of the systemic acquired resistance (SAR) response. In Arabidopsis the NPR1 protein can regulate SAR by interacting with members of the TGA class of basic, leucine-zipper transcription factors to alter pathogenesis-related (PR) gene expression. Overexpression of (At)NPR1 in Arabidopsis enhances resistance to multiple pathogens. Similarly, overexpression of (At)NPR1 in rice enhances resistance to the bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo). These results suggest that components of the (At)NPR1-mediated SAR defense response may be conserved between monocots and dicots. To determine whether or not rice TGA factors are involved in disease resistance responses, the effect of altering the function of rice TGA2.1 was analyzed in transgenic plants. Transgenic rice overexpressing an rTGA2.1 mutant, that can no longer bind DNA, and transgenic rice that have the endogenous rTGA2.1 silenced by dsRNA-mediated silencing were generated. Both types of transgenic rice displayed increased tolerance to Xoo, were dwarfed, and had altered accumulation of PR genes. The results presented in this study suggest that wild-type rTGA2.1 has primarily a negative role in rice basal defense responses to bacterial pathogens.

Overexpression of a rice NPR1 homolog leads to constitutive activation of defense response and hypersensitivity to light

Arabidopsis NPR1/NIM1 is a key regulator of systemic acquired resistance (SAR), which confers lasting broad-spectrum resistance. Previous reports indicate that rice has a disease-resistance pathway similar to the Arabidopsis SAR pathway. Here we report the isolation and characterization of a rice NPR1 homologue (NH1). Transgenic rice plants overexpressing NH1 (NH1ox) acquire high levels of resistance to Xanthomonas oryzae pv. oryzae. The resistance phenotype is heritable and correlates with the presence of the transgene and reduced bacterial growth. Northern analysis shows that NH1ox rice spontaneously activates defense genes, contrasting with NPR1-overexpressing Arabidopsis, where defense genes are not activated until induction. Wild-type NH1, but not a point mutant corresponding to npr1-1, interacts strongly with the rice transcription factor rTGA2.2 in yeast two-hybrid. Greenhouse-grown NH1ox plants develop lesion-mimic spots on leaves at preflowering stage although no other developmental effects are observed. However, when grown in growth chambers (GCs) under low light, NH1ox plants are dwarfed, indicating elevated sensitivity to light. The GC-grown NH1ox plants show much higher salicylic acid (SA) levels than the wild type, whereas greenhouse-grown NH1ox plants contain lower SA. These results indicate that NH1 may be involved in the regulation of SA in response to environmental changes.

Induction of Protein Secretory Pathway Is Required for Systemic Acquired Resistance

In plants, systemic acquired resistance (SAR) is established as a result of NPR1-regulated expression of pathogenesis-related (PR) genes. Using gene expression profiling in Arabidopsis, we found that in addition to controlling the expression of PR genes, NPR1 also directly controls the expression of the protein secretory pathway genes. Up-regulation of these genes is essential for SAR, because mutations in some of them diminished the secretion of PR proteins (for example, PR1), resulting in reduced resistance. We provide evidence that NPR1 coordinately regulates these secretion-related genes through a previously undescribed cis-element. Activation of this cis-element is controlled by a transcription factor that is translocated into the nucleus upon SAR induction.

Transgenic tomato plants expressing the Arabidopsis NPR1 gene display enhanced resistance to a spectrum of fungal and bacterial diseases

Development of effective disease-resistance to a broad-range of pathogens in crops usually requires tremendous resources and effort when traditional breeding approaches are taken. Genetic engineering of disease-resistance in crops has become popular and valuable in terms of cost and efficacy. Due to long-lasting and broad-spectrum of effectiveness against pathogens, employment of systemic acquired resistance (SAR) for the genetic engineering of crop disease-resistance is of particular interest. In this report, we explored the potential of using SAR-related genes for the genetic engineering of enhanced resistance to multiple diseases in tomato. The Arabidopsis NPR1 (nonexpresser of PR genes) gene was introduced into a tomato cultivar, which possesses heat-tolerance and resistance to tomato mosaic virus (ToMV). The transgenic lines expressing NPR1 were normal as regards overall morphology and horticultural traits for at least four generations. Disease screens against eight important tropical diseases revealed that, in addition to the innate ToMV-resistance, the tested transgenic lines conferred significant level of enhanced resistance to bacterial wilt (BW) and Fusarium wilt (FW), and moderate degree of enhanced resistance to gray leaf spot (GLS) and bacterial spot (BS). Transgenic lines that accumulated higher levels of NPR1 proteins exhibited higher levels and a broader spectrum of enhanced resistance to the diseases, and enhanced disease-resistance was stably inherited. The spectrum and degree of these NPR1-transgenic lines are more significant compared to that of transgenic tomatoes reported to date. These transgenic lines may be further explored as future tomato stocks, aiming at building up resistance to a broader spectrum of diseases.

BLADE-ON-PETIOLE-dependent signaling controls leaf and floral patterning in Arabidopsis

NONEXPRESSOR OF PR GENES1 (NPR1) is a key regulator of the plant defense response known as systemic acquired resistance. Accumulation of the signal molecule salicylic acid (SA) leads to a change in intracellular redox potential, enabling NPR1 to enter the nucleus and interact with TGACG sequence-specific binding protein (TGA) transcription factors, which in turn bind to SA-responsive elements in the promoters of defense genes. Here, we show that two NPR1-like genes, BLADE-ON-PETIOLE1 (BOP1) and BOP2, function redundantly to control growth asymmetry, an important aspect of patterning in leaves and flowers. Phenotypes in the double mutant include leafy petioles, loss of floral organ abscission, and asymmetric flowers subtended by a bract. We demonstrate that BOP2 is localized to both the nucleus and the cytoplasm, but unlike NPR1, it is highly expressed in young floral meristems and in yeast interacts preferentially with the TGA transcription factor encoded by PERIANTHIA (PAN). In support of a biological relevance for this interaction, we show that bop1 bop2 and pan mutants share a pentamerous arrangement of first whorl floral organs, a patterning defect that is retained in bop1 bop2 pan triple mutants. Our data provide evidence that BOP proteins control patterning via direct interactions with TGA transcription factors and demonstrate that a signaling mechanism similar to that formally associated with plant defense is likely used for the control of developmental patterning.

Salicylic acid (SA)-dependent gene activation can be uncoupled from cell death-mediated gene activation: the SA-inducible NIMIN-1 and NIMIN-2 promoters, unlike the PR-1a promoter, do not respond to cell death signals in tobacco

SUMMARY Tobacco pathogenesis-related (PR) genes of group 1 are induced during pathogen defence (hypersensitive response, HR, and systemic acquired resistance, SAR), after exogenous application of salicylic acid (SA), and by developmental cues. Likewise, SA enhances transcripts for Arabidopsis NIMIN-1 and NIMIN-2, which interact with NPR1/NIM1, a key regulator of SAR. To further illuminate gene activation during pathogen defence, reporter gene expression from the NIMIN-1 and NIMIN-2 promoters was analysed in transgenic tobacco plants in direct comparison to PR-1 gene expression. NIMIN[GUS] chimeric genes were highly sensitive to SA, whereas NIMIN[GUS], unlike PR1a[GUS], expression was only weak in necrotic tissue exhibiting HR. Furthermore, PR-1a, but not NIMIN, promoter constructs were activated systemically in response to local cell death elicited by expression of the proapoptotic Bax gene. Conversely, NIMIN-1[GUS] expression was completely suppressed during pathogen defence in plants depleted from SA, whereas PR-1 proteins still accumulated in necrotic tissue. These findings demonstrate that SA-dependent gene activation can be uncoupled from cell death-induced gene activation. Whereas PR-1a induction during the HR and SAR responses is mediated by HR-associated signals and SA, activation of the NIMIN-1 and NIMIN-2 promoters in infected tobacco relies on SA, but not on cell death signals.

Discovery of functional genes for systemic acquired resistance in Arabidopsis thaliana through integrated data mining

Various data mining techniques combined with sequence motif information in the promoter region of genes were applied to discover functional genes that are involved in the defense mechanism of systemic acquired resistance (SAR) in Arabidopsis thaliana. A series of K-Means clustering with difference-in-shape as distance measure was initially applied. A stability measure was used to validate this clustering process. A decision tree algorithm with the discover-and-mask technique was used to identify a group of most informative genes. Appearance and abundance of various transcription factor binding sites in the promoter region of the genes were studied. Through the combination of these techniques, we were able to identify 24 candidate genes involved in the SAR defense mechanism. The candidate genes fell into 2 highly resolved categories, each category showing significantly unique profiles of regulatory elements in their promoter regions. This study demonstrates the strength of such integration methods and suggests a broader application of this approach.

Interaction of NIMIN1 with NPR1 modulates PR gene expression in Arabidopsis

The Arabidopsis thaliana NONEXPRESSER OF PR GENES1 (NPR1, also known as NIM1) protein is an essential positive regulator of salicylic acid (SA)-induced PATHOGENESIS-RELATED (PR) gene expression and systemic acquired resistance (SAR). PR gene activity is regulated at the level of redox-dependent nuclear transport of NPR1. NPR1 interacts with members of the TGA family of transcription factors that are known to bind to SA-responsive elements in the PR-1 promoter. In an attempt to identify proteins involved in SA-mediated signal transduction, we previously described the isolation of three novel genes encoding distinct albeit structurally related proteins designated NIMIN1 (for NIM1-INTERACTING1), NIMIN2, and NIMIN3 that interact with NPR1 in the yeast two-hybrid system. Here, we show that NIMIN1 and NPR1 can be copurified from plant extracts, providing biochemical evidence for their interaction. We provide functional evidence for this interaction by describing transgenic plants constitutively expressing high amounts of NIMIN1. These plants show reduced SA-mediated PR gene induction and a compromised SAR, thus mimicking the described phenotype conferred by npr1. Moreover, they showed reduced RESISTANCE gene-mediated protection. These effects were dependent on the ability of NIMIN1 to interact with NPR1. Mutant plants with a T-DNA insertion in NIMIN1 as well as transgenic plants with reduced NIMIN1 mRNA levels showed hyperactivation of PR-1 gene expression after SA treatment but no effect on the disease resistance phenotype. Our results strongly suggest that NIMIN1 negatively regulates distinct functions of NPR1, providing a mechanism to modulate specific features of SAR.

Interaction of NIMIN1 with NPR1 modulates PR gene expression in Arabidopsis

The Arabidopsis thaliana NONEXPRESSER OF PR GENES1 (NPR1, also known as NIM1) protein is an essential positive regulator of salicylic acid (SA)-induced PATHOGENESIS-RELATED (PR) gene expression and systemic acquired resistance (SAR). PR gene activity is regulated at the level of redox-dependent nuclear transport of NPR1. NPR1 interacts with members of the TGA family of transcription factors that are known to bind to SA-responsive elements in the PR-1 promoter. In an attempt to identify proteins involved in SA-mediated signal transduction, we previously described the isolation of three novel genes encoding distinct albeit structurally related proteins designated NIMIN1 (for NIM1-INTERACTING1), NIMIN2, and NIMIN3 that interact with NPR1 in the yeast two-hybrid system. Here, we show that NIMIN1 and NPR1 can be copurified from plant extracts, providing biochemical evidence for their interaction. We provide functional evidence for this interaction by describing transgenic plants constitutively expressing high amounts of NIMIN1. These plants show reduced SA-mediated PR gene induction and a compromised SAR, thus mimicking the described phenotype conferred by npr1. Moreover, they showed reduced RESISTANCE gene-mediated protection. These effects were dependent on the ability of NIMIN1 to interact with NPR1. Mutant plants with a T-DNA insertion in NIMIN1 as well as transgenic plants with reduced NIMIN1 mRNA levels showed hyperactivation of PR-1 gene expression after SA treatment but no effect on the disease resistance phenotype. Our results strongly suggest that NIMIN1 negatively regulates distinct functions of NPR1, providing a mechanism to modulate specific features of SAR.

Salicylic acid-inducible Arabidopsis CK2-like activity phosphorylates TGA2

We demonstrate that TGA2, TGA5 and TGA6, and TGA3 to a lesser extent, are phosphorylated by an activity in rabbit reticulocytes. Using deletion and point mutagenesis of TGA2, three amino acid (aa) residues, (11)Ser, (12)Thr and (16)Thr, were found to be critical for efficient phosphorylation by a kinase(s) in rabbit reticulocytes. These three residues also were important for phosphorylation by recombinant human Casein Kinase II (CK2) and by a CK2-like kinase in Arabidopsis leaf extracts. Salicylic acid (SA) treatment enhanced the phosphorylation of recombinant TGA2 in vitro; it also enhanced phosphorylation of a TGA2-GFP fusion protein in vivo. By contrast, in vivo phosphorylation of a TGA2-A-GFP fusion protein, in which the (11)Ser, (12)Thr and (16)Thr residues were mutated to non-phosphorylable alanine, was only poorly if at all stimulated by SA treatment. Mutation of the putative CK2 phosphorylation motif did not affect nuclear localization of TGA2. However, the DNA binding activity of TGA2 was reduced by CK2 treatment, whereas that of TGA2-A was unaffected; TGA2's DNA binding activity after incubation in a rabbit reticulocyte lysate also was substantially lower than that of comparably treated TGA2-A. Taken together, these results suggest that phosphorylation at the putative CK2 phosphorylation site negatively regulates the DNA binding activity of TGA2. Analysis of transgenic Arabidopsis overexpressing TGA2-GFP or TGA2-A-GFP, in the absence of SA treatment, revealed that they accumulated similarly elevated levels of PR-1 gene transcripts. Possible reasons why mutations in the putative CK2 phosphorylation site had little effect on PR-1 induction by TGA2 are discussed.

Constitutively elevated salicylic acid signals glutathione-mediated nickel tolerance in Thlaspi nickel hyperaccumulators

Progress is being made in understanding the biochemical and molecular basis of nickel (Ni)/zinc (Zn) hyperaccumulation in Thlaspi; however, the molecular signaling pathways that control these mechanisms are not understood. We observed that elevated concentrations of salicylic acid (SA), a molecule known to be involved in signaling induced pathogen defense responses in plants, is a strong predictor of Ni hyperaccumulation in the six diverse Thlaspi species investigated, including the hyperaccumulators Thlaspi goesingense, Thlaspi rosulare, Thlaspi oxyceras, and Thlaspi caerulescens and the nonaccumulators Thlaspi arvense and Thlaspi perfoliatum. Furthermore, the SA metabolites phenylalanine, cinnamic acid, salicyloyl-glucose, and catechol are also elevated in the hyperaccumulator T. goesingense when compared to the nonaccumulators Arabidopsis (Arabidopsis thaliana) and T. arvense. Elevation of free SA levels in Arabidopsis, both genetically and by exogenous feeding, enhances the specific activity of serine acetyltransferase, leading to elevated glutathione and increased Ni resistance. Such SA-mediated Ni resistance in Arabidopsis phenocopies the glutathione-based Ni tolerance previously observed in Thlaspi, suggesting a biochemical linkage between SA and Ni tolerance in this genus. Intriguingly, the hyperaccumulator T. goesingense also shows enhanced sensitivity to the pathogen powdery mildew (Erysiphe cruciferarum) and fails to induce SA biosynthesis after infection. Nickel hyperaccumulation reverses this pathogen hypersensitivity, suggesting that the interaction between pathogen resistance and Ni tolerance and hyperaccumulation may have played a critical role in the evolution of metal hyperaccumulation in the Thlaspi genus.

Salicylic acid-dependent expression of host genes in compatible Arabidopsis-virus interactions

Plant viruses elicit the expression of common sets of genes in susceptible hosts. Studies in Arabidopsis (Arabidopsis thaliana) and tomato (Lycopersicon esculentum) indicate that at least one-third of the genes induced in common by viruses have been previously associated with plant defense and stress responses. The genetic and molecular requirements for the induction of these stress and defense-related genes during compatible host-virus interactions were investigated with a panel of Arabidopsis mutant and transgenic plants defective in one or more defense signaling pathways. pad4, eds5, NahG, npr1, jar1, ein2, sid2, eds1, and wild-type Columbia-0 and Wassilewskija-2 plants were infected with two different viruses, cucumber mosaic virus and oilseed rape mosaic virus. Gene expression was assayed by a high-throughput fiber-optic bead array consisting of 388 genes and by RNA gel blots. These analyses demonstrated that, in compatible host-virus interactions, the expression of the majority of defense-related genes is induced by a salicylic acid-dependent, NPR1-independent signaling pathway with a few notable exceptions that did require NPR1. Interestingly, none of the mutant or transgenic plants showed enhanced susceptibility to either cucumber mosaic virus or oilseed rape mosaic virus based on both symptoms and virus accumulation. This observation is in contrast to the enhanced disease susceptibility phenotypes that these mutations or transgenes confer to some bacterial and fungal pathogens. These experimental results suggest that expression of many defense-related genes in compatible host plants might share components of signaling pathways involved in incompatible host-pathogen interactions, but their increased expression has no negative effect on viral infection.

Regulation of the Arabidopsis defense transcriptome

Transcriptional re-programming is a key step of plant defense in response to pathogen recognition. Microarray analyses combined with genetic and biochemical approaches are now enabling us to study basic principles and details of regulatory mechanisms controlling the defense transcriptome in Arabidopsis. Recent results show that signaling pathways used by different defense systems converge and target overlapping gene sets. Furthermore, a quantitative mechanism common to multiple defense systems modulates transcript levels of these defense-associated genes. Most importantly, some transcription factors have been proven to play a pivotal role in disease resistance. Regulatory circuits linking signaling and gene regulation are emerging, suggesting that a complex interplay of transcriptional activators and repressors fine-tunes expression of the defense transcriptome.

Contrasting mechanisms of defense against biotrophic and necrotrophic pathogens

It has been suggested that effective defense against biotrophic pathogens is largely due to programmed cell death in the host, and to associated activation of defense responses regulated by the salicylic acid-dependent pathway. In contrast, necrotrophic pathogens benefit from host cell death, so they are not limited by cell death and salicylic acid-dependent defenses, but rather by a different set of defense responses activated by jasmonic acid and ethylene signaling. This review summarizes results from Arabidopsis-pathogen systems regarding the contributions of various defense responses to resistance to several biotrophic and necrotrophic pathogens. While the model above seems generally correct, there are exceptions and additional complexities.

An Arabidopsis NPR1-like gene, NPR4, is required for disease resistance

The Arabidopsis genome contains six NPR1-related genes. Given the pivotal role played by NPR1 in controlling salicylic acid (SA)-mediated gene expression and disease resistance, functional characterization of other family members appears to be justified. Reverse genetics was used to analyze the role of one NPR1-like gene, which we called NPR4. The NPR4 protein shares 36% identity with NPR1 and interacts with the same spectrum of TGA transcription factors in yeast two-hybrid assays. Plants with T-DNA insertions in NPR4 are more susceptible to the virulent bacterial pathogen Pseudomonas syringe pv. tomato DC3000. This phenotype is complemented by expression of the wild type NPR4 coding region. As determined by the parasite reproduction, the npr4-1 mutant is more susceptible to the fungal pathogen Erysiphe cichoracearum, but does not differ markedly from wild type in its interaction with virulent and avirulent strains of the oomycete Peronospora parasitica. In leaves of wild-type plants, NPR4 mRNA levels increase following pathogen challenge or SA treatment, and decrease rapidly following methyl jasmonic acid (MeJA) treatment. Transcripts of the pathogenesis-related (PR) genes PR-1, PR-2, and PR-5 are only marginally reduced in the npr4-1 mutant following pathogen challenge or SA treatment. This reduction of PR gene expression is more pronounced when leaves are challenged with the bacterial pathogen following SA treatment. Expression of the jasmonic acid-dependent pathway marker gene PDF1.2 is compromised in npr4-1 leaves following application of MeJA or a combination of SA and MeJA. These results indicate that NPR4 is required for basal defense against pathogens, and that it may be implicated in the cross-talk between the SA- and JA-dependent signaling pathways.

Nucleocytoplasmic Trafficking in Plant Cells

In a eukaryotic cell, the nuclear envelope (NE) separates genetic information from the environment of biosynthesis and metabolism. Transfer of macromolecules across the NE involves the nuclear pores--large multisubunit protein complexes--and machinery that facilitates rapid, directional, and selective transport. While core elements of the transport process are conserved between kingdoms, different solutions to similar problems have also evolved. Although the structure and composition of the yeast and mammalian nuclear pore have been unraveled recently, the plant nuclear pore remains largely enigmatic. Like any other process, nucleocytoplasmic transport can be regulated. Several examples from plants are discussed that promise insights into the regulation of signaling pathways. While controlling the partitioning of cellular components, the nuclear envelope also presents an obstacle to viruses and transforming agents that need access to the genome, and different mechanisms have evolved to overcome this obstacle. Finally, the recent recognition of the importance of small RNAs for gene regulation emphasizes the need to understand small RNA nuclear export and the levels of its regulation. This review attempts to wed our molecular-mechanistic understanding of nucleocytoplasmic trafficking drawn from all model systems with the intriguing examples of regulated nucleocytoplasmic partitioning in plants.

NPR1, all things considered

Recent work has shown that the Arabidopsis NPR1 protein not only plays an essential role in salicylic acid (SA)-mediated systemic acquired resistance and rhizobacterium-triggered induced systemic resistance, but also is involved in crosstalk inhibition of jasmonic acid (JA)-mediated defense responses. Molecular characterization has revealed that activation of NPR1 and certain TGA transcription factors occurs under the reducing conditions that follow an initial oxidative burst after the induction of defense responses. In addition to NPR1 and TGA, the single-stranded DNA-binding transcription factor AtWhy1 and the WRKY70 transcription factor were recently found to be involved in SA-mediated defense and SA-JA crosstalk, respectively.

Light conditions influence specific defence responses in incompatible plant-pathogen interactions: uncoupling systemic resistance from salicylic acid and PR-1 accumulation

In incompatible plant-pathogen interactions, disease resistance is generated by rapid activation of a multitude of plant defence reactions. Little is known about the dependency of these resistance responses on external factors. The plasticity of plant defence mechanisms in terms of light conditions is studied here. Interaction of Arabidopsis thaliana (L.) Heynh. with an avirulent strain of Pseudomonas syringae pv. maculicola in the dark resulted in increased apoplastic bacterial growth and therefore reduced local resistance as compared to an infection process in the presence of light. Several characteristic defence reactions, including activation of phenylalanine ammonia-lyase, accumulation of salicylic acid (SA), expression of the pathogenesis-related protein PR-1 and the development of a microscopically defined hypersensitive response, proved to be light dependent. In contrast, the extent of the oxidative burst, as estimated by induction of the protectant gene glutathione- S-transferase, was not weakened by the absence of light. Moreover, pathogen-induced accumulation of jasmonic acid, production of the phytoalexin camalexin and transcriptional induction of a pathogen-inducible myrosinase were even more pronounced in the dark. Apart from affecting local defence responses, light also influenced the establishment of systemic acquired resistance (SAR). SAR development in response to infection by avirulent bacteria was completely lost when the primary infection process occurred in the absence of light. SAR developed both under medium (70 micromol photons m(-2) s(-1)) and strong (500 micromol photons m(-2) s(-1)) light conditions but was in the latter case not associated with an accumulation of SA and PR-1 in systemic leaves, demonstrating that SAR can be executed independently from these molecular SAR markers. Our results are consistent with the notion that SA accumulation in infected primary leaves is necessary for induction of systemic resistance and indicate that defence mechanisms different from SA signalling and PR-protein action exist in systemic tissue to confer resistance during SAR.

NPR1: the spider in the web of induced resistance signaling pathways

The plant hormones salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are major players in the regulation of signaling networks that are involved in induced defense responses against pathogens and insects. During the past two years, significant progress has been made in understanding the function of NON-EXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1), a key regulator of systemic acquired resistance (SAR), that is essential for transducing the SA signal to activate PATHOGENESIS-RELATED (PR) gene expression. SA-mediated redox changes in Arabidopsis cells regulate both the functioning of NPR1 and its binding to TGA1, a member of the TGA family of transcription factors that activate SA-responsive elements in the promoters of PR genes upon binding with NPR1. Apart from its role in regulating SAR in the nucleus, a novel cytosolic function of NPR1 in cross-communication between SA- and JA-dependent defense signaling pathways has been identified. Other advances in induced resistance signaling, such as the implication that ET is involved in the generation of systemic signal molecules, the suggestion of the involvement of lipid-derived molecules in long-distance signaling, and the identification of new components of various systemic defense signaling pathways, shed new light on how plants actively defend themselves against harmful organisms.

Gene expression signatures from three genetically separable resistance gene signaling pathways for downy mildew resistance

Resistance gene-dependent disease resistance to pathogenic microorganisms is mediated by genetically separable regulatory pathways. Using the GeneChip Arabidopsis genome array, we compared the expression profiles of approximately 8,000 Arabidopsis genes following activation of three RPP genes directed against the pathogenic oomycete Peronospora parasitica. Judicious choice of P. parasitica isolates and loss of resistance plant mutants allowed us to compare the responses controlled by three genetically distinct resistance gene-mediated signaling pathways. We found that all three pathways can converge, leading to up-regulation of common sets of target genes. At least two temporal patterns of gene activation are triggered by two of the pathways examined. Many genes defined by their early and transient increases in expression encode proteins that execute defense biochemistry, while genes exhibiting a sustained or delayed expression increase predominantly encode putative signaling proteins. Previously defined and novel sequence motifs were found to be enriched in the promoters of genes coregulated by the local defense-signaling network. These putative promoter elements may operate downstream from signal convergence points.