The Arabidopsis NPR1 gene that controls systemic acquired resistance encodes a novel protein containing ankyrin repeats

Unraveling regulatory networks in plant defense using microarrays

DNA microarrays are being used to comprehensively examine gene expression networks during the plant defense response that is triggered when a plant encounters a pathogen or an elicitor molecule. In addition to identifying new genes induced during defense, these studies are providing new insights into the complex pathways governing defense gene regulation.

Resistance gene complexes: evolution and utilization

More than 30 genes have been characterized from different plant species that provide resistance to a variety of different pathogen and pest species. The structures of most are consistent with a role in pathogen recognition and defense response signaling. Resistance genes are very abundant in plant genomes and most belong to tightly linked gene families. Evolution of R genes is driven by selection on allelic variation created by mutation and re-assorted by recombination between alleles and sometimes between different gene family members. Selection favors genes that can recognize pathogen avr gene products that are present in pathogen populations. Selection at linked gene families favors haplotypes with useful combinations of genes but a limited physiological cost to the plant. Future utilization of R genes will include transfer between related genera and identification or construction of genes that condition durable resistance to variable pathogens. Genes with durable resistance may interact with conserved pathogen elicitors or condition resistance responses that are independent of specific Avr gene interactions.

Arabidopsis map kinase 4 negatively regulates systemic acquired resistance

Transposon inactivation of Arabidopsis MAP kinase 4 produced the mpk4 mutant exhibiting constitutive systemic acquired resistance (SAR) including elevated salicylic acid (SA) levels, increased resistance to virulent pathogens, and constitutive pathogenesis-related gene expression shown by Northern and microarray hybridizations. MPK4 kinase activity is required to repress SAR, as an inactive MPK4 form failed to complement mpk4. Analysis of mpk4 expressing the SA hydroxylase NahG and of mpk4/npr1 double mutants indicated that SAR expression in mpk4 is dependent upon elevated SA levels but is independent of NPR1. PDF1.2 and THI2.1 gene induction by jasmonate was blocked in mpk4 expressing NahG, suggesting that MPK4 is required for jasmonic acid-responsive gene expression.

hxc2, an Arabidopsis mutant with an altered hypersensitive response to Xanthomonas campestris pv. campestris

A chemical mutagenized population of Arabidopsis Col-0-gl plants was screened for an altered hypersensitive response (HR) after spray inoculation with an HR-inducing isolate of Xanthomonas campestris pv. campestris (strain 147). Three classes of mutant were identified: those exhibiting an HR- phenotype or partial loss of HR; hyper-responsive mutants showing necrotic lesions rapidly leading to the collapse of leaves; and susceptible mutants. One mutant belonging to the susceptible class, hxc-2, was extensively characterized. The compatible phenotype observed several days after initiation of the interaction was confirmed by measurement of in planta bacterial growth and use of bacterial strains constitutively expressing the GUS reporter gene. In the same way, accumulation of autofluorescent compounds, salicylic acid production and defence gene expression in the mutant were found to be similar to that displayed by the susceptible ecotype. Inoculation of hxc-2 with different avirulent bacteria suggests that the mutation is specific for the interaction with the Xcc 147 strain, although the mutation has been shown to affect a single dominant locus, different from the resistance locus defined by genetic analysis of resistance to Xcc 147. Genetic mapping of the mutation indicated that it is located on chromosome III, defining a previously unknown resistance function in response to X. c. campestris.

Are diverse signalling pathways integrated in the regulation of arabidopsis antioxidant defence gene expression in response to excess excitation energy?

When low-light-grown Arabidopsis rosettes are partially exposed to excess light (EL), the unexposed leaves become acclimated to excess excitation energy (EEE) and consequent photo-oxidative stress. This phenomenon, termed systemic acquired acclimation (SAA), is associated with redox changes in the proximity of photosystem II, changes in foliar H2O2 content and induction of antioxidant defences. The induction of extra-plastidial antioxidant systems is important in the protection of the chloroplast under EL conditions. A larger range of transcripts encoding different antioxidant defence enzymes may be induced in the systemically acclimated leaves and these include those encoded by the glutathione peroxidase (GPX2) and glutathione-S-transferase (GST) genes, which are also highly induced in the hypersensitive response and associated systemic acquired resistance (SAR) in incompatible plant-pathogen interactions. Furthermore, the expression of the SAR-inducible pathogenesis-related protein gene, PR2, is enhanced in SAA leaves. Wounded leaf tissue also shows enhanced systemic induction of a cytosolic ascorbate peroxidase gene (APX2) under EL conditions. These and other considerations, suggest H2O2 and other reactive oxygen species (ROS) could be the common factor in signalling pathways for diverse environmental stresses. These effects may be mediated by changes in the level and redox state of the cellular glutathione pool. Mutants with constitutive expression of a normally EL-inducible APX2 gene have much reduced levels of foliar glutathione. The expression of APX1 and APX3, encoding cytosolic and peroxisome-associated isoforms, respectively, are also under phytochrome-A-mediated control. The expression of these genes is tightly linked to the greening of plastids in etiolated seedlings. These data suggest that part of the developmental processes that bring about the acclimation of leaves to high light includes the configuration of antioxidant defences. Therefore, the linkage between immediate responses of leaves to EL, acclimation of chloroplasts to EEE and the subsequent changes to leaf form and function in high light could be mediated by the activity of foliar antioxidant defences and changes in the concentration of ROS.

Coordinated plant defense responses in Arabidopsis revealed by microarray analysis

Disease resistance is associated with a plant defense response that involves an integrated set of signal transduction pathways. Changes in the expression patterns of 2,375 selected genes were examined simultaneously by cDNA microarray analysis in Arabidopsis thaliana after inoculation with an incompatible fungal pathogen Alternaria brassicicola or treatment with the defense-related signaling molecules salicylic acid (SA), methyl jasmonate (MJ), or ethylene. Substantial changes (up- and down-regulation) in the steady-state abundance of 705 mRNAs were observed in response to one or more of the treatments, including known and putative defense-related genes and 106 genes with no previously described function or homology. In leaf tissue inoculated with A. brassicicola, the abundance of 168 mRNAs was increased more than 2.5-fold, whereas that of 39 mRNAs was reduced. Similarly, the abundance of 192, 221, and 55 mRNAs was highly (>2.5-fold) increased after treatment with SA, MJ, and ethylene, respectively. Data analysis revealed a surprising level of coordinated defense responses, including 169 mRNAs regulated by multiple treatments/defense pathways. The largest number of genes coinduced (one of four induced genes) and corepressed was found after treatments with SA and MJ. In addition, 50% of the genes induced by ethylene treatment were also induced by MJ treatment. These results indicated the existence of a substantial network of regulatory interactions and coordination occurring during plant defense among the different defense signaling pathways, notably between the salicylate and jasmonate pathways that were previously thought to act in an antagonistic fashion.

Interplay of signaling pathways in plant disease resistance

Plants are under constant threat of infection by pathogens armed with a diverse array of effector molecules to colonize their host. Plants have, in turn, evolved sophisticated detection and response systems that decipher pathogen signals and induce appropriate defenses. Genetic analysis of plant mutants impaired in mounting a resistance response to invading pathogens has uncovered a number of distinct, but interconnecting, signaling networks that are under both positive and negative control. These pathways operate, at least partly, through the action of small signaling molecules such as salicylate, jasmonate and ethylene. The interplay of signals probably allows the plant to fine-tune defense responses in both local and systemic tissue.

Uncoupling salicylic acid-dependent cell death and defense-related responses from disease resistance in the Arabidopsis mutant acd5

Salicylic acid (SA) is required for resistance to many diseases in higher plants. SA-dependent cell death and defense-related responses have been correlated with disease resistance. The accelerated cell death 5 mutant of Arabidopsis provides additional genetic evidence that SA regulates cell death and defense-related responses. However, in acd5, these events are uncoupled from disease resistance. acd5 plants are more susceptible to Pseudomonas syringae early in development and show spontaneous SA accumulation, cell death, and defense-related markers later in development. In acd5 plants, cell death and defense-related responses are SA dependent but they do not confer disease resistance. Double mutants with acd5 and nonexpressor of PR1, in which SA signaling is partially blocked, show greatly attenuated cell death, indicating a role for NPR1 in controlling cell death. The hormone ethylene potentiates the effects of SA and is important for disease symptom development in Arabidopsis. Double mutants of acd5 and ethylene insensitive 2, in which ethylene signaling is blocked, show decreased cell death, supporting a role for ethylene in cell death control. We propose that acd5 plants mimic P. syringae-infected wild-type plants and that both SA and ethylene are normally involved in regulating cell death during some susceptible pathogen infections.

Salicylic acid has a role in regulating gene expression during leaf senescence

Leaf senescence is a complex process that is controlled by multiple developmental and environmental signals and is manifested by induced expression of a large number of different genes. In this paper we describe experiments that show, for the first time, that the salicylic acid (SA)-signalling pathway has a role in the control of gene expression during developmental senescence. Arabidopsis plants defective in the SA-signalling pathway (npr1 and pad4 mutants and NahG transgenic plants) were used to investigate senescence-enhanced gene expression, and a number of genes showed altered expression patterns. Senescence-induced expression of the cysteine protease gene SAG12, for example, was conditional on the presence of SA, together with another unidentified senescence-specific factor. Changes in gene expression patterns were accompanied by a delayed yellowing and reduced necrosis in the mutant plants defective in SA-signalling, suggesting a role for SA in the cell death that occurs at the final stage of senescence. We propose the presence of a minimum of three senescence-enhanced signalling factors in senescing leaves, one of which is SA. We also suggest that a combination of signalling factors is required for the optimum expression of many genes during senescence.

Riboflavin induces disease resistance in plants by activating a novel signal transduction pathway

ABSTRACT The role of riboflavin as an elicitor of systemic resistance and an activator of a novel signaling process in plants was demonstrated. Following treatment with riboflavin, Arabidopsis thaliana developed systemic resistance to Peronospora parasitica and Pseudomonas syringae pv. Tomato, and tobacco developed systemic resistance to Tobacco mosaic virus (TMV) and Alternaria alternata. Riboflavin, at concentrations necessary for resistance induction, did not cause cell death in plants or directly affect growth of the culturable pathogens. Riboflavin induced expression of pathogenesis-related (PR) genes in the plants, suggesting its ability to trigger a signal transduction pathway that leads to systemic resistance. Both the protein kinase inhibitor K252a and mutation in the NIM1/NPR1 gene which controls transcription of defense genes, impaired responsiveness to riboflavin. In contrast, riboflavin induced resistance and PR gene expression in NahG plants, which fail to accumulate salicylic acid (SA). Thus, riboflavin-induced resistance requires protein kinase signaling mechanisms and a functional NIM1/NPR1 gene, but not accumulation of SA. Riboflavin is an elicitor of systemic resistance, and it triggers resistance signal transduction in a distinct manner.

Nitric oxide and salicylic acid signaling in plant defense

Salicylic acid (SA) plays a critical signaling role in the activation of plant defense responses after pathogen attack. We have identified several potential components of the SA signaling pathway, including (i) the H(2)O(2)-scavenging enzymes catalase and ascorbate peroxidase, (ii) a high affinity SA-binding protein (SABP2), (iii) a SA-inducible protein kinase (SIPK), (iv) NPR1, an ankyrin repeat-containing protein that exhibits limited homology to IkappaBalpha and is required for SA signaling, and (v) members of the TGA/OBF family of bZIP transcription factors. These bZIP factors physically interact with NPR1 and bind the SA-responsive element in promoters of several defense genes, such as the pathogenesis-related 1 gene (PR-1). Recent studies have demonstrated that nitric oxide (NO) is another signal that activates defense responses after pathogen attack. NO has been shown to play a critical role in the activation of innate immune and inflammatory responses in animals. Increases in NO synthase (NOS)-like activity occurred in resistant but not susceptible tobacco after infection with tobacco mosaic virus. Here we demonstrate that this increase in activity participates in PR-1 gene induction. Two signaling molecules, cGMP and cyclic ADP ribose (cADPR), which function downstream of NO in animals, also appear to mediate plant defense gene activation (e.g., PR-1). Additionally, NO may activate PR-1 expression via an NO-dependent, cADPR-independent pathway. Several targets of NO in animals, including guanylate cyclase, aconitase, and mitogen-activated protein kinases (e.g., SIPK), are also modulated by NO in plants. Thus, at least portions of NO signaling pathways appear to be shared between plants and animals.

Positive and negative regulation of salicylic acid-dependent cell death and pathogen resistance in Arabidopsis lsd6 and ssi1 mutants

Salicylic acid (SA) is a key defense molecule in higher plants that is required for resistance to diverse pathogens. A number of mutants of Arabidopsis with elevated resistance to pathogens and constitutive activation of defense-related genes and cell death have been shown to require SA for all of their phenotypes. These mutants potentially identify interesting regulatory genes that control diverse SA responses. When dominant mutations confer SA-dependent phenotypes, it is important to know the genetic basis of dominance in order to draw conclusions on the possible mechanisms of action of the genes identified. Here I characterize the basis of the dominant phenotypes conferred by the ssi1 and lsd6 mutations. I show that ssi1 is haploinsufficient, while lsd6 is a gain-of-function mutation. Thus, SA-dependent responses are under both negative and positive regulation.

Tobacco transcription factor TGA2.2 is the main component of as-1-binding factor ASF-1 and is involved in salicylic acid- and auxin-inducible expression of as-1-containing target promoters

In higher plants, activating sequence-1 (as-1) of the cauliflower mosaic virus 35 S promoter mediates both salicylic acid (SA)- and auxin-inducible transcriptional activation. Originally found in promoters of several viral and bacterial plant pathogens, as-1-like elements are also functional elements of plant promoters activated in the course of a defense response upon pathogen attack. Nuclear as-1-binding factor (ASF-1) and cellular salicylic acid response protein (SARP) bind specifically to as-1. Four different tobacco bZIP transcription factors (TGA1a, PG13, TGA2.1, and TGA2.2) are potential components of either ASF-1 or SARP. Here we show that ASF-1 and SARP are very similar in their composition. TGA2.2 is a major component of either complex, as shown by supershift analysis and Western blot analysis of DNA affinity-purified SARP. Minor amounts of a protein immunologically related to TGA2.1 were detected, whereas TGA1a was not detectable. Overexpression of either TGA2.2 or a dominant negative TGA2.2 mutant affected both SA and auxin (2, 4D) inducibility of various target promoters encoding as-1-like elements, albeit to different extents. This indicates that TGA2.2 is a component of the enhancosome assembling on these target promoters, both under elevated SA and 2,4D concentrations. However, the effect of altered TGA2.2 levels on gene expression was more pronounced upon SA treatment than upon 2,4D treatment.

Cactin, a conserved protein that interacts with the Drosophila IkappaB protein cactus and modulates its function

Rel transcription factors function in flies and vertebrates in immunity and development. Although Rel proteins regulate diverse processes, the control of their function is conserved. In a two-hybrid screen for additional components of the pathway using the Drosophila I-kappaB protein Cactus as a bait, we isolated a novel coiled-coil protein with N-terminal Arg-Asp (RD)- like motifs that we call Cactin. Like the other components of this pathway, Cactin is evolutionarily conserved. Over-expression of cactin in a cactus(A2) heterozygous background results in the enhancement of the cactus phenotype. Both the embryonic lethality and ventralization are strongly increased, suggesting that cactin functions in the Rel pathway controlling the formation of dorsal-ventral embryonic polarity.

Downy mildew (Peronospora parasitica) resistance genes in Arabidopsis vary in functional requirements for NDR1, EDS1, NPR1 and salicylic acid accumulation

To better understand the genetic requirements for R gene-dependent defense activation in Arabidopsis, we tested the effect of several defense response mutants on resistance specified by eight RPP genes (for resistance to Peronospora parasitica) expressed in the Col-0 background. In most cases, resistance was not suppressed by a mutation in the SAR regulatory gene NPR1 or by expression of the NahG transgene. Thus, salicylic acid accumulation and NPR1 function are not necessary for resistance mediated by these RPP genes. In addition, resistance conferred by two of these genes, RPP7 and RPP8, was not significantly suppressed by mutations in either EDS1 or NDR1. RPP7 resistance was also not compromised by mutations in EIN2, JAR1 or COI1 which affect ethylene or jasmonic acid signaling. Double mutants were therefore tested. RPP7 and RPP8 were weakly suppressed in an eds1-2/ndr1-1 background, suggesting that these RPP genes operate additively through EDS1, NDR1 and as-yet-undefined signaling components. RPP7 was not compromised in coi1/npr1 or coi1/NahG backgrounds. These observations suggest that RPP7 initiates resistance through a novel signaling pathway that functions independently of salicylic acid accumulation or jasmonic acid response components.

The WRKY superfamily of plant transcription factors

The WRKY proteins are a superfamily of transcription factors with up to 100 representatives in Arabidopsis. Family members appear to be involved in the regulation of various physio-logical programs that are unique to plants, including pathogen defense, senescence and trichome development. In spite of the strong conservation of their DNA-binding domain, the overall structures of WRKY proteins are highly divergent and can be categorized into distinct groups, which might reflect their different functions.

Exploiting the full potential of disease-resistance genes for agricultural use

Effective and sustained control of fungal pathogens and nematodes is an important issue for all agricultural systems. Global losses caused by pathogens are estimated to be 12% of the potential crop production [1], despite the continued release of new resistant cultivars and pesticides. Furthermore, fungi are continually becoming resistant to existing resistance genes and fungicides, and a few of the pesticides are being withdrawn from the market for environmental reasons. In addition to reducing crop yield, fungal diseases often lower crop quality by producing toxins that affect humans and human health. Additional methods of disease control are therefore highly desirable. Breeding programs based on plant disease-resistance genes are being optimized by incorporating molecular marker techniques and biotechnology. These efforts can be expected to result in the first launches of new disease-resistant crops within the next five years.

A gene controlling sex in grapevines placed on a molecular marker-based genetic map

Genetic maps of Vitis (2n = 38) have been constructed from an interspecific hybrid population of 58 seedlings of the cross 'Horizon' ('Seyval' × 'Schuyler') × Illinois 547-1 (V. cinerea B9 × V. rupestris B38). The maps were initially constructed based on 277 RAPD (random amplified polymorphic DNA) markers using a double-pseudotestcross strategy. Subsequently, 25 microsatellites, 4 CAPS (cleaved amplified polymorphic sequence), and 12 AFLP (amplified fragment length polymorphism) markers were added to the maps. Another 120 markers, mostly those segregating 3:1, were also assigned but not positioned on the linkage groups in the two maps. The 'Horizon' map consisted of 153 markers covering 1199 cM, with an average map distance of 7.6 cM between markers. The Illinois 547-1 map had 179 markers covering 1470 cM, with an average map distance of 8.1 cM. There were 20 linkage groups in each map, one more than the basic number of chromosomes in grapes. Ten linkage groups in each map were identified as homologous using 16 microsatellite and 2 CAPS markers polymorphic in both parents. A single locus controlling sex in grapes mapped close to a microsatellite marker. These maps provide enough coverage of the genome for QTL (quantitative trait loci) analysis and as a starting point for positional gene cloning in grapes.

Key words: Vitis, RAPD, microsatellite, SSR, CAPS.

Nitric oxide modulates the activity of tobacco aconitase

Recent evidence suggests an important role for nitric oxide (NO) signaling in plant-pathogen interactions. Additional elucidation of the role of NO in plants will require identification of NO targets. Since aconitases are major NO targets in animals, we examined the effect of NO on tobacco (Nicotiana tabacum) aconitase. The tobacco aconitases, like their animal counterparts, were inhibited by NO donors. The cytosolic aconitase in animals, in addition to being a key redox and NO sensor, is converted by NO into an mRNA binding protein (IRP, or iron-regulatory protein) that regulates iron homeostasis. A tobacco cytosolic aconitase gene (NtACO1) whose deduced amino acid sequence shared 61% identity and 76% similarity with the human IRP-1 was cloned. Furthermore, residues involved in mRNA binding by IRP-1 were conserved in NtACO1. These results reveal additional similarities between the NO signaling mechanisms used by plants and animals.

NPR1 differentially interacts with members of the TGA/OBF family of transcription factors that bind an element of the PR-1 gene required for induction by salicylic acid

NPR1 is a critical component of the salicylic acid (SA)-mediated signal transduction pathway leading to the induction of defense genes, such as the pathogenesis-related (PR)-1 gene, and enhanced disease resistance. Using a yeast two-hybrid screen, we identified several NPR1-interacting proteins (NIPs). Two of these NIPs are members of the TGA/OBF family of basic leucine zipper (bZIP) transcription factors; this family has been implicated in the activation of SA-responsive genes, including PR-1. Six TGA family members were tested and shown to differentially interact with NPR1: TGA2 and TGA3 showed strong affinity for NPR1; TGA5 and TGA6 exhibited weaker affinity; and TGA1 and TGA4 displayed little or no detectable interaction with NPR1, respectively. Interestingly, the amino-termini of these factors were found to decrease their stability in yeast and differentially affect their apparent affinity toward NPR1. The interacting regions on NPR1 and the TGA factors were also defined. Each of four point mutations in NPR1 that disrupt SA signaling in Arabidopsis completely blocked interaction of NPR1 with TGA2 and TGA3. TGA2 and TGA3 were also found to bind the SA-responsive element of the Arabidopsis PR-1 promoter. These results directly link NPR1 to SA-induced PR-1 expression through members of the TGA family of transcription factors.

Induction of competence for elicitation of defense responses in cucumber hypocotyls requires proteasome activity

The epidermal cells of hypocotyls from etiolated cucumber seedlings are not constitutively competent for elicitation of the rapid H2O2 defense response. However, elicitor competence developed while conditioning the surface-abraded seedlings by rotating them in buffer for 4 h. Competence development was greatly potentiated by inducers of systemic acquired resistance and suppressed by specific inhibitors of proteasome activity, clastolactacystin beta-lactone (LAC) and carboxybenzoyl-L-leucyl-L-leucyl-L-leucinal (LLL). In the freshly abraded seedlings, chitinase gene activation became evident approximately 4 h after elicitor addition. Accumulation of chitinase mRNA was enhanced upon conditioning prior to elicitation and was inhibited by LAC and LLL, indicating that the process which leads to H2O2 elicitation competence is also superimposed on the elicitation of chitinase mRNA. LAC and LLL caused an accumulation of ubiquitin-conjugated proteins and enhanced the expression of a proteasome alpha-subunit, suggesting that proteasome activity was specifically inhibited and that the effect observed on gene expression was not due to impaired gene induction in general. Together, our results suggest that the ubiquitin-proteasome system may play a crucial role in a process which switches the signaling pathway for diverse plant defense responses into a functional state, as is known for many basic cellular processes in both animals and yeast.

Unravelling R gene-mediated disease resistance pathways in Arabidopsis

Abstract Molecular genetic approaches were adopted in the model crucifer, Arabidopsis thaliana, to unravel components of RPP5- and RPP1-mediated disease resistance to the oomycete pathogen, Peronospora parasitica. The products of RPP5 and three genes comprising the RPP1 complex locus belong to a major subclass of nucleotide-binding/leucine-rich repeat (NB-LRR) resistance (R) protein that has amino-terminal homology to the cytoplasmic domains of Drosophila and mammalian Toll and interleukin-1 family receptors (the so called 'TIR' domain). Similarities in the domain architecture of these proteins and animal regulators of programmed cell death have also been observed. Mutational screens revealed a number of genes that are required for RPP5-conditioned resistance. Among these are EDS1 and PAD4. Both EDS1 and PAD4 precede the function of salicylic acid-mediated plant responses. The EDS1 and PAD4 genes were cloned and found to encode proteins with similarity to the catalytic site of eukaryotic lipases, suggesting that they may function by hydrolysing a lipid-based substrate.

Arabidopsis thaliana PAD4 encodes a lipase-like gene that is important for salicylic acid signaling

The Arabidopsis PAD4 gene previously was found to be required for expression of multiple defense responses including camalexin synthesis and PR-1 gene expression in response to infection by the bacterial pathogen Pseudomonas syringae pv. maculicola. This report describes the isolation of PAD4. The predicted PAD4 protein sequence displays similarity to triacyl glycerol lipases and other esterases. The PAD4 transcript was found to accumulate after P. syringae infection or treatment with salicylic acid (SA). PAD4 transcript levels were very low in infected pad4 mutants. Treatment with SA induced expression of PAD4 mRNA in pad4-1, pad4-3, and pad4-4 plants but not in pad4-2 plants. Induction of PAD4 expression by P. syringae was independent of the regulatory factor NPR1 but induction by SA was NPR1-dependent. Taken together with the previous observation that pad4 mutants have a defect in accumulation of SA upon pathogen infection, these results suggest that PAD4 participates in a positive regulatory loop that increases SA levels, thereby activating SA-dependent defense responses.

Induced plant defence responses: scientific and commercial development possibilities

Recent work has demonstrated that plants have endogenous defence mechanisms that can be induced as a response to attack by insects and pathogens. There are two well-studied examples of these induced defence responses. Systemic acquired resistance (SAR) results in increased resistance to a broad spectrum of pathogens throughout a plant in response to localized necrosis caused by pathogen infection. The second example is the systemic induction of proteinase inhibitors to deter feeding by herbivores following an initial event of feeding. In addition, there is now preliminary evidence for other induced defence response pathways. By understanding the breadth of induced defence responses and the mechanisms used to control these pathways, novel plant protection strategies may be developed for use in agronomic settings. Rather than reducing crop losses caused by pests or pathogens by using chemicals that are designed to kill the offending organism, the plant's own defence mechanisms can be used to limit damage due to pests. Novel crop protection strategies based on genetic or chemical regulation of these induced responses show great potential. The first example of a crop protection product that acts by inducing an endogenous defence response pathway is now on the market. Bion reduces the level of pathogen infection in plants by activating SAR.

NRSA-1: a resistance gene homolog expressed in roots of non-host plants following parasitism by Striga asiatica (witchweed)

Studies of the initial interactions of Striga asiatica with the non-host plant species Tagetes erecta (marigold) established that parasite penetration through the root is arrested most frequently in the cortex. The arrest of parasite ingress is associated with browning and necrosis of root cortical cells flanking the invading endophyte and with increased intracellular wall appositions on the root cell walls directly adjacent to the plant-parasite interface. Using a polymerase chain reaction-based differential cDNA amplification strategy followed by 5'-RACE, we have identified several gene products whose expression is induced in marigold roots during attempted parasitism by Striga. Among these was a 917 bp cDNA encoding a 221 amino acid protein with significant homology to proteins encoded by disease resistance genes from other plant species, including N, RPP5, L6 and M. This cDNA was subsequently used to isolate a nuclear gene, designated NRSA-1, for non-host resistance to Striga asiatica. NRSA-1 is a member of a small gene family in marigold consisting of two to four members. RNA gel blot analysis showed that NRSA-1 transcripts accumulate to high levels in roots near the site of Striga invasion within 120 h after parasite attachment, and appear at lower levels throughout the rest of the plant under Striga parasitism. NRSA-1 expression is rapidly induced by treatment with jasmonic acid (JA), but not by mechanical wounding, treatment with salicylic acid, paraquat or ABA. A possible role for NRSA-1 in the non-host resistance mechanism is discussed.

Harpin induces disease resistance in Arabidopsis through the systemic acquired resistance pathway mediated by salicylic acid and the NIM1 gene

Harpin, the product of the hrpN gene of Erwinia amylovora, elicits the hypersensitive response and disease resistance in many plants. Harpin and known inducers of systemic acquired resistance (SAR) were tested on five genotypes of Arabidopsis thaliana to assess the role of SAR in harpin-induced resistance. In wild-type plants, harpin elicited systemic resistance to Peronospora parasitica and Pseudomonas syringae pv. tomato, accompanied by induction of the SAR genes PR-1 and PR-2. However, in experiments with transgenic Arabidopsis plants containing the nahG gene which prevents accumulation of salicylic acid (SA), harpin neither elicited resistance nor activated SAR gene expression. Harpin also failed to activate SAR when applied to nim1 (non-inducible immunity) mutants, which are defective in responding to SA and regulation of SAR. In contrast, mutants compromised in responsiveness to methyl jasmonate and ethylene developed the same resistance as did wild-type plants. Thus, harpin elicits disease resistance through the NIM1-mediated SAR signal transduction pathway in an SA-dependent fashion. The site of action of harpin in the SAR regulatory pathway is upstream of SA.

Identification and cloning of a negative regulator of systemic acquired resistance, SNI1, through a screen for suppressors of npr1-1

Systemic acquired resistance (SAR) is a plant immune response induced after a local infection by necrotizing pathogens. The Arabidopsis NPR1 gene is a positive regulator of SAR, essential for transducing the SAR signal salicylic acid (SA). Mutations in the NPR1 gene abolish the SA-induced expression of pathogenesis-related (PR) genes and resistance to pathogens. To identify additional regulators of SAR, we screened for suppressors of npr1-1. In the npr1-1 background, the sni1 (suppressor of npr1-1, inducible 1) mutant shows near wild-type levels of PR1 expression and resistance to pathogens after induction. Restoration of SAR in npr1-1 by the recessive sni1 mutation indicates that wild-type SNI1 may function as a negative regulator of SAR. We cloned the SNI1 gene and found that it encodes a leucine-rich nuclear protein.

Functional analysis of plant disease resistance genes and their downstream effectors

Plant disease resistance (R) genes encode proteins that both determine recognition of specific pathogen-derived avirulence (Avr) proteins and initiate signal transduction pathways leading to complex defense responses. Recent developments suggest that recognition specificity of R proteins is determined by either a protein kinase domain or by a region consisting of leucine-rich repeats. R genes conferring resistance to bacterial, viral, and fungal pathogens appear to use multiple signaling pathways, some of which involve distinct proteins and others which converge upon common downstream effectors. Manipulation of R genes and their signaling pathways by transgenic expression is a promising strategy to improve disease resistance in plants.

Genes controlling expression of defense responses in Arabidopsis

In the past year, two regulatory defense-related genes, EDS1l and COl1, have been cloned. Several other genes with regulatory functions have been identified by mutation, including DND1, PAD4, CPR6, and SSl1. It has become clear that jasmonate signaling plays an important role in defense response signaling, and that the jasmonate and salicylic acid signaling pathways are interconnected.

Oyster IKK-like protein shares structural and functional properties with its mammalian homologues

In our search for genes involved in oyster immunity we isolated a cDNA encoding a polypeptide closely related to the mammalian IkappaB kinase (IKK) family. IKK proteins play a central role in cell signaling by regulating nuclear factor-kappaB (NF-kappaB) activation. We report here the cloning of an oyster IKK-like protein (oIKK) which possesses the characteristic organization of the mammalian IKK proteins, namely an amino-terminal kinase domain followed by a leucine zipper region and a carboxyl-terminal helix-loop-helix motif. When transfected into human cell lines, oIKK activated the expression of NF-kappaB-controlled reporter gene, whereas transfections with mutants of oIKK deleted within the kinase domain or within the helix-loop-helix motif respectively abolished and greatly reduced reporter gene activation. These results indicate that oIKK can replace the hIKK-alpha in catalyzing NF-kappaB nuclear translocation, and in triggering gene expression. Our results sustain the concept of an evolutionarily conserved signaling machinery in which IKK plays a major role.

Interaction of NPR1 with basic leucine zipper protein transcription factors that bind sequences required for salicylic acid induction of the PR-1 gene

The Arabidopsis thaliana NPR1 has been shown to be a key regulator of gene expression during the onset of a plant disease-resistance response known as systemic acquired resistance. The npr1 mutant plants fail to respond to systemic acquired resistance-inducing signals such as salicylic acid (SA), or express SA-induced pathogenesis-related (PR) genes. Using NPR1 as bait in a yeast two-hybrid screen, we identified a subclass of transcription factors in the basic leucine zipper protein family (AHBP-1b and TGA6) and showed that they interact specifically in yeast and in vitro with NPR1. Point mutations that abolish the NPR1 function in A. thaliana also impair the interactions between NPR1 and the transcription factors in the yeast two-hybrid assay. Furthermore, a gel mobility shift assay showed that the purified transcription factor protein, AHBP-1b, binds specifically to an SA-responsive promoter element of the A. thaliana PR-1 gene. These data suggest that NPR1 may regulate PR-1 gene expression by interacting with a subclass of basic leucine zipper protein transcription factors.

Salicylic acid induces rapid inhibition of mitochondrial electron transport and oxidative phosphorylation in tobacco cells

Salicylic acid (SA) is known to induce alternative pathway respiration by activating expression of the alternative oxidase gene. In the present study we report a rapid mode of action by SA on plant mitochondrial functions. SA at concentrations as low as 20 &mgr;M induced inhibition of both ATP synthesis and respiratory O2 uptake within minutes of incubation in tobacco (Nicotiana tabacum) cell cultures. Biologically active SA analogs capable of inducing pathogenesis-related genes and enhanced resistance also caused rapid inhibition of ATP synthesis and respiratory O2 uptake, whereas biologically inactive analogs did not. Inhibition of ATP synthesis and respiratory O2 uptake by SA was insensitive to the protein synthesis inhibitor cycloheximide, but was substantially reduced by the antioxidant N-acetylcysteine, suggesting a possible role for reactive oxygen species in the inhibition of mitochondrial functions. With exogenous NADH as the respiratory substrate, mitochondria isolated from SA-treated tobacco cell cultures were found to have normal capacities for both ATP synthesis and respiratory O2 uptake; direct incubation of isolated mitochondria with SA had no significant effect on these mitochondrial functions. These results indicate that (a) the respiration capacities of isolated mitochondria do not correspond to the in vivo respiration activities in SA-treated cell cultures and (b) the SA-induced inhibition of respiration in tobacco cell cultures may involve other components that are not present in isolated mitochondria. Given the recently demonstrated roles of mitochondria in plant disease resistance and animal apoptosis, this rapid inhibition by SA of mitochondrial functions may play a role in SA-mediated biological processes, including plant defense responses.

Interactions between tobacco mosaic virus and the tobacco N gene

The interaction between tobacco mosaic virus (TMV) and tobacco harbouring the N gene is a classical system for studying gene-for-gene interactions in disease resistance. The N gene confers resistance to TMV by mediating defence responses that function to limit viral replication and movement. We isolated the N gene and determined that N belongs to the nucleotide-binding-site-leucine-rich-repeat (NBS-LRR) class of plant disease resistance genes, and encodes both full-length and truncated proteins. Sequence homologies and mutagenesis studies indicated a signalling role for the N protein similar to that seen for proteins involved in defence responses in insects and mammals. The N gene confers resistance to TMV in transgenic tomato, demonstrating the use of the NBS-LRR class of disease resistance genes in engineering crop resistance. From the pathogen side of this interaction, the TMV 126 kDa replicase protein has been implicated as the avirulence factor that triggers N-mediated defence responses. We employed Agrobacterium-mediated expression strategies to demonstrate that expression of the putative helicase region of the replicase protein is sufficient to elicit N-mediated defences. The thermosensitivity of the N-mediated response to TMV is retained when induced by expression of this replicase fragment. Thus, both components of this gene-for-gene interaction are now available for studies that address the molecular mechanisms involved in N-mediated TMV resistance.

Gentisic Acid As a Pathogen-Inducible Signal, Additional to Salicylic Acid for Activation of Plant Defenses in Tomato

Citrus exocortis viroid (CEVd) and tomato mosaic virus (ToMV), which produce a systemic non-necrotizing infection in tomato (Lycopersicon esculentum cv. Rutgers), strongly induced the accumulation of a phenolic compound that we have characterized as 2,5-dihydroxybenzoic acid (gentisic acid, GA) by nuclear magnetic resonance, following purification by high-performance liquid chromatography. Levels of free and total GA increased more than 150-fold in response to CEVd and ToMV infections. Unlike these non-necrotizing infections, the necrotizing reaction elicited by Pseudomonas syringae pv. syringae in this host did not produce any accumulation of GA. It is also shown that, in healthy leaf tissues, benzoic acid (BA) and salicylic acid (SA) were rapidly converted to GA, SA being the immediate precursor of GA, according to radiolabeling studies. Interestingly, exogenous GA elicited accumulation of the previously described CEVd-induced antifungal pathogenesis-related (PR) proteins P23, P32, and P34. These proteins were not induced by exogenous SA, which is able to elicit other CEVd-induced PR proteins in tomato. These results suggest that GA acts as a pathogeninduced signal, additional to SA, for activation of plant defense genes in tomato.

Inhibition of protoporphyrinogen oxidase expression in Arabidopsis causes a lesion-mimic phenotype that induces systemic acquired resistance

We have used an antisense expression technology in Arabidopsis based on the yeast GAL4/UAS transactivation system (Guyer et al., Genetics, 1998; 149:633-639) to reduce levels of protoporphyrinogen IX oxidase (PPO), the last common enzyme of the biosynthesis of the haem group and chlorophyll. Plants expressing the antisense PPO gene presented growth alterations and their leaves showed necrotic lesions that appeared similar to lesions characteristic of the pathogen-induced hypersensitive reaction, and seen in the so-called lesion-mimic mutants. Plants expressing the antisense gene also had high endogenous salicylic acid levels, constitutive expression of the PR-1 gene, and were resistant to Peronospora parasitica, consistent with the activation of systemic acquired resistance (SAR). Treatment of wild-type plants with sublethal concentrations of herbicides that inhibit PPO also induced defence responses that conferred enhanced tolerance to P. parasitica. This effect was not observed in NahG and nim1 plants, which are compromised in their ability to activate SAR. These results demonstrate that genetic or chemical disruption of a metabolic pathway can lead to the induction of a set of defence responses including activation of SAR.

Salicylic acid-independent plant defence pathways

Salicylic acid is an important signalling molecule involved in both locally and systemically induced disease resistance responses. Recent advances in our understanding of plant defence signalling have revealed that plants employ a network of signal transduction pathways, some of which are independent of salicylic acid. Evidence is emerging that jasmonic acid and ethylene play key roles in these salicylic acid-independent pathways. Cross-talk between the salicylic acid-dependent and the salicylic acid-independent pathways provides great regulatory potential for activating multiple resistance mechanisms in varying combinations.

Fold prediction and evolutionary analysis of the POZ domain: structural and evolutionary relationship with the potassium channel tetramerization domain

Using iterative database searches, a statistically significant sequence similarity was detected between the POZ (poxvirus and zinc finger) domains found in a variety of proteins involved in animal transcription regulation, cytoskeleton organization, and development, and the tetramerization domain of animal potassium channels. Using the crystal structure of the Aplysia Shaker channel tetramerization domain as a template, the common structure of the POZ domain class was predicted. Examination of the structure resulted in the identification of several structural features and specific amino acid residues that may be involved in conserved protein-protein interactions mediated by the POZ domains as well as those that may contribute to the specificity of these interactions. Phylogenetic analysis of the POZ domains suggests that the common ancestor of the crown group eukaryotes already possessed this domain; POZ domains have undergone independent expansion in plants and in different animal lineages.

Separate jasmonate-dependent and salicylate-dependent defense-response pathways in Arabidopsis are essential for resistance to distinct microbial pathogens

The endogenous plant hormones salicylic acid (SA) and jasmonic acid (JA), whose levels increase on pathogen infection, activate separate sets of genes encoding antimicrobial proteins in Arabidopsis thaliana. The pathogen-inducible genes PR-1, PR-2, and PR-5 require SA signaling for activation, whereas the plant defensin gene PDF1.2, along with a PR-3 and PR-4 gene, are induced by pathogens via an SA-independent and JA-dependent pathway. An Arabidopsis mutant, coi1, that is affected in the JA-response pathway shows enhanced susceptibility to infection by the fungal pathogens Alternaria brassicicola and Botrytis cinerea but not to Peronospora parasitica, and vice versa for two Arabidopsis genotypes (npr1 and NahG) with a defect in their SA response. Resistance to P. parasitica was boosted by external application of the SA-mimicking compound 2, 6-dichloroisonicotinic acid [Delaney, T., et al. (1994) Science 266, 1247-1250] but not by methyl jasmonate (MeJA), whereas treatment with MeJA but not 2,6-dichloroisonicotinic acid elevated resistance to Alternaria brassicicola. The protective effect of MeJA against A. brassicicola was the result of an endogenous defense response activated in planta and not a direct effect of MeJA on the pathogen, as no protection to A. brassicicola was observed in the coi1 mutant treated with MeJA. These data point to the existence of at least two separate hormone-dependent defense pathways in Arabidopsis that contribute to resistance against distinct microbial pathogens.

Regulated nuclear targeting

Extracellular signals are transduced to the nucleus through respective signal transduction pathways. Evidence in animals and yeast indicates the importance of regulated nuclear targeting in these processes. Although little is known about plants in this regard, some plant signaling factors have recently been shown to translocate to the nucleus upon receipt of a signal.

Correlation of defense gene induction defects with powdery mildew susceptibility in Arabidopsis enhanced disease susceptibility mutants

We investigated the relative importance of specific Arabidopsis thaliana genes in conferring resistance to bacterial versus fungal pathogens. We first developed a pathosystem involving the infection of Arabidopsis accession Columbia with a virulent isolate of the obligate biotrophic fungal pathogen Erysiphe orontii. E. orontii elicited the accumulation of mRNAs corresponding to the defense-related genes PR1, BGL2 (PR2), PR5 and GST1, but did not elicit production of the phytoalexin camalexin or the accumulation of defensin (PDF1.2) or thionin (THI2.1) mRNAs. We tested a set of 15 previously isolated Arabidopsis phytoalexin deficient (pad), non-expresser of PR (npr) and enhanced disease susceptibility (eds) mutants that are more susceptible to Pseudomonas syringae for their susceptibility to E. orontii. Four of these mutants (pad4-1, npr1-1, eds5-1 and a double npr1-1 eds5-1 mutant) as well as Arabidopsis lines carrying a nahG transgene exhibited enhanced susceptibility to E. orontii and reduced levels of PR gene expression. Comparison of the PR gene induction patterns in response to E. orontii in the various mutants and in the nahG transgenics suggests the existence of NPR1-independent salicylate-dependent and NPR1-independent salicylate-independent defense gene activation pathways. Eleven other eds and pad mutants did not show measurable enhanced susceptibility to E. orontii, suggesting that these mutants are defective in factors that are not important for the limitation of E. orontii growth.

Caspases and programmed cell death in the hypersensitive response of plants to pathogens

The hypersensitive response (HR) is induced by certain plant pathogens and involves programmed cell death (PCD) to restrict the spread of pathogens from the infection site [1]. Concurrent with the induction of cell death, the host activates a defense response [2]. The cell death associated with the HR in several plant-pathogen systems has morphological similarities to animal apoptosis [3,4], which suggests that cell death mechanisms in plants and animals may share common components that lead to similar cellular events. Caspases are conserved cysteine proteases that regulate animal PCD [5]; caspase activity or an involvement of caspases in cell death has yet to be reported in plants. In this work, we investigated the participation of caspases in HR cell death. Caspase-specific peptide inhibitors, Ac-YVAD-CMK [6] and Ac-DEVD-CHO [7], could abolish bacteria-induced plant PCD but did not significantly affect the induction of other aspects of HR, such as the expression of defense genes. This result confirmed our previous model that cell death can be uncoupled from defense gene activation during HR [8]. Caspase-like proteolytic activity was detected in tobacco tissues that were developing HR following infection with tobacco mosaic virus (TMV). Our results provide evidence for the presence of caspase-like plant protease(s) that participate in HR cell death.

Functional analysis of regulatory sequences controlling PR-1 gene expression in Arabidopsis

The Arabidopsis PR-1 gene is one of a suite of genes induced co-ordinately during the onset of systemic acquired resistance (SAR), a plant defense pathway triggered by pathogen infection or exogenous application of chemicals such as salicylic acid (SA) and 2,6-dichloroisonicotinic acid (INA). We have characterized cis-acting regulatory elements in the PR-1 promoter involved in INA induction using deletion analysis, linker-scanning mutagenesis, and in vivo footprinting. Compared to promoter fragments of 815 bp or longer (which show greater than 10-fold inducibility after INA treatment), induction of a 698 bp long promoter fragment is reduced by half and promoter fragments of 621 bp or shorter have lost all inducibility. Additionally, two 10-bp linker-scanning mutations centered at 640 bp and 610 bp upstream from the transcription initiation site are each sufficient to abolish chemical inducibility of a GUS reporter fusion. The -640 linker-scanning mutation encompasses a region highly homologous to recognition sites for transcription factors of the basic leucine zipper class, while the -610 linker-scanning mutation contains a sequence similar to a consensus recognition site for the transcription factor NF-kappa B. Furthermore, several inducible in vivo footprints located at or nearby these motifs demonstrate significant and highly reproducible changes in DNA accessibility following SAR induction. This in vivo signature of protein-DNA interactions after INA induction is tightly correlated with the functionally important regions of the promoter identified by mutation analysis.

The hypersensitive response. A programmed cell death associated with plant resistance

In plants, the hypersensitive response (HR) is defined as a rapid cell death occurring in response to pathogen attack, and is closely related to active resistance. Initiation of the HR process begins with the recognition of the pathogen by the plant, which is mediated mainly by the pathogen avirulence genes and the plant resistance genes. Then, complex signal transduction pathways intervene, involving changes in protein phosphorylation, production of reactive oxygen species and modification of ion fluxes. Components required for the regulation of the HR cell death are now being identified genetically by the isolation of mutants, in contrast to those involved in the execution of the cell death programme, which are still largely unknown. Further genetic and biochemical analyses will undoubtedly answer the question as to whether this form of programmed cell death (PCD) can be compared with other forms of PCD in plants and with apoptosis in animals.

Four classes of salicylate-induced tobacco genes

We have identified and characterized fragments of 15 salicylic acid (SA) early response genes. The kinetics of induction and response to cycloheximide (CHX) treatment allowed classification of genes into four groups. Classes I-III are characterized by immediate-early responses, showing increased accumulation of mRNA within 30 min of SA treatment. Moreover, CHX did not block induction of these genes, indicating that latent cellular factors mediate the SA response. Class IV genes were induced more slowly, but still within 2 to 3 h of SA treatment, and required protein synthesis for expression. Although identified in this study as SA-responsive genes, several could also be induced by other compounds. Two genes were characterized in more detail, including isolation of cDNA sequences and additional analysis of gene expression. Sequence analysis revealed that the class I gene, C18-1, is the previously identified ethylene response element binding protein 1 (EREBP1), an ethylene-induced transcription factor for basic pathogenesis-related (PR) genes, whereas the class III gene, G8-1, is a novel sequence. G8-1 was found to be strongly induced only by SA and its active analogs and was exquisitely sensitive to low SA concentrations. These and other genes were found to be activated at early times following tobacco mosaic virus infection of resistant tobacco genotypes.

Different requirements for EDS1 and NDR1 by disease resistance genes define at least two R gene-mediated signaling pathways in Arabidopsis

The Arabidopsis genes EDS1 and NDR1 were shown previously by mutational analysis to encode essential components of race-specific disease resistance. Here, we examined the relative requirements for EDS1 and NDR1 by a broad spectrum of Resistance (R) genes present in three Arabidopsis accessions (Columbia, Landsberg-erecta, and Wassilewskija). We show that there is a strong requirement for EDS1 by a subset of R loci (RPP2, RPP4, RPP5, RPP21, and RPS4), conferring resistance to the biotrophic oomycete Peronospora parasitica, and to Pseudomonas bacteria expressing the avirulence gene avrRps4. The requirement for NDR1 by these EDS1-dependent R loci is either weak or not measurable. Conversely, three NDR1-dependent R loci, RPS2, RPM1, and RPS5, operate independently of EDS1. Another RPP locus, RPP8, exhibits no strong exclusive requirement for EDS1 or NDR1 in isolate-specific resistance to P. parasitica, although resistance is compromised weakly by eds1. Similarly, resistance conditioned by two EDS1-dependent RPP genes, RPP4 and RPP5, is impaired partially by ndr1, implicating a degree of pathway cross-talk. Our results provide compelling evidence for the preferential utilization of either signaling component by particular R genes and thus define at least two disease resistance pathways. The data also suggest that strong dependence on EDS1 or NDR1 is governed by R protein structural type rather than pathogen class.

A benzothiadiazole primes parsley cells for augmented elicitation of defense responses

Systemic acquired resistance is an important component of the disease-resistance arsenal of plants, and is associated with an enhanced potency for activating local defense responses upon pathogen attack. Here we demonstrate that pretreatment with benzothiadiazole (BTH), a synthetic activator of acquired resistance in plants, augmented the sensitivity for low-dose elicitation of coumarin phytoalexin secretion by cultured parsley (Petroselinum crispum L.) cells. Enhanced coumarin secretion was associated with potentiated activation of genes encoding Phe ammonia-lyase (PAL). The augmentation of PAL gene induction was proportional to the length of pretreatment with BTH, indicating time-dependent priming of the cells. In contrast to the PAL genes, those for anionic peroxidase were directly induced by BTH in the absence of elicitor, thus confirming a dual role for BTH in the activation of plant defenses. Strikingly, the ability of various chemicals to enhance plant disease resistance correlated with their capability to potentiate parsley PAL gene elicitation, emphasizing an important role for defense response potentiation in acquired plant disease resistance.

Transgenic approaches to microbial disease resistance in crop plants

Recent progress in the genetic dissection of plant disease resistance signaling pathways has opened a number of new avenues towards engineering pathogen resistance in crops. Genes controlling race-specific and broad-spectrum resistance responses have been cloned, and novel induced resistance pathways have been identified in model and crop systems. Advances continue to be made in identification of antifungal proteins with effects inhibitory to either pathogen development or accumulation of associated mycotoxins.