Systemic acquired resistance

The BTB/POZ domain of the Arabidopsis disease resistance protein NPR1 interacts with the repression domain of TGA2 to negate its function

TGA2 and NONEXPRESSER OF PR GENES1 (NPR1) are activators of systemic acquired resistance (SAR) and of the SAR marker gene pathogenesis-related-1 (PR-1) in Arabidopsis thaliana. TGA2 is a transcriptional repressor required for basal repression of PR-1, but during SAR, TGA2 recruits NPR1 as part of an enhanceosome. Transactivation by the enhanceosome requires the NPR1 BTB/POZ domain. However, the NPR1 BTB/POZ domain does not contain an autonomous transactivation domain; thus, its molecular role within the enhanceosome remains elusive. We now show by gel filtration analyses that TGA2 binds DNA as a dimer, tetramer, or oligomer. Using in vivo plant transcription assays, we localize the repression domain of TGA2 to the N terminus and demonstrate that this domain is responsible for modulating the DNA binding activity of the oligomer both in vitro and in vivo. We confirm that the NPR1 BTB/POZ domain interacts with and negates the molecular function of the TGA2 repression domain by excluding TGA2 oligomers from cognate DNA. These data distinguish the NPR1 BTB/POZ domain from other known BTB/POZ domains and establish its molecular role in the context of the Arabidopsis PR-1 gene enhanceosome.

Regulation of cell death and innate immunity by two receptor-like kinases in Arabidopsis

Upon recognition of bacterial flagellin, the plant receptor FLS2 heterodimerizes with brassinosteroid insensitive 1-associated receptor kinase 1 (BAK1) and activates plant defense responses. Because constitutive activation of defense responses is detrimental, plant resistance signaling pathways must be negatively controlled, although the mechanisms involved are unclear. We identified Arabidopsis BIR1 as a BAK1-interacting receptor-like kinase. Knocking out BIR1 leads to extensive cell death, activation of constitutive defense responses, and impairment in the activation of MPK4, a negative regulator of plant resistance (R) protein signaling, by flagellin. sobir1-1, a mutant obtained in a screen for suppressors of the bir1-1 phenotype, rescued cell death observed in bir1-1. SOBIR1 encodes another receptor-like kinase whose overexpression activates cell death and defense responses. Our data suggest that BIR1 negatively regulates multiple plant resistance signaling pathways, one of which is the SOBIR1-dependent pathway identified here.

The infective cycle of Cabbage leaf curl virus (CaLCuV) is affected by CRUMPLED LEAF (CRL) gene in Arabidopsis thaliana

Background

Geminiviruses are single-stranded DNA viruses that cause serious crop losses worldwide. Successful infection by these pathogens depends extensively on virus-host intermolecular interactions that allow them to express their gene products, to replicate their genomes and to move to adjacent cells and throughout the plant.

Results

To identify host genes that show an altered regulation in response to Cabbage leaf curl virus (CaLCuV) infection, a screening of transposant Arabidopsis thaliana lines was carried out. Several genes were identified to be virus responsive and one, Crumpled leaf (CRL) gene, was selected for further characterization. CRL was previously reported by Asano et al., (2004) to affect the morphogenesis of all plant organs and the division of plastids. We report here that CRL expression, during CaLCuV infection, shows a short but strong induction at an early stage (3-5 days post inoculation, dpi). To study the role of CRL in CaLCuV infection, CRL over-expressing and silenced transgenic plants were generated. We compared the replication, movement and infectivity of CaLCuV in transgenic and wild type plants.

Conclusion

Our results showed that CRL over-expressing plants showed an increased susceptibility to CaLCuV infection (as compared to wt plants) whereas CRL-silenced plants, on the contrary, presented a reduced susceptibility to viral infection. The possible role of CRL in the CaLCuV infection cycle is discussed.

Accumulation of elicitor activity in the intercellular fluid of the Arabidopsis thaliana len3 mutant

Activation of the hypersensitive response (HR) triggers localized acquired resistance (LAR) and systemic acquired resistance (SAR). Recently we found that Arabidopsis thaliana lesion initiation 3 (len3) plants develop lesions on leaves without pathogen attack, constitutively express PR genes, and accumulate elevated levels of SA. Hence we hypothesized that a signal important for LAR and SAR accumulates in the intercellular fluids (IFs) of the len3 plants. Infiltration of the IF from len3 plants induced PR-2 expression in local leaves but not in the systemic leaves of the wild type plants, suggesting that the elicitor activity of the IF contributes to LAR but not to SAR. Induction of PR-2 was dependent on SA signaling and ET signaling, and the elicitor in the IF was associated with molecules in the range of >100 kDa. These results suggest that len3 plants accumulate the elicitor in the IF, and that this might play a role in the establishment of LAR.

Ectopic expression of MgSM1, a Cerato-platanin family protein from Magnaporthe grisea, confers broad-spectrum disease resistance in Arabidopsis

Proteins belonging to the newly identified Cerato-platanin (CP) family have been shown to have elicitor activity in inducing disease resistance responses in various plants. In this study, we characterized a gene, MgSM1, from Magnaporthe grisea, encoding a putative small protein belonging to the CP family. MgSM1 was constitutively expressed not only in different fungal growth stages but also during its infection process in rice plants. Agrobacterium-mediated transient expression of MgSM1 in Arabidopsis resulted in hypersensitive response in the infiltrated local leaves and enhanced disease resistance against Botrytis cinerea and Pseudomonas syringae pv. tomato (Pst) DC3000 in upper leaves of plants, accompanyed by up-regulated expression of defense genes (PR-1, PR-5 and PDF1.2). Transgenic Arabidopsis plants expressing MgSM1 under control of a dexamethasone (DEX)-inducible promoter were generated. Expression of MgSM1 in transgenic plants was induced by exogenous application of DEX. MgSM1-expressing plants showed normal growth with application of <10 microm DEX. After DEX induction, the MgSM1-expressing plants showed enhanced disease resistance against B. cinerea, Alternaria brassicicola and Psto DC3000 as well as up-regulated expression of some of defense genes. Moreover, accumulation of reactive oxygen species was observed in MgSM1-expressing plants. These results collectively suggest that ectopic expression of MgSM1 in transgenic plants confers broad-spectrum resistance against different types of pathogens. Our study also provides a novel strategy to generate environment-friendly crops with enhanced broad-spectrum resistance through ectopic expression of microbe-derived disease resistance-inducing proteins.

A pair of allelic WRKY genes play opposite roles in rice-bacteria interactions

Although allelic diversity of genes has been reported to play important roles in different physiological processes, information on allelic diversity of defense-responsive genes in host-pathogen interactions is limited. Here, we report that a pair of allelic genes, OsWRKY45-1 and OsWRKY45-2, which encode proteins with a 10-amino acid difference, play opposite roles in rice (Oryza sativa) resistance against bacterial pathogens. Bacterial blight caused by Xanthomonas oryzae pv oryzae (Xoo), bacterial streak caused by Xanthomonas oryzae pv oryzicola (Xoc), and fungal blast caused by Magnaporthe grisea are devastating diseases of rice worldwide. OsWRKY45-1-overexpressing plants showed increased susceptibility and OsWRKY45-1-knockout plants showed enhanced resistance to Xoo and Xoc. In contrast, OsWRKY45-2-overexpressing plants showed enhanced resistance and OsWRKY45-2-suppressing plants showed increased susceptibility to Xoo and Xoc. Interestingly, both OsWRKY45-1- and OsWRKY45-2-overexpressing plants showed enhanced resistance to M. grisea. OsWRKY45-1-regulated Xoo resistance was accompanied by increased accumulation of salicylic acid and jasmonic acid and induced expression of a subset of defense-responsive genes, while OsWRKY45-2-regulated Xoo resistance was accompanied by increased accumulation of jasmonic acid but not salicylic acid and induced expression of another subset of defense-responsive genes. These results suggest that both OsWRKY45-1 and OsWRKY45-2 are positive regulators in rice resistance against M. grisea, but the former is a negative regulator and the latter is a positive regulator in rice resistance against Xoo and Xoc. The opposite roles of the two allelic genes in rice-Xoo interaction appear to be due to their mediation of different defense signaling pathways.

Molecular analysis of menadione-induced resistance against biotic stress in Arabidopsis

Menadione sodium bisulphite (MSB) is a water-soluble derivative of vitamin K3, or menadione, and has been previously demonstrated to function as a plant defence activator against several pathogens in several plant species. However, there are no reports of the role of this vitamin in the induction of resistance in the plant model Arabidopsis thaliana. In the current study, we demonstrate that MSB induces resistance by priming in Arabidopsis against the virulent strain Pseudomonas syringae pv. tomato DC3000 (Pto) without inducing necrosis or visible damage. Changes in gene expression in response to 0.2 mm MSB were analysed in Arabidopsis at 3, 6 and 24 h post-treatment using microarray technology. In general, the treatment with MSB does not correlate with other publicly available data, thus MSB produces a unique molecular footprint. We observed 158 differentially regulated genes among all the possible trends. More up-regulated genes are included in categories such as 'response to stress' than the background, and the behaviour of these genes in different treatments confirms their role in response to biotic and abiotic stress. In addition, there is an over-representation of the G-box in their promoters. Some interesting functions are represented among the individual up-regulated genes, such as glutathione S-transferases, transcription factors (including putative regulators of the G-box) and cytochrome P450s. This work provides a wide insight into the molecular cues underlying the effect of MSB as a plant resistance inducer.

Distinct changes in soybean xylem sap proteome in response to pathogenic and symbiotic microbe interactions

BACKGROUND

Plant systemic signaling characterized by the long distance transport of molecules across plant organs involves the xylem and phloem conduits. Root-microbe interactions generate systemic signals that are transported to aerial organs via the xylem sap. We analyzed the xylem sap proteome of soybean seedlings in response to pathogenic and symbiotic interactions to identify systemic signaling proteins and other differentially expressed proteins.

RESULTS

We observed the increase of a serine protease and peroxidase in the xylem sap in response to Phytophthora sojae elicitor treatment. The high molecular weight fraction of soybean xylem sap was found to promote the growth of Neurospora crassa in vitro at lower concentrations and inhibit growth at higher concentrations. Sap from soybean plants treated with a P. sojae elicitor had a significantly higher inhibitory effect than sap from control soybean plants. When soybean seedlings were inoculated with the symbiont Bradyrhizobium japonicum, the abundance of a xyloglucan transendoglycosyl transferase protein increased in the xylem sap. However, RNAi-mediated silencing of the corresponding gene did not significantly affect nodulation in soybean hairy root composite plants.

CONCLUSION

Our study identified a number of sap proteins from soybean that are differentially induced in response to B. japonicum and P. sojae elicitor treatments and a majority of them were secreted proteins.

Jasmonate signaling in plant interactions with resistance-inducing beneficial microbes

Beneficial soil-borne microorganisms can induce an enhanced defensive capacity in above-ground plant parts that provides protection against a broad spectrum of microbial pathogens and even insect herbivores. The phytohormones jasmonic acid (JA) and ethylene emerged as important regulators of this induced systemic resistance (ISR). ISR triggered by plant growth-promoting rhizobacteria and fungi is often not associated with enhanced biosynthesis of these hormones, nor with massive changes in defense-related gene expression. Instead, ISR-expressing plants are primed for enhanced defense. Priming is characterized by a faster and stronger expression of cellular defense responses that become activated only upon pathogen or insect attack, resulting in an enhanced level of resistance to the invader encountered. Recent advances in induced defense signaling research revealed regulators of ISR and suggest a model in which (JA)-related transcription factors play a central role in establishing the primed state.

A temporal analysis of antioxidative defense responses in the phloem of Picea abies after attack by Ips typographus

The temporal gradation of antioxidants was investigated on the phloem tissue of Norway spruce [Picea abies (L.) Karst.] in response to weather conditions and colonization levels of Ips typographus L. (Col., Scolytidae). Two weeks after pheromone dispensers were placed on trees, the initial reaction of Norway spruce to bark beetle attack resulted in moderately lowered levels of total glutathione (tGSH) and total cysteine. Likewise, the total ascorbic acid dropped slightly below the control levels, whereas the concentration of dehydroascorbic acid increased in comparison to the first sampling date. This transient degradation and oxidation of the glutathione and ascorbate system was accompanied by moderately increased concentrations of total phenolics. One month later, the shift in antioxidant balance after moderate attack differed quantitatively from the reaction after massive attack. An intensification of antioxidant defense occurred within moderately affected bark. Total cysteine and tGSH contents were markedly raised, whereas the concentrations of total ascorbic acid and total phenolics were slightly increased by moderate attack. On the other hand, massive bark beetle colonization caused a strong decrease in tGSH and total phenolics, whereas total cysteine and total ascorbic acid values remained at control level. Dependent upon the intensity and the success of the attack, a progressive degradation of antioxidants was determined at later sampling dates, which was accompanied by an obvious oxidation of the ascorbate and glutathione pools. With an unsuccessful defense upon massive attack, the thiols and total phenolics did not reach a new steady state, but deteriorated until the end of the brood beetles' development. In contrast, the dynamic antioxidative response within the moderately affected trees indicated an acclimation stage in the middle of July. It was characterized by a higher accumulation of tGSH, total ascorbic acid and total phenolics as well as a more reduced redox state of glutathione. A sequence of changes in the endogenous levels of antioxidant defense molecules in the bark beetle-affected Norway spruce showed consistency with the general ecophysiological stress-response concept, and provided important avenues for evaluating the role and effectiveness of antioxidants in systemic acquired resistance against the complex interactive effects of bark beetle attack and environmental factors.

Dissection of salicylic acid-mediated defense signaling networks

The small phenolic molecule salicylic acid (SA) plays a key role in plant defense. Significant progress has been made recently in understanding SA-mediated defense signaling networks. Functional analysis of a large number of genes involved in SA biosynthesis and regulation of SA accumulation and signal transduction has revealed distinct but interconnecting pathways that orchestrate the control of plant defense. Further studies utilizing combinatorial approaches in genetics, molecular biology, biochemistry and genomics will uncover finer details of SA-mediated defense networks as well as further insights into the crosstalk of SA with other defense signaling pathways. The complexity of defense networks illustrates the capacity of plants to integrate multiple developmental and environmental signals into a tight control of the costly defense responses.

Abscisic acid has a key role in modulating diverse plant-pathogen interactions

We isolated an activation-tagged Arabidopsis (Arabidopsis thaliana) line, constitutive disease susceptibility2-1D (cds2-1D), that showed enhanced bacterial growth when challenged with various Pseudomonas syringae strains. Systemic acquired resistance and systemic PATHOGENESIS-RELATED GENE1 induction were also compromised in cds2-1D. The T-DNA insertion adjacent to NINE-CIS-EPOXYCAROTENOID DIOXYGENASE5 (NCED5), one of six genes encoding the abscisic acid (ABA) biosynthetic enzyme NCED, caused a massive increase in transcript level and enhanced ABA levels >2-fold. Overexpression of NCED genes recreated the enhanced disease susceptibility phenotype. NCED2, NCED3, and NCED5 were induced, and ABA accumulated strongly following compatible P. syringae infection. The ABA biosynthetic mutant aba3-1 showed reduced susceptibility to virulent P. syringae, and ABA, whether through exogenous application or endogenous accumulation in response to mild water stress, resulted in increased bacterial growth following challenge with virulent P. syringae, indicating that ABA suppresses resistance to P. syringae. Likewise ABA accumulation also compromised resistance to the biotrophic oomycete Hyaloperonospora arabidopsis, whereas resistance to the fungus Alternaria brassicicola was enhanced in cds2-1D plants and compromised in aba3-1 plants, indicating that ABA promotes resistance to this necrotroph. Comparison of the accumulation of salicylic acid and jasmonic acid in the wild type, cds2-1D, and aba3-1 plants challenged with P. syringae showed that ABA promotes jasmonic acid accumulation and exhibits a complex antagonistic relationship with salicylic acid. Our findings provide genetic evidence that the abiotic stress signal ABA also has profound roles in modulating diverse plant-pathogen interactions mediated at least in part by cross talk with the jasmonic acid and salicylic acid biotic stress signal pathways.

Nuclear pore complex component MOS7/Nup88 is required for innate immunity and nuclear accumulation of defense regulators in Arabidopsis

Plant immune responses depend on dynamic signaling events across the nuclear envelope through nuclear pores. Nuclear accumulation of certain resistance (R) proteins and downstream signal transducers are critical for their functions, but it is not understood how these processes are controlled. Here, we report the identification, cloning, and analysis of Arabidopsis thaliana modifier of snc1,7 (mos7-1), a partial loss-of-function mutation that suppresses immune responses conditioned by the autoactivated R protein snc1 (for suppressor of npr1-1, constitutive 1). mos7-1 single mutant plants exhibit defects in basal and R protein-mediated immunity and in systemic acquired resistance but do not display obvious pleiotropic defects in development, salt tolerance, or plant hormone responses. MOS7 is homologous to human and Drosophila melanogaster nucleoporin Nup88 and resides at the nuclear envelope. In animals, Nup88 attenuates nuclear export of activated NF-kappaB transcription factors, resulting in nuclear accumulation of NF-kappaB. Our analysis shows that nuclear accumulation of snc1 and the defense signaling components Enhanced Disease Susceptibility 1 and Nonexpresser of PR genes 1 is significantly reduced in mos7-1 plants, while nuclear retention of other tested proteins is unaffected. The data suggest that specifically modulating the nuclear concentrations of certain defense proteins regulates defense outputs.

Direct induced resistance in Oryza sativa to Spodoptera frugiperda

Rice, Oryza sativa, is the most important staple food for a significant portion of the world's population. Despite the importance of rice, however, induced resistance to insects has not been thoroughly studied in rice; in fact, to our knowledge, direct induced resistance after injury by chewing insects has not been shown in rice. We conducted a series of experiments designed to characterize direct induced resistance in rice after feeding by larvae of the fall armyworm (Spodoptera frugiperda J. E. Smith) and application of jasmonic acid. Weight gains and relative growth rates of fall armyworm larvae were lower when fed leaves from plants previously damaged by armyworms than when fed leaves from undamaged plants. This response was stronger at a systemic spatial scale; that is, the induced resistance was stronger in newly emerged leaves not present at the time plants were damaged than in damaged leaves themselves. Armyworm growth rates were also reduced on foliage from plants treated with jasmonic acid, a hormone known to mediate plant responses to wounding. The response to injury by armyworm larvae and to exogenous jasmonic acid was stronger in transgenic rice plants in which levels of salicylic acid (a signaling molecule that inhibits jasmonic acid) were suppressed. These results show the existence of a direct induced resistance response in rice and suggest that this response to injury by a chewing insect may be mediated by jasmonic acid.

ETHYLENE INSENSITIVE3 and ETHYLENE INSENSITIVE3-LIKE1 repress SALICYLIC ACID INDUCTION DEFICIENT2 expression to negatively regulate plant innate immunity in Arabidopsis

Pathogen/microbe-associated molecular patterns (PAMPs/MAMPs) trigger plant immunity that forms the first line inducible defenses in plants. The regulatory mechanism of MAMP-triggered immunity, however, is poorly understood. Here, we show that Arabidopsis thaliana transcription factors ETHYLENE INSENSITIVE3 (EIN3) and ETHYLENE INSENSITIVE3-LIKE1 (EIL1), previously known to mediate ethylene signaling, also negatively regulate PAMP-triggered immunity. Plants lacking EIN3 and EIL1 display enhanced PAMP defenses and heightened resistance to Pseudomonas syringae bacteria. Conversely, plants overaccumulating EIN3 are compromised in PAMP defenses and exhibit enhanced disease susceptibility to Pseudomonas syringae. Microarray analysis revealed that EIN3 and EIL1 negatively control PAMP response genes. Further analyses indicated that SALICYLIC ACID INDUCTION DEFICIENT2 (SID2), which encodes isochorismate synthase required for pathogen-induced biosynthesis of salicylic acid (SA), is a key target of EIN3 and EIL1. Consistent with this, the ein3-1 eil1-1 double mutant constitutively accumulates SA in the absence of pathogen attack, and a mutation in SID2 restores normal susceptibility in the ein3 eil1 double mutant. EIN3 can specifically bind SID2 promoter sequence in vitro and in vivo. Taken together, our data provide evidence that EIN3/EIL1 directly target SID2 to downregulate PAMP defenses.

Salicylic acid-induced resistance to Fusarium oxysporum f. sp. lycopersici in tomato

We demonstrated that exogenous application of 200 microM salicylic acid through root feeding and foliar spray could induce resistance against Fusarium oxysporum f. sp. Lycopersici (Fol) in tomato. Endogenous accumulation of free salicylic acid in tomato roots was detected by HPLC and identification was confirmed by LC-MS/MS analysis. At 168h of salicylic acid treatment through roots, the endogenous salicylic acid level in the roots increased to 1477ngg(-1) FW which was 10 times higher than control plants. Similarly, the salicylic acid content was 1001ngg(-1) FW at 168h of treatment by foliar spray, which was 8.7 times higher than control plants. The activities of phenylalanine ammonia lyase (PAL, EC 4.3.1.5) and peroxidase (POD, EC 1.11.1.7) were 5.9 and 4.7 times higher, respectively than the control plants at 168h of salicylic acid feeding through the roots. The increase in PAL and POD activities was 3.7 and 3.3 times higher, respectively at 168h of salicylic acid treatments through foliar spray than control plants. The salicylic acid-treated tomato plants challenged with Fol exhibited significantly reduced vascular browning and leaf yellowing wilting. The mycelial growth of Fol was not significantly affected by salicylic acid. Significant increase in basal level of salicylic acid in noninoculated plants indicated that tomato root system might have the capacity to assimilate and distribute salicylic acid throughout the plant. The results indicated that the induced resistance observed in tomato against Fol might be a case of salicylic acid-dependent systemic acquired resistance.

Enhanced disease susceptibility 1 and salicylic acid act redundantly to regulate resistance gene-mediated signaling

Resistance (R) protein-associated pathways are well known to participate in defense against a variety of microbial pathogens. Salicylic acid (SA) and its associated proteinaceous signaling components, including enhanced disease susceptibility 1 (EDS1), non-race-specific disease resistance 1 (NDR1), phytoalexin deficient 4 (PAD4), senescence associated gene 101 (SAG101), and EDS5, have been identified as components of resistance derived from many R proteins. Here, we show that EDS1 and SA fulfill redundant functions in defense signaling mediated by R proteins, which were thought to function independent of EDS1 and/or SA. Simultaneous mutations in EDS1 and the SA-synthesizing enzyme SID2 compromised hypersensitive response and/or resistance mediated by R proteins that contain coiled coil domains at their N-terminal ends. Furthermore, the expression of R genes and the associated defense signaling induced in response to a reduction in the level of oleic acid were also suppressed by compromising SA biosynthesis in the eds1 mutant background. The functional redundancy with SA was specific to EDS1. Results presented here redefine our understanding of the roles of EDS1 and SA in plant defense.

Suppression of the rice fatty-acid desaturase gene OsSSI2 enhances resistance to blast and leaf blight diseases in rice

Fatty acids and their derivatives play important signaling roles in plant defense responses. It has been shown that suppressing a gene for stearoyl acyl carrier protein fatty-acid desaturase (SACPD) enhances the resistance of Arabidopsis (SSI2) and soybean to multiple pathogens. In this study, we present functional analyses of a rice homolog of SSI2 (OsSSI2) in disease resistance of rice plants. A transposon insertion mutation (Osssi2-Tos17) and RNAi-mediated knockdown of OsSSI2 (OsSSI2-kd) reduced the oleic acid (18:1) level and increased that of stearic acid (18:0), indicating that OsSSI2 is responsible for fatty-acid desaturase activity. These plants displayed spontaneous lesion formation in leaf blades, retarded growth, slight increase in endogenous free salicylic acid (SA) levels, and SA/benzothiadiazole (BTH)-specific inducible genes, including WRKY45, a key regulator of SA/BTH-induced resistance, in rice. Moreover, the OsSSI2-kd plants showed markedly enhanced resistance to the blast fungus Magnaporthe grisea and leaf-blight bacteria Xanthomonas oryzae pv. oryzae. These results suggest that OsSSI2 is involved in the negative regulation of defense responses in rice, as are its Arabidopsis and soybean counterparts. Microarray analyses identified 406 genes that were differentially expressed (>or=2-fold) in OsSSI2-kd rice plants compared with wild-type rice and, of these, approximately 39% were BTH responsive. Taken together, our results suggest that induction of SA-responsive genes, including WRKY45, is likely responsible for enhanced disease resistance in OsSSI2-kd rice plants.

Activation tagging of ADR2 conveys a spreading lesion phenotype and resistance to biotrophic pathogens

An Arabidopsis PR1::luciferase (LUC) transgenic line was transformed with activation T-DNA tags and the resulting population screened for dominant gain-of-function mutants exhibiting constitutive LUC activity. LUC imaging identified activated disease resistance 2 (adr2), which exhibited slowly spreading lesions in the absence of pathogen challenge. Molecular, genetic and histochemical analysis was employed to characterize this mutant in detail. adr2 plants constitutively expressed defence-related and antioxidant genes. Moreover, this line accrued increased quantities of salicylic acid (SA) and exhibited heightened mitogen-activated protein kinase activity. adr2 plants exhibited increased resistance against numerous biotrophic but not necrotrophic pathogens. The adr2 phenotype resulted from the overexpression of a Toll interleukin receptor (TIR) nucleotide binding site (NBS) leucine rich repeat (LRR) gene (At1g56510). Constitutive PR1 expression was completely abolished in adr2 nahG, adr2 npr1 and adr2 eds1 double mutants. Furthermore, heightened resistance against Hyaloperonospora arabidopsis Noco2 was compromised in adr2 nahG and adr2 eds1 double mutants but not in adr2 npr1, adr2 coi1 or adr2 etr1 plants. These data imply that adr2-mediated resistance operates through an Enhanced Disease Susceptibility (EDS) and SA-dependent defence signalling network which functions independently from COI1 or ETR1.

NPR1 in Plant Defense: It's Not over 'til It's Turned over

NPR1 is a key transcriptional coregulator in plant defense responses. In this issue, Spoel et al. (2009) demonstrate that proteasome-mediated degradation of NPR1 in the nucleus promotes efficient expression of defense response genes following infection and prevents spurious activation of defensive responses in the absence of infection.

Identification and expression analysis of WRKY transcription factor genes in canola (Brassica napus L.) in response to fungal pathogens and hormone treatments

BACKGROUND

Members of plant WRKY transcription factor families are widely implicated in defense responses and various other physiological processes. For canola (Brassica napus L.), no WRKY genes have been described in detail. Because of the economic importance of this crop, and its evolutionary relationship to Arabidopsis thaliana, we sought to characterize a subset of canola WRKY genes in the context of pathogen and hormone responses.

RESULTS

In this study, we identified 46 WRKY genes from canola by mining the expressed sequence tag (EST) database and cloned cDNA sequences of 38 BnWRKYs. A phylogenetic tree was constructed using the conserved WRKY domain amino acid sequences, which demonstrated that BnWRKYs can be divided into three major groups. We further compared BnWRKYs to the 72 WRKY genes from Arabidopsis and 91 WRKY from rice, and we identified 46 presumptive orthologs of AtWRKY genes. We examined the subcellular localization of four BnWRKY proteins using green fluorescent protein (GFP) and we observed the fluorescent green signals in the nucleus only.The responses of 16 selected BnWRKY genes to two fungal pathogens, Sclerotinia sclerotiorum and Alternaria brassicae, were analyzed by quantitative real time-PCR (qRT-PCR). Transcript abundance of 13 BnWRKY genes changed significantly following pathogen challenge: transcripts of 10 WRKYs increased in abundance, two WRKY transcripts decreased after infection, and one decreased at 12 h post-infection but increased later on (72 h). We also observed that transcript abundance of 13/16 BnWRKY genes was responsive to one or more hormones, including abscisic acid (ABA), and cytokinin (6-benzylaminopurine, BAP) and the defense signaling molecules jasmonic acid (JA), salicylic acid (SA), and ethylene (ET). We compared these transcript expression patterns to those previously described for presumptive orthologs of these genes in Arabidopsis and rice, and observed both similarities and differences in expression patterns.

CONCLUSION

We identified a set of 13 BnWRKY genes from among 16 BnWRKY genes assayed, that are responsive to both fungal pathogens and hormone treatments, suggesting shared signaling mechanisms for these responses. This study suggests that a large number of BnWRKY proteins are involved in the transcriptional regulation of defense-related genes in response to fungal pathogens and hormone stimuli.

Proteasome-mediated turnover of the transcription coactivator NPR1 plays dual roles in regulating plant immunity

Systemic acquired resistance (SAR) is a broad-spectrum plant immune response involving profound transcriptional changes that are regulated by the coactivator NPR1. Nuclear translocation of NPR1 is a critical regulatory step, but how the protein is regulated in the nucleus is unknown. Here, we show that turnover of nuclear NPR1 protein plays an important role in modulating transcription of its target genes. In the absence of pathogen challenge, NPR1 is continuously cleared from the nucleus by the proteasome, which restricts its coactivator activity to prevent untimely activation of SAR. Surprisingly, inducers of SAR promote NPR1 phosphorylation at residues Ser11/Ser15, and then facilitate its recruitment to a Cullin3-based ubiquitin ligase. Turnover of phosphorylated NPR1 is required for full induction of target genes and establishment of SAR. These in vivo data demonstrate dual roles for coactivator turnover in both preventing and stimulating gene transcription to regulate plant immunity.

Networking by small-molecule hormones in plant immunity

Plants live in complex environments in which they intimately interact with a broad range of microbial pathogens with different lifestyles and infection strategies. The evolutionary arms race between plants and their attackers provided plants with a highly sophisticated defense system that, like the animal innate immune system, recognizes pathogen molecules and responds by activating specific defenses that are directed against the invader. Recent advances in plant immunity research have provided exciting new insights into the underlying defense signaling network. Diverse small-molecule hormones play pivotal roles in the regulation of this network. Their signaling pathways cross-communicate in an antagonistic or synergistic manner, providing the plant with a powerful capacity to finely regulate its immune response. Pathogens, on the other hand, can manipulate the plant's defense signaling network for their own benefit by affecting phytohormone homeostasis to antagonize the host immune response.

Plant hormones are versatile chemical regulators of plant growth

The plant hormones are a structurally unrelated collection of small molecules derived from various essential metabolic pathways. These compounds are important regulators of plant growth and mediate responses to both biotic and abiotic stresses. During the last ten years there have been many exciting advances in our understanding of plant hormone biology, including new discoveries in the areas of hormone biosynthesis, transport, perception and response. Receptors for many of the major hormones have now been identified, providing new opportunities to study the chemical specificity of hormone signaling. These studies also reveal a surprisingly important role for the ubiquitin-proteasome pathway in hormone signaling. In addition, recent work confirms that hormone signaling interacts at multiple levels during plant growth and development. In the future, a major challenge will be to understand how the information conveyed by these simple compounds is integrated during plant growth.

Proteomics approach to identify unique xylem sap proteins in Pierce's disease-tolerant Vitis species

Pierce's disease (PD) is a destructive bacterial disease of grapes caused by Xylella fastidiosa which is xylem-confined. The tolerance level to this disease varies among Vitis species. Our research was aimed at identifying unique xylem sap proteins present in PD-tolerant Vitis species. The results showed wide variation in the xylem sap protein composition, where a set of polypeptides with pI between 4.5 and 4.7 and M(r) of 31 kDa were present in abundant amount in muscadine (Vitis rotundifolia, PD-tolerant), in reduced levels in Florida hybrid bunch (Vitis spp., PD-tolerant) and absent in bunch grapes (Vitis vinifera, PD-susceptible). Liquid chromatography/mass spectrometry/mass spectrometry analysis of these proteins revealed their similarity to beta-1, 3-glucanase, peroxidase, and a subunit of oxygen-evolving enhancer protein 1, which are known to play role in defense and oxygen generation. In addition, the amount of free amino acids and soluble sugars was found to be significantly lower in xylem sap of muscadine genotypes compared to V. vinifera genotypes, indicating that the higher nutritional value of bunch grape sap may be more suitable for Xylella growth. These data suggest that the presence of these unique proteins in xylem sap is vital for PD tolerance in muscadine and Florida hybrid bunch grapes.

Transgenic approaches for pre-harvest control of mycotoxin contamination in crop plants

Mycotoxins are fungal metabolites that can contaminate food and feed crops worldwide and are responsible for toxic effects in animals and humans that consume contaminated commodities. Regulatory guidelines and limits for mycotoxins have been set by the US Food and Drug Administration (FDA) and food safety agencies of other countries for both import and export of affected commodities. Mycotoxin contamination of foods and feeds can also cause serious economic hardships to producers, processors, and the consumer. Therefore, there has been a concerted effort by researchers worldwide to develop strategies for the effective control of mycotoxin contamination of crops, particularly at the pre-harvest stage. Strategies currently being utilised to combat pre-harvest mycotoxin contamination include: (1) use of non-toxigenic biocontrol strains; (2) improved agricultural practices; (3) application of agrochemicals; (4) plant breeding for resistance; and (5) genetic engineering of resistance genes into crop plants. This article highlights research on the genetic engineering of plants for resistance to invasion by mycotoxigenic fungi as well as detoxification of mycotoxins. Emphasis is placed on the most economically relevant fungi and the mycotoxins they produce. These include aflatoxins produced mainly by Aspergillus flavus and A. parasiticus, trichothecenes produced mainly by Fusarium graminearum, and to a lesser extent, fumonisins produced by F. verticillioides. Information is also presented on the use of genomics and proteomics technologies as a means of identifying genes and proteins that can be utilised in transgenic approaches to control the growth of mycotoxigenic fungi and the mycotoxins that they produce in food and feed crops.

Jasmonate- and salicylate-mediated plant defense responses to insect herbivores, pathogens and parasitic plants

While extensive research has examined plant defense responses to attack by herbivores and pathogens, plant responses to parasitism by other plants are not well characterized. The expression of induced plant defenses is mediated by complex signaling networks in which the plant hormones jasmonic acid (JA) and salicylic acid (SA) play key roles. In general, JA-mediated signaling pathways are implicated in the regulation of antiherbivore defenses, while the SA pathway is associated with defense responses against pathogens. However, there are many exceptions to this basic framework, and recent work suggests that interactions between the JA and SA pathways may play important roles in fine-tuning defense responses. The few studies that have addressed plant responses to parasitism by other plants suggest that both salicylates and jasmonates can mediate effective defenses. Here, a review is given of recent work on these defense signaling molecules with relevance for understanding their role in plant responses to parasitism.

Early genomic responses to salicylic acid in Arabidopsis

Salicylic acid (SA) is a stress-induced hormone involved in the activation of defense genes. Here we analyzed the early genetic responses to SA of wild type and npr1-1 mutant Arabidopsis seedlings, using Complete Arabidopsis Transcriptome MicroArray (CATMAv2) chip. We identified 217 genes rapidly induced by SA (early SAIGs); 193 by a NPR1-dependent and 24 by a NPR1-independent pathway. These two groups of genes also differed in their functional classification, expression profiles and over-representation of cis-elements, supporting differential pathways for their activation. Examination of the expression patterns for selected early SAIGs from both groups indicated that their activation by SA required TGA2/5/6 subclass of transcription factors. These genes were also activated by Pseudomonas syringae pv. tomato AvrRpm1, suggesting that they might play a role in defense against bacteria. This study gives a global idea of the early response to SA in Arabidopsis seedlings, expanding our knowledge about SA function in plant defense.

Phospholipase D activation is an early component of the salicylic acid signaling pathway in Arabidopsis cell suspensions

Salicylic acid (SA) plays a central role in defense against pathogen attack, as well as in germination, flowering, senescence, and the acquisition of thermotolerance. In this report we investigate the involvement of phospholipase D (PLD) in the SA signaling pathway. In presence of exogenous primary alcohols, the production of phosphatidic acid by PLD is diverted toward the formation of phosphatidylalcohols through a reaction called transphosphatidylation. By in vivo metabolic phospholipid labeling with (33)P(i), PLD activity was found to be induced 45 min after addition of SA. We show that incubation of Arabidopsis (Arabidopsis thaliana) cell suspensions with primary alcohols inhibited the induction of two SA-responsive genes, PATHOGENESIS-RELATED1 and WRKY38, in a dose-dependent manner. This inhibitory effect was more pronounced when the primary alcohols were more hydrophobic. Secondary or tertiary alcohols had no inhibitory effect. These results provide compelling arguments for PLD activity being upstream of the induction of these genes by SA. A subsequent study of n-butanol effects on the SA-responsive transcriptome identified 1,327 genes differentially expressed upon SA treatment. Strikingly, the SA response of 380 of these genes was inhibited by n-butanol but not by tert-butanol. A detailed analysis of the regulation of these genes showed that PLD could act both positively and negatively, either on gene induction or gene repression. The overlap with the previously described phosphatidylinositol-4-kinase pathway is discussed.

Analysis of temperature modulation of plant defense against biotrophic microbes

Plant-pathogen interactions are known to be affected by environmental factors including temperature; however, the temperature effects have not been systematically studied in plant disease resistance. Here, we characterized the effects of a moderate increase in temperature on resistance to bacterial pathogen Pseudomonas syringae and two viral elicitors in Arabidopsis thaliana and Nicotiana benthamiana. Both the basal and the resistance (R) gene-mediated defense responses to Pseudomonas syringae are found to be inhibited by a moderately high temperature, and hypersensitive responses induced by R genes against two viruses are also reduced by an increase of temperature. These indicate that temperature modulation of defense responses to biotrophic and hemibiotrophic pathogens might be a general phenomenon. We further investigated the roles of two small signaling molecules, salicylic acid and jasmonic acid, as well as two defense regulators, EDS1 and PAD4, in this temperature modulation. These components, though modulated by temperature or involved in temperature regulation or both, are not themselves determinants of temperature sensitivity in the defense responses analyzed. The inhibition of plant defense response by a moderately high temperature may thus be mediated by other defense signaling components or a combination of multiple factors.

Insights into the defense-related events occurring in plant cells following perception of surfactin-type lipopeptide from Bacillus subtilis

Multiple strains of Bacillus subtilis were demonstrated to stimulate plant defense responses, and cyclic lipopeptides may be involved in the elicitation of this induced systemic resistance phenomenon. Here, we further investigated molecular events underlying the interaction between such lipopeptides and plant cells. Addition of surfactin but not fengycin or iturin in the micromolar range to tobacco cell suspensions induced defense-related early events such as extracellular medium alkalinization coupled with ion fluxes and reactive oxygen species production. Surfactin also stimulated the defense enzymes phenylalanine ammonia lyase and lipoxygenase and modified the pattern of phenolics produced by the elicited cells. The occurrence of these surfactin-elicited early events is closely related to Ca(2+) influx and dynamic changes in protein phosphorylation but is not associated with any marked phytotoxicity or adverse effect on the integrity and growth potential of the treated tobacco cells. Reduced activity of some homologues also indicates that surfactin perception is dictated by structural clues in both the acyl moiety and cyclic peptide part. Our results suggest that these molecules could interact without irreversible pore formation but in a way sufficient to induce disturbance or transient channeling in the plasma membrane that can, in turn, activate a biochemical cascade of molecular events leading to defensive responses. The present study sheds new light not only on defense-related events induced following recognition of amphiphilic lipopeptides from Bacillus spp. but also more globally on the way elicitors from beneficial bacteria can be perceived by host plant cells.

GDSL lipase-like 1 regulates systemic resistance associated with ethylene signaling in Arabidopsis

Systemic resistance is induced by necrotizing pathogenic microbes and non-pathogenic rhizobacteria and confers protection against a broad range of pathogens. Here we show that Arabidopsis GDSL LIPASE-LIKE 1 (GLIP1) plays an important role in plant immunity, eliciting both local and systemic resistance in plants. GLIP1 functions independently of salicylic acid but requires ethylene signaling. Enhancement of GLIP1 expression in plants increases resistance to pathogens including Alternaria brassicicola, Erwinia carotovora and Pseudomonas syringae, and limits their growth at the infection site. Furthermore, local treatment with GLIP1 proteins is sufficient for the activation of systemic resistance, inducing both resistance gene expression and pathogen resistance in systemic leaves. The PDF1.2-inducing activity accumulates in petiole exudates in a GLIP1-dependent manner and is fractionated in the size range of less than 10 kDa as determined by size exclusion chromatography. Our results demonstrate that GLIP1-elicited systemic resistance is dependent on ethylene signaling and provide evidence that GLIP1 may mediate the production of a systemic signaling molecule(s).

Xanthomonas T3S Effector XopN Suppresses PAMP-Triggered Immunity and Interacts with a Tomato Atypical Receptor-Like Kinase and TFT1

XopN is a virulence factor from Xanthomonas campestris pathovar vesicatoria (Xcv) that is translocated into tomato (Solanum lycopersicum) leaf cells by the pathogen's type III secretion system. Xcv DeltaxopN mutants are impaired in growth and have reduced ability to elicit disease symptoms in susceptible tomato leaves. We show that XopN action in planta reduced pathogen-associated molecular pattern (PAMP)-induced gene expression and callose deposition in host tissue, indicating that XopN suppresses PAMP-triggered immune responses during Xcv infection. XopN is predicted to have irregular, alpha-helical repeats, suggesting multiple protein-protein interactions in planta. Consistent with this prediction, XopN interacted with the cytosolic domain of a Tomato Atypical Receptor-Like Kinase1 (TARK1) and four Tomato Fourteen-Three-Three isoforms (TFT1, TFT3, TFT5, and TFT6) in yeast. XopN/TARK1 and XopN/TFT1 interactions were confirmed in planta by bimolecular fluorescence complementation and pull-down analysis. Xcv DeltaxopN virulence defects were partially suppressed in transgenic tomato leaves with reduced TARK1 mRNA levels, indicating that TARK1 plays an important role in the outcome of Xcv-tomato interactions. These data provide the basis for a model in which XopN binds to TARK1 to interfere with TARK1-dependent signaling events triggered in response to Xcv infection.

A guide to the integrated application of on-line data mining tools for the inference of gene functions at the systems level

Genes function in networks to achieve a common biological response. Thus, inferences into the biological role of individual genes can be gained by analyzing their association with other genes with more precisely defined functions. Here, we present a guide, using the well-characterized Arabidopsis thaliana pathogenesis-related protein 2 gene (PR-2) as an example, to document how the sequential use of web-based tools can be applied to integrate information from different databases and associate the function of an individual gene with a network of genes and additionally identify specific biological processes in which they collectively function. The analysis begins by performing a global expression correlation analysis to build a functionally associated gene network. The network is subsequently analyzed for Gene Ontology enrichment, stimuli and mutant-specific transcriptional responses and enriched putative promoter regulatory elements that may be responsible for their correlated relationships. The results for the PR-2 gene are entirely consistent with the published literature documenting the accuracy of this type of analysis. Furthermore, this type of analysis can also be performed on other organisms with the appropriate data available and will greatly assist in understanding individual gene functions in a systems context.

Phloem sap of tomato plants contains a DIR1 putative ortholog

Arabidopsis thaliana defective in induced resistance 1 (At-DIR1) has been characterized as a protein responsible for the generation or transmission of the still unknown signal involved in systemic acquired resistance. This acidic apoplastic protein is a member of the family of lipid transfer proteins and was detected in vascular fluids. To our knowledge, no DIR1-like protein has been described in other plant species. Hence, we have performed data mining to identify a putative ortholog of DIR1 in tomato. This strategy allowed the detection of a few gene products displaying sequence similarity to At-DIR1 whose structural features were further analysed in silico. The best match (unigene SGN-327306) encoded a protein with an acidic pI, a peculiar characteristic of DIR1 among lipid transfer proteins, and was hence selected as a putative tomato ortholog of At-DIR1. This sequence, named Le-DIR1, served for the design of a specific antigenic peptide and the generation of polyclonal antibodies. The antiserum anti-Le-DIR1 recognized a peptide of the expected size (7kDa) in phloem sap of tomato plants, hence confirming the existence of the predicted protein in vascular fluids. This result supports the notion of the existence of common systemic acquired resistance (SAR) signaling molecules in different species.

Use of a synthetic salicylic acid analog to investigate the roles of methyl salicylate and its esterases in plant disease resistance

We previously demonstrated that salicylic acid-binding protein 2 (SABP2) of tobacco is an integral component of systemic acquired resistance (SAR). SABP2 is a methyl salicylate (MeSA) esterase that has high affinity for SA, which feedback inhibits its esterase activity. MeSA esterase activity is required in distal, healthy tissue of pathogen-infected plants to hydrolyze MeSA, which functions as a long-distance, phloem-mobile SAR signal; this hydrolysis releases the biologically active defense hormone SA. In this study, we examined the inhibitory interaction of SA with SABP2, and identified a synthetic SA analog, 2,2,2,2'-tetra-f luoroacetophenone (tetraFA) that, like SA, competitively inhibits the activity of SABP2 and targets esterases, which utilize MeSA as a substrate. However, in contrast to SA, tetraFA does not induce downstream defense responses and, therefore, is effective in planta at blocking SAR development in tobacco mosaic virus (TMV)-infected tobacco and Pseudomonas syringae-infected Arabidopsis. These results confirm the importance of SABP2 and MeSA for SAR development in tobacco and establish similar roles for MeSA and the orthologs of SABP2 in Arabidopsis. Moreover, they demonstrate that tetraFA can be used to determine whether MeSA and its corresponding esterase(s) play a role in SAR signaling in other plant species. In planta analyses using tetraFA, in conjunction with leaf detachment assays and MeSA quantification, were used to assess the kinetics with which MeSA is generated in pathogen-infected leaves, transmitted through the phloem, and processed in the distal healthy leaves. In TMV-infected tobacco, these studies revealed that critical amounts of MeSA are generated, transmitted, and processed between 48 and 72 h post primary infection.

E3 ubiquitin ligases and plant innate immunity

In yeast and in animals the ubiquitin-proteasome system (UPS) is responsible for removing or modifying most abnormal peptides and also short-lived cellular regulators. The UPS therefore influences many processes such as the cell cycle, signal transduction, transcription, and stress responses including defence. In recent years, similar regulatory roles have been identified in plants. In Arabidopsis, mutations in the ubiquitin-proteasome pathway block development, circadian rhythms, photomorphogenesis, floral homeosis, hormone responses, senescence, and pathogen invasion. Plants have evolved an armoury of defence mechanisms that allow them to counter infection. These encompass both basal responses, triggered by recognition of conserved pathogen-associated molecular patterns, and pathogen-specific responses, mediated via pathogen- and plant-specific gene-for-gene recognition events. The role of E3 ubiquitin ligases in mediating plant defence signalling is reviewed and examples where pathogens impinge on the host's ubiquitination machinery acting as molecular mimics to undermine defence are also highlighted.

Methyl salicylate production and jasmonate signaling are not essential for systemic acquired resistance in Arabidopsis

Systemic acquired resistance (SAR) develops in response to local microbial leaf inoculation and renders the whole plant more resistant to subsequent pathogen infection. Accumulation of salicylic acid (SA) in noninfected plant parts is required for SAR, and methyl salicylate (MeSA) and jasmonate (JA) are proposed to have critical roles during SAR long-distance signaling from inoculated to distant leaves. Here, we address the significance of MeSA and JA during SAR development in Arabidopsis thaliana. MeSA production increases in leaves inoculated with the SAR-inducing bacterial pathogen Pseudomonas syringae; however, most MeSA is emitted into the atmosphere, and only small amounts are retained. We show that in several Arabidopsis defense mutants, the abilities to produce MeSA and to establish SAR do not coincide. T-DNA insertion lines defective in expression of a pathogen-responsive SA methyltransferase gene are completely devoid of induced MeSA production but increase systemic SA levels and develop SAR upon local P. syringae inoculation. Therefore, MeSA is dispensable for SAR in Arabidopsis, and SA accumulation in distant leaves appears to occur by de novo synthesis via isochorismate synthase. We show that MeSA production induced by P. syringae depends on the JA pathway but that JA biosynthesis or downstream signaling is not required for SAR. In compatible interactions, MeSA production depends on the P. syringae virulence factor coronatine, suggesting that the phytopathogen uses coronatine-mediated volatilization of MeSA from leaves to attenuate the SA-based defense pathway.

Mitogen-activated protein kinases 3 and 6 are required for full priming of stress responses in Arabidopsis thaliana

In plants and animals, induced resistance (IR) to biotic and abiotic stress is associated with priming of cells for faster and stronger activation of defense responses. It has been hypothesized that cell priming involves accumulation of latent signaling components that are not used until challenge exposure to stress. However, the identity of such signaling components has remained elusive. Here, we show that during development of chemically induced resistance in Arabidopsis thaliana, priming is associated with accumulation of mRNA and inactive proteins of mitogen-activated protein kinases (MPKs), MPK3 and MPK6. Upon challenge exposure to biotic or abiotic stress, these two enzymes were more strongly activated in primed plants than in nonprimed plants. This elevated activation was linked to enhanced defense gene expression and development of IR. Strong elicitation of stress-induced MPK3 and MPK6 activity is also seen in the constitutive priming mutant edr1, while activity was attenuated in the priming-deficient npr1 mutant. Moreover, priming of defense gene expression and IR were lost or reduced in mpk3 or mpk6 mutants. Our findings argue that prestress deposition of the signaling components MPK3 and MPK6 is a critical step in priming plants for full induction of defense responses during IR.

Genome-wide analysis of differentially expressed genes during the early stages of tomato infection by a potyvirus

Plant responses against pathogens cause up- and downward shifts in gene expression. To identify differentially expressed genes in a plant-virus interaction, susceptible tomato plants were inoculated with the potyvirus Pepper yellow mosaic virus (PepYMV) and a subtractive library was constructed from inoculated leaves at 72 h after inoculation. Several genes were identified as upregulated, including genes involved in plant defense responses (e.g., pathogenesis-related protein 5), regulation of the cell cycle (e.g., cytokinin-repressed proteins), signal transduction (e.g., CAX-interacting protein 4, SNF1 kinase), transcriptional regulators (e.g., WRKY and SCARECROW transcription factors), stress response proteins (e.g., Hsp90, DNA-J, 20S proteasome alpha subunit B, translationally controlled tumor protein), ubiquitins (e.g., polyubiquitin, ubiquitin activating enzyme 2), among others. Downregulated genes were also identified, which likewise display identity with genes involved in several metabolic pathways. Differential expression of selected genes was validated by macroarray analysis and quantitative real-time polymerase chain reaction. The possible roles played by some of these genes in the viral infection cycle are discussed.

Temporal global expression data reveal known and novel salicylate-impacted processes and regulators mediating powdery mildew growth and reproduction on Arabidopsis

Salicylic acid (SA) is a critical mediator of plant innate immunity. It plays an important role in limiting the growth and reproduction of the virulent powdery mildew (PM) Golovinomyces orontii on Arabidopsis (Arabidopsis thaliana). To investigate this later phase of the PM interaction and the role played by SA, we performed replicated global expression profiling for wild-type and SA biosynthetic mutant isochorismate synthase1 (ics1) Arabidopsis from 0 to 7 d after infection. We found that ICS1-impacted genes constitute 3.8% of profiled genes, with known molecular markers of Arabidopsis defense ranked very highly by the multivariate empirical Bayes statistic (T(2) statistic). Functional analyses of T(2)-selected genes identified statistically significant PM-impacted processes, including photosynthesis, cell wall modification, and alkaloid metabolism, that are ICS1 independent. ICS1-impacted processes include redox, vacuolar transport/secretion, and signaling. Our data also support a role for ICS1 (SA) in iron and calcium homeostasis and identify components of SA cross talk with other phytohormones. Through our analysis, 39 novel PM-impacted transcriptional regulators were identified. Insertion mutants in one of these regulators, PUX2 (for plant ubiquitin regulatory X domain-containing protein 2), results in significantly reduced reproduction of the PM in a cell death-independent manner. Although little is known about PUX2, PUX1 acts as a negative regulator of Arabidopsis CDC48, an essential AAA-ATPase chaperone that mediates diverse cellular activities, including homotypic fusion of endoplasmic reticulum and Golgi membranes, endoplasmic reticulum-associated protein degradation, cell cycle progression, and apoptosis. Future work will elucidate the functional role of the novel regulator PUX2 in PM resistance.

Induced resistance triggered by Piriformospora indica

The root endophytic Basidiomycete Piriformospora indica forms a specific type of mycorrhiza symbiosis with a broad spectrum of plant species, including the Brassicaceae. A recent report on the interaction of P. indica with Arabidopsis thaliana suggests that the fungus induces a mode of resistance to microbial pathogens reminiscent of Induced Systemic Resistance (ISR) first discovered with non-pathogenic rhizobacteria. The characteristics of P. indica mediated resistance are the dependency on JA-signalling and the cytosolic function of the master regulator protein Non-expressor-of-PR-genes 1 (NPR1), a low level of altered systemic gene expression in leaves before pathogen challenge, the induction of the JA-inducible marker gene vegetative storage protein 1 (VSP1) after pathogen challenge, and an independency of the resistance phenotype from salicylate biosynthesis and signalling. We discuss here two more factors regarding the P. indica-mediated ISR response: the role of the plant hormone ethylene as well as a possible contribution of the recently discovered close association of P. indica with the α-proteobacterium Rhizobium radiobacter.

Role of the zinc-finger and basic motifs of chrysanthemum virus B p12 protein in nucleic acid binding, protein localization and induction of a hypersensitive response upon expression from a viral vector

The genomes of carlaviruses encode cysteine-rich proteins (CRPs) of unknown function. The 12 kDa CRP of chrysanthemum virus B (CVB), p12, has been shown previously to induce a hypersensitive response (HR) when expressed from potato virus X (PVX). This study demonstrated that a p12-induced HR was preceded by induction of a number of genes related to pathogenesis, stress and systemic acquired resistance. p12 localized predominantly to the nucleus. Interestingly, it was found that p12 bound both RNA and DNA in vitro, but notably exhibited a preference for DNA in the presence of Zn(2+) ions. Mutational analysis of the p12 conserved sequence motifs demonstrated that the basic motif is required for p12 translocation to the nucleus, thus representing part of the protein nuclear localization signal, whereas the predicted zinc finger motif is needed for both Zn(2+)-dependent DNA binding and eliciting an HR in PVX-infected leaves. Collectively, these results link, for the first time, nuclear localization of the protein encoded by a cytoplasmically replicating virus and its DNA-binding capacity with HR induction. Furthermore, these data suggest that p12 may mediate induction of the host genes by binding to the plant genomic DNA, and emphasize that CVB p12 is functionally distinct from other known nuclear-localized proteins encoded by the plant positive-stranded RNA viruses.

Activation of the salicylic acid signaling pathway enhances Clover yellow vein virus virulence in susceptible pea cultivars

The wild-type strain (Cl-WT) of Clover yellow vein virus (ClYVV) systemically induces cell death in pea cv. Plant introduction (PI) 118501 but not in PI 226564. A single incompletely dominant gene, Cyn1, controls systemic cell death in PI 118501. Here, we show that activation of the salicylic acid (SA) signaling pathway enhances ClYVV virulence in susceptible pea cultivars. The kinetics of virus accumulation was not significantly different between PI 118501 (Cyn1) and PI 226564 (cyn1); however, the SA-responsive chitinase gene (SA-CHI) and the hypersensitive response (HR)-related gene homologous to tobacco HSR203J were induced only in PI 118501 (Cyn1). Two mutant viruses with mutations in P1/HCPro, which is an RNA-silencing suppressor, reduced the ability to induce cell death and SA-CHI expression. The application of SA and of its analog benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH) partially complemented the reduced virulence of mutant viruses. These results suggest that high activation of the SA signaling pathway is required for ClYVV virulence. Interestingly, BTH could enhance Cl-WT symptoms in PI 226564 (cyn1). However, it could not enhance symptoms induced by White clover mosaic virus and Bean yellow mosaic virus. Our report suggests that the SA signaling pathway has opposing functions in compatible interactions, depending on the virus-host combination.