Systemic acquired resistance

A mutant CHS3 protein with TIR-NB-LRR-LIM domains modulates growth, cell death and freezing tolerance in a temperature-dependent manner in Arabidopsis

Low temperature is one of environmental factors that restrict plant growth homeostasis and plant-pathogen interactions. Recent studies suggest a link between temperature responses and defense responses; however, the underlying molecular mechanisms remain unclear. In this study, the chilling sensitive 3 (chs3-1) mutant in Arabidopsis was characterized. chs3-1 plants showed arrested growth and chlorosis when grown at 16 degrees C or when shifted from 22 to 4 degrees C. chs3-1 plants also exhibited constitutively activated defense responses at 16 degrees C, which were alleviated at a higher temperature (22 degrees C). Map-based cloning of CHS3 revealed that it encodes an unconventional disease resistance (R) protein belonging to the TIR-NB-LRR class with a zinc-binding LIM domain (Lin-11, Isl-1 and Mec-3 domains) at the carboxyl terminus. The chs3-1 mutation in the conserved LIM-containing domain led to the constitutive activation of the TIR-NB-LRR domain. Consistently, the growth and defense phenotypes of chs3-1 plants were completely suppressed by eds1, sgt1b and rar1, partially by pad4 and nahG, but not by npr1 and ndr1. Intriguingly, chs3-1 plants grown at 16 degrees C showed enhanced tolerance to freezing temperatures. This tolerance was correlated with growth defect and cell death phenotypes caused by activated defense responses. Other mutants with activated defense responses, including cpr1, cpr5 and slh1 also displayed enhanced freezing tolerance. These findings revealed a role of an unconventional mutant R gene in plant growth, defense response and cold stress, suggesting a mutual interaction between cold signaling and defense responses.

Involvement of salicylate and jasmonate signaling pathways in Arabidopsis interaction with Fusarium graminearum

Fusarium graminearum is the principal causative agent of Fusarium head blight (FHB), a devastating disease of wheat and barley. This fungus can also colonize Arabidopsis thaliana. Disease resistance was enhanced in transgenic wheat and Arabidopsis plants that constitutively overexpress the NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1) gene, which regulates salicylic acid (SA) signaling and modulates the activation of jasmonic acid (JA)-dependent defenses. Here, we provide several lines of evidence that reveal an important role for SA and JA signaling in Arabidopsis defense against F. graminearum. SA level was elevated in fungus-inoculated leaves, and SA application and biologically activated systemic acquired resistance enhanced resistance. Furthermore, the disruption of SA accumulation and signaling in the sid2 mutant and NahG transgenic plant, and the npr1 and wrky18 mutants, respectively, resulted in heightened susceptibility to this fungus in leaves and inflorescence. JA signaling was activated in parallel with SA signaling in the fungus-challenged plants. However, the hyperresistance of the JA pathway mutants opr3, coi1, and jar1 indicates that this pathway contributes to susceptibility. Genetic and biochemical experiments indicate that the JA pathway promotes disease by attenuating the activation of SA signaling in fungus-inoculated plants. However, the hypersusceptibility of the jar1 npr1 double mutant compared with the npr1 mutant suggests that JAR1 also contributes to defense, signifying a dichotomous role of JA and a JAR1-dependent mechanism in this interaction.

Resistance loci affecting distinct stages of fungal pathogenesis: use of introgression lines for QTL mapping and characterization in the maize--Setosphaeria turcica pathosystem

BACKGROUND

Studies on host-pathogen interactions in a range of pathosystems have revealed an array of mechanisms by which plants reduce the efficiency of pathogenesis. While R-gene mediated resistance confers highly effective defense responses against pathogen invasion, quantitative resistance is associated with intermediate levels of resistance that reduces disease progress. To test the hypothesis that specific loci affect distinct stages of fungal pathogenesis, a set of maize introgression lines was used for mapping and characterization of quantitative trait loci (QTL) conditioning resistance to Setosphaeria turcica, the causal agent of northern leaf blight (NLB). To better understand the nature of quantitative resistance, the identified QTL were further tested for three secondary hypotheses: (1) that disease QTL differ by host developmental stage; (2) that their performance changes across environments; and (3) that they condition broad-spectrum resistance.

RESULTS

Among a set of 82 introgression lines, seven lines were confirmed as more resistant or susceptible than B73. Two NLB QTL were validated in BC4F2 segregating populations and advanced introgression lines. These loci, designated qNLB1.02 and qNLB1.06, were investigated in detail by comparing the introgression lines with B73 for a series of macroscopic and microscopic disease components targeting different stages of NLB development. Repeated greenhouse and field trials revealed that qNLB1.06(Tx303) (the Tx303 allele at bin 1.06) reduces the efficiency of fungal penetration, while qNLB1.02(B73) (the B73 allele at bin 1.02) enhances the accumulation of callose and phenolics surrounding infection sites, reduces hyphal growth into the vascular bundle and impairs the subsequent necrotrophic colonization in the leaves. The QTL were equally effective in both juvenile and adult plants; qNLB1.06(Tx303) showed greater effectiveness in the field than in the greenhouse. In addition to NLB resistance, qNLB1.02(B73) was associated with resistance to Stewart's wilt and common rust, while qNLB1.06(Tx303) conferred resistance to Stewart's wilt. The non-specific resistance may be attributed to pleiotropy or linkage.

CONCLUSIONS

Our research has led to successful identification of two reliably-expressed QTL that can potentially be utilized to protect maize from S. turcica in different environments. This approach to identifying and dissecting quantitative resistance in plants will facilitate the application of quantitative resistance in crop protection.

Abscisic acid interacts antagonistically with salicylic acid signaling pathway in rice-Magnaporthe grisea interaction

Plant hormones play pivotal signaling roles in plant-pathogen interactions. Here, we report characterization of an antagonistic interaction of abscisic acid (ABA) with salicylic acid (SA) signaling pathways in the rice-Magnaporthe grisea interaction. Exogenous application of ABA drastically compromised the rice resistance to both compatible and incompatible M. grisea strains, indicating that ABA negatively regulates both basal and resistance gene-mediated blast resistance. ABA markedly suppressed the transcriptional upregulation of WRKY45 and OsNPR1, the two key components of the SA signaling pathway in rice, induced by SA or benzothiadiazole or by blast infection. Overexpression of OsNPR1 or WRKY45 largely negated the enhancement of blast susceptibility by ABA, suggesting that ABA acts upstream of WRKY45 and OsNPR1 in the rice SA pathway. ABA-responsive genes were induced during blast infection in a pattern reciprocal to those of WRKY45 and OsPR1b in the compatible rice-blast interaction but only marginally in the incompatible one. These results suggest that the balance of SA and ABA signaling is an important determinant for the outcome of the rice-M. grisea interaction. ABA was detected in hyphae and conidia of M. grisea as well as in culture media, implying that blast-fungus-derived ABA could play a role in triggering ABA signaling at host infection sites.

Metabolic profiling reveals local and systemic responses of host plants to nematode parasitism

The plant parasitic beet cyst nematode Heterodera schachtii induces syncytial feeding structures in Arabidopsis roots. The feeding structures form strong sink tissues that have been suggested to be metabolically highly active. In the present study, metabolic profiling and gene targeted expression analyses were performed in order to study the local and systemic effects of nematode infection on the plant host. The results showed increased levels of many amino acids and phosphorylated metabolites in syncytia, as well as high accumulation of specific sugars such as 1-kestose that do not accumulate naturally in Arabidopsis roots. A correlation-based network analysis revealed highly activated and coordinated metabolism in syncytia compared to non-infected control roots. An integrated analysis of the central primary metabolism showed a clear coherence of metabolite and transcript levels, indicating transcriptional regulation of specific pathways. Furthermore, systemic effects of nematode infection were demonstrated by correlation-based network analysis as well as independent component analysis. 1-kestose, raffinose, alpha,alpha-trehalose and three non-identified analytes showed clear systemic accumulation, indicating future potential for diagnostic and detailed metabolic analyses. Our studies open the door towards understanding the complex remodelling of plant metabolism in favour of the parasitizing nematode.

Dual-function transcription factors and their entourage: unique and unifying themes governing two pathogenesis-related genes

Much of what we, as plant molecular biologists studying gene regulation, know comes from paradigms characterized or developed in mammalian systems. Although plants, animals, and fungi have been diverging for a very long time, a great deal of the machineries and components discovered in yeast and mammals seem to have been maintained in plants. Nevertheless, despite this apparent conservation, evolutionary pressures on the mechanisms of gene regulation are likely to be different between these kingdoms, given their different environmental constraints. As such, it is imperative for plant molecular biologists to develop their own paradigms, even on seemingly conserved systems. It is with this intent that we compare and contrast the regulation of two pathogenesis-related genes, the arabidopsis PR-1 and potato PR-10a genes. The transcription factors regulating these genes present prime paradigms for the study of plant signal- and context-dependent dual-function transcription factors.

Intersections between immune responses and morphological regulation in plants

Successful plant pathogens have developed strategies to interfere with the defence mechanisms of their host plants through evolution. Conversely, host plants have evolved systems to counteract pathogen attack. Some pathogens induce pathogenic symptoms on plants that include morphological changes in addition to interference with plant growth. Recent studies, based on molecular biology and genetics using Arabidopsis thaliana, have revealed that factors derived from pathogens can modulate host systems and/or host factors that play important roles in the morphological regulation of host plants. Other reports, meanwhile, have shown that factors known to have roles in plant morphology also function in plant immune responses. Evolutionary conservation of these factors and systems implies that host-pathogen interactions and the evolution they drive have yielded tight links between morphological processes and immune responses. In this review, recent findings about these topics are introduced and discussed.

Proteomic analysis of salicylate-induced proteins of pea (Pisum sativum L.) leaves

The effect of 50 microM salicylic acid on soluble proteins of pea (Pisum sativum L.) leaves was studied by proteomic analysis. Thirty-two salicylate-induced proteins were found, and 13 of these were identified using MALDI TOF MS. Salicylate-induced increased content was shown for the first time for the family 18 glycoside hydrolase, alpha-amylase, 33 kDa protein of photosystem II, lipid-desaturase-like protein, and glutamine amidotransferase. Increased content of protective proteins of direct antipathogenic action such as chitinase and beta-1,3-glucanases was also noted.

DL-beta-aminobutyric acid-induced resistance of potato against Phytophthora infestans requires salicylic acid but not oxylipins

Inducing systemic resistance responses in crop plants is a promising alternative way of disease management. To understand the underlying signaling events leading to induced resistance, functional analyses of plants defective in defined signaling pathway steps are required. We used potato, one of the economically most-important crop plants worldwide, to examine systemic resistance against the devastating late blight pathogen Phytophthora infestans, induced by treatment with dl-beta-aminobutyric acid (BABA). Transgenic plants impaired in either the 9-lipoxygenase pathway, which produces defense-related compounds, or the 13-lipoxygenase pathway, which generates jasmonic acid-derived signals, expressed wild-type levels of BABA-induced resistance. Plants incapable of accumulating salicylic acid (SA), on the other hand, failed to mount this type of induced resistance. Consistently, treatment of these plants with the SA analog 2,6-dichloroisonicotinic acid restored BABA-induced resistance. Together, these results demonstrate the indispensability of a functional SA pathway for systemic resistance in potato induced by BABA.

An endoplasmic reticulum response pathway mediates programmed cell death of root tip induced by water stress in Arabidopsis

Drought induces root death in plants; however, the nature and characteristics of root cell death and its underlying mechanisms are poorly understood. Here, we provide a systematic analysis of cell death in the primary root tips in Arabidopsis during water stress. Root tip cell death occurs when high water deficit is reached. The dying cells were first detected in the apical meristem of the primary roots and underwent active programmed cell death (PCD). Transmission electron microscopic analysis shows that the cells undergoing induced death had unambiguous morphological features of autophagic cell death, including an increase in vacuole size, degradation of organelles, and collapse of the tonoplast and the plasma membrane. The results suggest that autophagic PCD occurs as a response to severe water deficit. Significant accumulation of reactive oxygen species (ROS) was detected in the stressed root tips. Expression of BAX inhibitor-1 (AtBI1) was increased in response to water stress, and atbi1-1 displayed accelerated cell death, indicating that AtBI1 and the endoplasmic reticulum (ER) stress response pathway both modulate water stress-induced PCD. These findings form the basis for further investigations into the mechanisms underlying the PCD and its role in developmental plasticity of root system architecture and subsequent adaptation to water stress.

CRYPTOCHROME 1 is implicated in promoting R protein-mediated plant resistance to Pseudomonas syringae in Arabidopsis

Plants have evolved complex mechanisms to defend themselves against pathogens. It has been shown that several defense responses are influenced by light, and the red/far-red light photoreceptor phytochromes (PHY) modulate plant defense responses in Arabidopsis. Blue light receptor cryptochromes (CRY) work together with PHY to regulate many light-controlled responses, including photomorphogenesis, floral induction, and entrainment of the circadian clock. We report here that the Arabidopsis blue light photoreceptor CRY1 positively regulates inducible resistance to Pseudomonas syringae under continuous light conditions. By challenging plants with P. syringae pv. tomato (Pst.) DC3000 carrying avrRpt2, we demonstrate that effector-triggered local resistance is down-regulated in the cry1 mutant, leading to more pathogen multiplication. In plants overexpressing CRY1 (CRY1-ovx), however, local resistance is significantly up-regulated. We also show that systemic acquired resistance (SAR) is positively regulated by CRY1, and that salicylic acid (SA)-induced pathogenesis-related gene PR-1 expression is reduced in the cry1 mutant, but enhanced in CRY1-ovx plants. However, our results indicate that CRY1 only modestly influences SA accumulation and has no effect on hypersensitive cell death. These results suggest that CRY1 may positively regulate R protein-mediated resistance to P. syringae with increased PR gene expression.

Identification of genes differentially expressed in cauliflower associated with resistance to Xanthomonas campestris pv. campestris

Black rot, caused by Xanthomonas campestris pv. campestris (Pammel) Dowson (Xcc), is one of the most damaging diseases of cauliflower and other crucifers. In order to investigate the molecular resistance mechanisms and to find the genes related to black rot resistance in cauliflower, a suppression subtractive hybridization (SSH) cDNA library was constructed using resistant line C712 and its susceptible near-isogenic line C731 as tester and driver, respectively. A total of 280 clones were obtained from the library by reverse northern blotting. Sequencing analysis and homology searching showed that these clones represent 202 unique sequences. The library included many defense/disease-resistant related genes, such as plant defensin gene PDF1.2, lipid transfer protein, thioredoxin h. Gene expression profiles of 12 genes corresponding to different functional categories were monitored by real-time RT-PCR. The results showed that the expression induction of these genes in the susceptible line C712 in response to Xcc was quicker and more intense, while in C731 the reaction was delayed and limited. Our results imply that these up-regulated genes might be involved in cauliflower responses against Xcc infection. Information obtained from this study could be used to understand the molecular mechanisms of disease response in cauliflower under Xcc stress.

Genetic transformation of cotton with a harpin-encoding gene hpaXoo confers an enhanced defense response against different pathogens through a priming mechanism

BACKGROUND

The soil-borne fungal pathogen Verticillium dahliae Kleb causes Verticillium wilt in a wide range of crops including cotton (Gossypium hirsutum). To date, most upland cotton varieties are susceptible to V. dahliae and the breeding for cotton varieties with the resistance to Verticillium wilt has not been successful.

RESULTS

Hpa1Xoo is a harpin protein from Xanthomonas oryzae pv. oryzae which induces the hypersensitive cell death in plants. When hpa1Xoo was transformed into the susceptible cotton line Z35 through Agrobacterium-mediated transformation, the transgenic cotton line (T-34) with an improved resistance to Verticillium dahliae was obtained. Cells of the transgenic T-34, when mixed with the conidia suspension of V. dahliae, had a higher tolerance to V. dahliae compared to cells of untransformed Z35. Cells of T-34 were more viable 12 h after mixing with V. dahliae conidia suspension. Immunocytological analysis showed that Hpa1Xoo, expressed in T-34, accumulated as clustered particles along the cell walls of T-34. In response to the infection caused by V. dahliae, the microscopic cell death and the generation of reactive oxygen intermediates were observed in leaves of T-34 and these responses were absent in leaves of Z35 inoculated with V. dahliae. Quantitative RT-PCR analysis indicated that five defense-related genes, ghAOX1, hin1, npr1, ghdhg-OMT, and hsr203J, were up-regulated in T-34 inoculated with V. dahliae. The up-regulations of these defense-relate genes were not observed or in a less extent in leaves of Z-35 after the inoculation.

CONCLUSIONS

Hpa1Xoo accumulates along the cell walls of the transgenic T-34, where it triggers the generation of H2O2 as an endogenous elicitor. T-34 is thus in a primed state, ready to protect the host from the pathogen. The results of this study suggest that the transformation of cotton with hpa1Xoo could be an effective approach for the development of cotton varieties with the improved resistance against soil-borne pathogens.

Transcriptional responses to polycyclic aromatic hydrocarbon-induced stress in Arabidopsis thaliana reveal the involvement of hormone and defense signaling pathways

BACKGROUND

Polycyclic aromatic hydrocarbons (PAHs) are toxic, widely-distributed, environmentally persistent, and carcinogenic byproducts of carbon-based fuel combustion. Previously, plant studies have shown that PAHs induce oxidative stress, reduce growth, and cause leaf deformation as well as tissue necrosis. To understand the transcriptional changes that occur during these processes, we performed microarray experiments on Arabidopsis thaliana L. under phenanthrene treatment, and compared the results to published Arabidopsis microarray data representing a variety of stress and hormone treatments. In addition, to probe hormonal aspects of PAH stress, we assayed transgenic ethylene-inducible reporter plants as well as ethylene pathway mutants under phenanthrene treatment.

RESULTS

Microarray results revealed numerous perturbations in signaling and metabolic pathways that regulate reactive oxygen species (ROS) and responses related to pathogen defense. A number of glutathione S-transferases that may tag xenobiotics for transport to the vacuole were upregulated. Comparative microarray analyses indicated that the phenanthrene response was closely related to other ROS conditions, including pathogen defense conditions. The ethylene-inducible transgenic reporters were activated by phenanthrene. Mutant experiments showed that PAH inhibits growth through an ethylene-independent pathway, as PAH-treated ethylene-insensitive etr1-4 mutants exhibited a greater growth reduction than WT. Further, phenanthrene-treated, constitutive ethylene signaling mutants had longer roots than the untreated control plants, indicating that the PAH inhibits parts of the ethylene signaling pathway.

CONCLUSIONS

This study identified major physiological systems that participate in the PAH-induced stress response in Arabidopsis. At the transcriptional level, the results identify specific gene targets that will be valuable in finding lead compounds and engineering increased tolerance. Collectively, the results open a number of new avenues for researching and improving plant resilience and PAH phytoremediation.

ROS in biotic interactions

Production of reactive oxygen species (ROS) is a hallmark of successful recognition of infection and activation of plant defenses. ROS play multifaceted signaling functions mediating the establishment of multiple responses and can act as local toxins. Controversy surrounds the origin of these ROS. Several enzymatic mechanisms, among them a plasma membrane NADPH oxidase and cell wall peroxidases, can be responsible for the ROS detected in the apoplast. However, high levels of ROS from metabolic origins and/or from downregulation of ROS-scavenging systems can also accumulate in different compartments of the plant cell. This compartmentalization could contribute to the specific functions attributed to ROS. Additionally, ROS interact with other signals and phytohormones, which could explain the variety of different scenarios where ROS signaling plays an important part. Interestingly, pathogens have developed ways to alter ROS accumulation or signaling to modify plant defenses. Although ROS have been mainly associated with pathogen attack, ROS are also detected in other biotic interactions including beneficial symbiotic interactions with bacteria or mycorrhiza, suggesting that ROS production is a common feature of different biotic interactions. Here, we present a comprehensive review describing the newer views in ROS signaling and function during biotic stress.

Functional analysis of a tomato salicylic acid methyl transferase and its role in synthesis of the flavor volatile methyl salicylate

Methyl salicylate (MeSA) is a volatile plant secondary metabolite that is an important contributor to taste and scent of many fruits and flowers. It is synthesized from salicylic acid (SA), a phytohormone that contributes to plant pathogen defense. MeSA is synthesized by members of a family of O-methyltransferases. In order to elaborate the mechanism of MeSA synthesis in tomato, we screened a set of O-methyltransferases for activity against multiple substrates. An enzyme that specifically catalyzes methylation of SA, SlSAMT, as well as enzymes that act upon jasmonic acid and indole-3-acetic acid were identified. Analyses of transgenic over- and under-producing lines validated the function of SlSAMT in vivo. The SlSAMT gene was mapped to a position near the bottom of chromosome 9. Analysis of MeSA emissions from an introgression population derived from a cross with Solanum pennellii revealed a quantitative trait locus (QTL) linked to higher fruit methyl salicylate emissions. The higher MeSA emissions associate with significantly higher SpSAMT expression, consistent with SAMT gene expression being rate limiting for ripening-associated MeSA emissions. Transgenic plants that constitutively over-produce MeSA exhibited only slightly delayed symptom development following infection with the disease-causing bacterial pathogen, Xanthomonas campestris pv. vesicatoria (Xcv). Unexpectedly, pathogen-challenged leaves accumulated significantly higher levels of SA as well as glycosylated forms of SA and MeSA, indicating a disruption in control of the SA-related metabolite pool. Taken together, the results indicate that SlSAMT is critical for methyl salicylate synthesis and methyl salicylate, in turn, likely has an important role in controlling SA synthesis.

Salicylic acid is involved in the Nb-mediated defense responses to Potato virus X in Solanum tuberosum

To evaluate the role of salicylic acid (SA) in Nb-mediated hypersensitive resistance to Potato virus X (PVX) avirulent strain ROTH1 in Solanum tuberosum, we have constructed SA-deficient transgenic potato plant lines by overexpressing the bacterial enzyme salicylate hydroxylase (NahG), which degrades SA. Evaluation of these transgenic lines revealed hydrogen peroxide accumulation and spontaneous lesion formation in an age- and light-dependent manner. In concordance, NahG potato plants were more sensitive to treatment with methyl viologen, a reactive oxygen species-generating compound. In addition, when challenged with PVX ROTH1, NahG transgenic lines showed a decreased disease-resistance response to infection and were unable to induce systemic acquired resistance. However, the avirulent viral effector, the PVX 25-kDa protein, does induce expression of the pathogenesis-related gene PR-1a in NahG potato plants. Taken together, our data indicate that SA is involved in local and systemic defense responses mediated by the Nb gene in Solanum tuberosum. This is the first report to show that basal levels of SA correlate with hypersensitive resistance to PVX.

Nucleoporin MOS7/Nup88 contributes to plant immunity and nuclear accumulation of defense regulators

Controlled nucleocytoplasmic trafficking is an important feature for fine-tuning signaling pathways in eukaryotic organisms. Nuclear pore complexes (NPCs) composed of nucleoporin proteins (Nups) are essential for the exchange of macromolecules across the nuclear envelope. A recent genetic screen in our laboratory identified a partial loss-of-function mutation in Arabidopsis MOS7/Nup88 that causes defects in basal immunity, Resistance (R) protein-mediated defense and systemic acquired resistance. In Drosophila and mammalian cells, exportin-mediated nuclear export of activated Rel/NFκB transcription factors is enhanced in nup88 mutants resulting in immune response failure. Consistent with Nup88 promoting nuclear retention of NFκB, our functional analyses revealed that MOS7/Nup88 is required for appropriate nuclear accumulation of the autoactivated R protein snc1, as well as the key immune regulators EDS1 and NPR1. These results suggest that controlling the nuclear concentrations of specific immune regulators is fundamental for defining defense outputs.

Heterologous expression and characterization of recombinant OsCDR1, a rice aspartic proteinase involved in disease resistance

The Oryza sativa constitutive disease resistance 1 (OsCDR1) gene product is an aspartic proteinase that has been implicated in disease resistance signaling. This apoplastic enzyme is a member of the group of 'atypical' plant aspartic proteinases. Recombinant OsCDR1 expressed in Escherichia coli exhibited protease activity against succinylated-casein substrate. Inactivating the enzyme through modification of an aspartate residue present in the deduced active site completely abolished its proteinase activity. Infiltration of the OsCDR1 fusion protein into leaves of Arabidopsis plants induced PR2 transcripts in both the infiltrated leaf (primary) and in non-treated secondary leaves while the inactive recombinant protein failed to induce either local or systemic PR2. These findings demonstrate that OsCDR1 is capable of inducing systemic defense responses in plants.

Agrobacterium induces expression of a host F-box protein required for tumorigenicity

Agrobacterium exports DNA into plant cells, eliciting neoplastic growths on many plant species. During this process, a Skp1-Cdc53-cullin-F-box (SCF) complex that contains the bacterial virulence F-box protein VirF facilitates genetic transformation by targeting for proteolysis proteins, the Agrobacterium protein VirE2 and the host protein VIP1, that coat the transferred DNA. However, some plant species do not require VirF for transformation. Here, we show that Agrobacterium induces expression of a plant F-box protein, which we designated VBF for VIP1-binding F-box protein, that can functionally replace VirF, regulating levels of the VirE2 and VIP1 proteins via a VBF-containing SCF complex. When expressed in Agrobacterium and exported into the plant cell, VBF functionally complements tumor formation by a strain lacking VirF. VBF expression is known to be induced by diverse pathogens, suggesting that Agrobacterium has co-opted a plant defense response and that bacterial VirF and plant VBF both contribute to targeted proteolysis that promotes plant genetic transformation.

Identification of Fe-excess-induced genes in rice shoots reveals a WRKY transcription factor responsive to Fe, drought and senescence

Fe participates in several important reactions in plant metabolism. However, Fe homeostasis in plants is not completely understood, and molecular studies on Fe-excess stress are scarce. Rice (Oryza sativa L. ssp. indica) is largely cultivated in submerged conditions, where the extremely reductive environment can lead to severe Fe overload. In this work, we used representational difference analysis (RDA) to isolate sequences up-regulated in rice shoots after exposure to Fe-excess. We isolated 24 sequences which have putative functions in distinct cellular processes, such as transcription regulation (OsWRKY80), stress response (OsGAP1, DEAD-BOX RNA helicase), proteolysis (oryzain-α, rhomboid protein), photosynthesis (chlorophyll a/b binding protein), sugar metabolism (β glucosidase) and electron transport (NADH ubiquinone oxireductase). We show that the putative WRKY transcription factor OsWRKY80 is up-regulated in rice leaves, stems and roots after Fe-excess treatment. This up-regulation is also observed after dark-induced senescence and drought stress, indicating that OsWRKY80 could be a general stress-responsive gene. To our knowledge, this is the first report of an Fe-excess-induced transcription factor in plants.

Herbaspirillum seropedicae rfbB and rfbC genes are required for maize colonization

In this study we disrupted two Herbaspirillum seropedicae genes, rfbB and rfbC, responsible for rhamnose biosynthesis and its incoporation into LPS. GC-MS analysis of the H. seropedicae wild-type strain LPS oligosaccharide chain showed that rhamnose, glucose and N-acetyl glucosamine are the predominant monosaccharides, whereas rhamnose and N-acetyl glucosamine were not found in the rfbB and rfbC strains. The electrophoretic pattern of the mutants LPS was drastically altered when compared with the wild type. Knockout of rfbB or rfbC increased the sensitivity towards SDS, polymyxin B sulfate and salicylic acid. The mutants attachment capacity to maize root surface plantlets was 100-fold lower than the wild type. Interestingly, the wild-type capacity to attach to maize roots was reduced to a level similar to that of the mutants when the assay was performed in the presence of isolated wild-type LPS, glucosamine or N-acetyl glucosamine. The mutant strains were also significantly less efficient in endophytic colonization of maize. Expression analysis indicated that the rfbB gene is upregulated by naringenin, apigenin and CaCl(2). Together, the results suggest that intact LPS is required for H. seropedicae attachment to maize root and internal colonization of plant tissues.

Arabidopsis auxin mutants are compromised in systemic acquired resistance and exhibit aberrant accumulation of various indolic compounds

Systemic acquired resistance is a widespread phenomenon in the plant kingdom that confers heightened and often enduring immunity to a range of diverse pathogens. Systemic immunity develops through activation of plant disease resistance protein signaling networks following local infection with an incompatible pathogen. The accumulation of the phytohormone salicylic acid in systemically responding tissues occurs within days after a local immunizing infection and is essential for systemic resistance. However, our knowledge of the signaling components underpinning signal perception and the establishment of systemic immunity are rudimentary. Previously, we showed that an early and transient increase in jasmonic acid in distal responding tissues was central to effective establishment of systemic immunity. Based upon predicted transcriptional networks induced in naive Arabidopsis (Arabidopsis thaliana) leaves following avirulent Pseudomonas syringae challenge, we show that a variety of auxin mutants compromise the establishment of systemic immunity. Linking together transcriptional and targeted metabolite studies, our data provide compelling evidence for a role of indole-derived compounds, but not auxin itself, in the establishment and maintenance of systemic immunity.

Ascorbic acid deficiency in arabidopsis induces constitutive priming that is dependent on hydrogen peroxide, salicylic acid, and the NPR1 gene

The ascorbic acid (AA)-deficient Arabidopsis thaliana vtc1-1 mutant exhibits increased resistance to the virulent bacterial pathogen Pseudomonas syringae. This response correlates with heightened levels of salicylic acid (SA), which induces antimicrobial pathogenesis-related (PR) proteins. To determine if SA-mediated, enhanced disease resistance is a general phenomenon of AA deficiency, to elucidate the signal that stimulates SA synthesis, and to identify the biosynthetic pathway through which SA accumulates, we studied the four AA-deficient vtc1-1, vtc2-1, vtc3-1, and vtc4-1 mutants. We also studied double mutants defective in the AA-biosynthetic gene VTC1 and the SA signaling pathway genes PAD4, EDS5, and NPR1, respectively. All vtc mutants were more resistant to P. syringae than the wild type. With the exception of vtc4-1, this correlated with constitutively upregulated H(2)O(2), SA, and messenger RNA levels of PR genes. Double mutants exhibited decreased SA levels and enhanced susceptibility to P. syringae compared with the wild type, suggesting that vtc1-1 requires functional PAD4, EDS5, and NPR1 for SA biosynthesis and pathogen resistance. We suggest that AA deficiency causes constitutive priming through a buildup of H(2)O(2) that stimulates SA accumulation, conferring enhanced disease resistance in vtc1-1, vtc2-1, and vtc3-1, whereas vtc4-1 might be sensitized to H(2)O(2) and SA production after infection.

Induced resistance to pests and pathogens in trees

Tree resistance can be enhanced by a variety of biotic and abiotic inducers, including nonpathogenic and pathogenic microbes, and herbivores, resulting in enhanced protection against further biotic injury. Induced resistance (IR) could be a valuable tool in sustainable pest management. IR has been actively studied in herbaceous plant species, and, in recent years, in woody plant species, and is fast emerging as an intriguing, eco-friendly concept for enhancing tree resistance. However, before application of IR becomes possible, there is a need to increase our knowledge of the mechanisms of defence in forest trees. A richer understanding of these phenomena will play a critical role in developing sustainable integrated pest management strategies. This review summarizes our current knowledge of IR in forest trees, focusing on inducible defence mechanisms, systemic induction of resistance and phytohormone signalling networks. We conclude by discussing the potential advantages and limitations of applying IR-based management tools in forest systems.

Molecular and histochemical characterisation of two distinct poplar Melampsora leaf rust pathosystems

In this study, we compared interactions of two Melampsora foliar rust species with poplar, which resulted in either limited or abundant pathogen proliferation. In the pathosystem exhibiting limited pathogen growth, a defence response was observed after invasion of poplar leaf tissues by the biotroph, with late and clear production of reactive oxygen species (ROS) and other products. Characterisation of the histological, biochemical and transcriptional events occurring in both pathosystems showed striking similarity with components of plant defence reactions observed during qualitative resistance. Key components associated with development of an active defence response, such as up-regulation of pathogenesis-related (PR) genes, were observed during infection. Moreover, the time course and strength of gene induction appear to be critical determinants for the outcome of the tree-pathogen interaction. This work provides basic biochemical characterisation and expression data for the study of so-called partial resistance in the poplar-rust pathosystem, which is also applicable to other plant-pathogen interactions resulting in quantitative disease resistance.

Functional analysis of a TGA factor-binding site located in the promoter region controlling salicylic acid-induced NIMIN-1 expression in Arabidopsis

TGA factors play a key role in plant defense by binding to the promoter region of defense genes, inducing expression. Salicylic acid (SA) induces the expression of the gene encoding NIMIN-1, which interacts with NPR1/NIM1, a key regulator of systemic acquired resistance. We investigated whether the TGA2-binding motif TGACG located upstream of the NIMIN-1 gene is necessary for SA induction of NIMIN-1 expression. A mutated version of the NIMIN-1 promoter was created by site-directed mutagenesis. We generated T-DNA constructs in which native NIMIN-1 and mutated promoters were fused to green fluorescent protein and beta-glucuronidase reporters. We produced transgenic Arabidopsis plants and observed NIMIN-1 promoter-driven green fluorescent protein expression in the roots, petiole and leaves. Constructs were agroinfiltrated into the leaves for transient quantitative assays of gene expression. Using quantitative real-time RT-PCR, we characterized the normal gene response to SA and compared it to the response of the mutant version of the NIMIN-1 promoter. Both the native NIMIN-1 construct and an endogenous copy of NIMIN-1 were induced by SA. However, the mutated promoter construct was much less sensitive to SA than the native NIMIN-1 promoter, indicating that this TGA2-binding motif is directly involved in the modulation of SA-induced NIMIN-1 expression in Arabidopsis.

RFC3 regulates cell proliferation and pathogen resistance in Arabidopsis

Replication factor C subunit 3 (RFC3) is one of the small subunits of the RFC complex originally purified from the HeLa cells that is essential for the in vitro replication of Simian virus 40 (SV40). Although RFC has been reported to be involved in DNA replication, DNA repair and check-point control of cell cycle progression in yeast, little is known about the precise function of each subunit of the RFC in plants. We recently reported the identification of rfc3-1, which carries a point mutation leading to plants with enhanced expression of Pathogenesis-Related (PR) genes and resistance against the virulent oomycete Hyaloperonospora arabidopsidis (H.a.) Noco2. The mutant is hypersensitive to SA and has enhanced pathogen resistance independent of Nonexpressor of PR genes 1 (NPR1). The rfc3-1 mutation caused a substitution from a nonpolar aliphatic amino acid (Gly-84) to a negatively charged amino acid (Asp) in functional domain III, which is one of eight conserved domains in the RFC. This may interfere with the interaction between RFC3 and other subunits, compromising the function of the protein complex, and leading to cell proliferation defects in the leaves and roots of Arabidopsis. Furthermore, enhanced expression of PR genes and induction of systemic acquired resistance in rfc3-1 may be caused by a partial loss of RFC function through its involvement in replication-coupled chromatin assembling.

Parasitism by Cuscuta pentagona sequentially induces JA and SA defence pathways in tomato

While plant responses to herbivores and pathogens are well characterized, responses to attack by other plants remain largely unexplored. We measured phytohormones and C(18) fatty acids in tomato attacked by the parasitic plant Cuscuta pentagona, and used transgenic and mutant plants to explore the roles of the defence-related phytohormones salicylic acid (SA) and jasmonic acid (JA). Parasite attachment to 10-day-old tomato plants elicited few biochemical changes, but a second attachment 10 d later elicited a 60-fold increase in JA, a 30-fold increase in SA and a hypersensitive-like response (HLR). Host age also influenced the response: neither Cuscuta seedlings nor established vines elicited a HLR in 10-day-old hosts, but both did in 20-day-old hosts. Parasites grew larger on hosts deficient in SA (NahG) or insensitive to JA [jasmonic acid-insensitive1 (jai1)], suggesting that both phytohormones mediate effective defences. Moreover, amounts of JA peaked 12 h before SA, indicating that defences may be coordinated via sequential induction of these hormones. Parasitism also induced increases in free linolenic and linoleic acids and abscisic acid. These findings provide the first documentation of plant hormonal signalling induced by a parasitic plant and show that tomato responses to C. pentagona display characteristics similar to both herbivore- and pathogen-induced responses.

Molecular characterization of a defense-related AMP-binding protein gene, OsBIABP1, from rice

We cloned and characterized a rice gene OsBIABP1 encoding an AMP-binding protein. The full-length cDNA of OsBIABP1 is 1912-bp long and is predicted to encode a 558-aa protein. OsBIABP1 contains a typical AMP-binding signature motif and shows high similarity to members of AMP-binding protein family. OsBIABP1 is expressed in stems, leaves and flowers of rice plants, but is not expressed, or expressed at a very low level, in rice roots. The expression of OsBIABP1 was induced by some defense-related signal molecules, e.g., salicylic acid (SA), benzothiadiazole, jasmonic acid (JA), and 1-amino cyclopropane-1-carboxylic acid, which mediate SA- and JA/ethylene (ET)-dependent defense signaling pathways, respectively. Furthermore, the expression of OsBIABP1 is activated by the infection of Magnaporthe oryzae, and the induced expression is quicker and stronger during early stages of pathogenesis in incompatible interaction than that in compatible interaction between rice and M. oryzae. Our results suggest that OsBIABP1 may be a defense-related AMP-binding protein that is involved in the regulation of defense response through SA and/or JA/ET signaling pathways.

Identification of a novel NPR1-like gene from Nicotiana glutinosa and its role in resistance to fungal, bacterial and viral pathogens

The NPR1 or NPR1-like genes play a pivotal role in systemic acquired resistance in plants. Here, we isolated and identified a novel tobacco (Nicotiana glutinosa) NPR1-like gene (designated as NgNPR3). The full-length cDNA is 2049 bp in length with a 1767 bp open reading frame which encodes a 588 amino acids protein with an estimated molecular mass of 66 kDa and a calculated pI of 7.14. Homology analysis suggested that the NgNPR3 protein shares significant similarity to AtNPR3 of Arabidopsis. Transient expression assay of NgNPR3-GFP fusion gene in onion epidermal cells revealed that the NgNPR3 protein was localized to the cytoplasm and moved into the nucleus after redox change. RT-PCR results indicated that NgNPR3 was up-regulated after treatment with SA, INA, H(2)O(2,) and MeJA, which play important roles in various resistance responses in tobacco. Transcriptional level of NgNPR3 was also up-regulated after inoculation with Rhizoctonia solani, Phytophthora parasitica, Alternaria alternata, Pseudomonas solanacearum, and potato virus Y (PVY), respectively. When NgNPR3 was overexpressed in N. tabacum cv. Samsun plants, the transgenic plants showed enhanced resistance to the pathogens A. alternate, P. solanacearum and PVY. Furthermore, NgNPR3-mediated disease resistance is dosage-dependent. Our results suggest that NgNPR3 could be a putative NPR1-like gene, and might play an important role in resistance to a broad range of pathogens in tobacco.

Arabidopsis thaliana class-II TGA transcription factors are essential activators of jasmonic acid/ethylene-induced defense responses

The three closely related Arabidopsis basic leucine zipper (bZIP) transcription factors TGA2, TGA5 and TGA6 are required for the establishment of the salicylic acid (SA)-dependent plant defense response systemic acquired resistance, which is effective against biotrophic pathogens. Here we show that the same transcription factors are essential for the activation of jasmonic acid (JA)- and ethylene (ET)-dependent defense mechanisms that counteract necrotrophic pathogens: the tga256 triple mutant is impaired in JA/ET-induced PDF1.2 and b-CHI expression, which correlates with a higher susceptibility against the necrotroph Botrytis cinerea. JA/ET induction of the trans-activators ERF1 and ORA59, which act upstream of PDF1.2, was slightly increased (ERF1) or unaffected (ORA59). PDF1.2 expression can be restored in the tga256 mutant by increased expression of ORA59, as observed in the tga256 jin1 quadruple mutant, which lacks the transcription factor JIN1/AtMYC2 that functions as a negative regulator of the JA/ET-dependent anti-fungal defense program. Whereas JA/ET-induced PDF1.2 expression is strongly suppressed by SA in wild-type plants, no negative effect of SA on PDF1.2 expression was observed in the tga256 jin1 quadruple mutant. These results imply that the antagonistic effects of TGA factors and JIN1/AtMYC2 on the JA/ET pathway are necessary to evoke the SA-mediated suppression of JA/ET-induced defense responses.

Inoculation of tomato plants (Solanum lycopersicum) with growth-promoting Bacillus subtilis retards whitefly Bemisia tabaci development

Root inoculation of tomato (Solanum lycopersicum) plants with a Bacillus subtilis strain BEB-DN (BsDN) isolated from the rhizosphere of cultivated potato plants was able to promote growth and to generate an induced systemic resistance (ISR) response against virus-free Bemisia tabaci. Growth promotion was evident 3 weeks after inoculation. No changes in oviposition density, preference and nymphal number in the early stages of B. tabaci development were observed between BsDN-treated plants and control plants inoculated with a non-growth promoting Bs strain (PY-79), growth medium or water. However, a long-term ISR response was manifested by a significantly reduced number of B. tabaci pupae developing into adults in BsDN-treated plants. The observed resistance response appeared to be a combination of jasmonic acid (JA) dependent and JA-independent responses, since the BsDN-related retardation effect on B. tabaci development was still effective in the highly susceptible spr2 tomato mutants with an impaired capacity for JA biosynthesis. A screening of 244 genes, 169 of which were previously obtained from subtractive-suppressive-hybridization libraries generated from B. tabaci-infested plants suggested that the BsDN JA-dependent ISR depended on an anti-nutritive effect produced by the simultaneous expression of genes coding principally for proteases and proteinase inhibitors, whereas the JA-independent ISR observed in the spr2 background curiously involved the up-regulation of several photosynthetic genes, key components of the phenyl-propanoid and terpenoid biosynthetic pathways and of the Hsp90 chaperonin, which probably mediated pest resistance response(s), in addition to the down-regulation of pathogenesis and hypersensitive response genes.

Systemic acquired resistance is induced by R gene-mediated responses independent of cell death

On infection by pathogens, plants initiate defence responses that are able to curtail infection locally. These responses are mediated either by receptor-like proteins that recognize pathogen-associated molecular patterns or by the protein products of disease resistance (R) genes. At the same time, primary defence responses often result in the generation of signals that induce what is known as systemic acquired resistance (SAR), such that defence responses are enhanced on secondary pathogen challenge in distal tissues. R protein-mediated SAR induction is normally accompanied by a type of programmed cell death known as the hypersensitive response (HR) and, in some instances, cell death alone has been implicated in the induction of SAR. This has raised the question of whether R protein-mediated signalling per se induces SAR or whether SAR is an indirect result of the induction of HR. Using the Rx gene of potato, which confers resistance to Potato Virus X in the absence of cell death, we have shown that the HR is dispensable for R protein-mediated induction of SAR and that Rx-induced SAR is mediated by the same salicylate-dependent pathway induced by other R proteins.

Induced systemic resistance and plant responses to fungal biocontrol agents

Biocontrol fungi (BCF) are agents that control plant diseases. These include the well-known Trichoderma spp. and the recently described Sebacinales spp. They have the ability to control numerous foliar, root, and fruit pathogens and even invertebrates such as nematodes. However, this is only a subset of their abilities. We now know that they also have the ability to ameliorate a wide range of abiotic stresses, and some of them can also alleviate physiological stresses such as seed aging. They can also enhance nutrient uptake in plants and can substantially increase nitrogen use efficiency in crops. These abilities may be more important to agriculture than disease control. Some strains also have abilities to improve photosynthetic efficiency and probably respiratory activities of plants. All of these capabilities are a consequence of their abilities to reprogram plant gene expression, probably through activation of a limited number of general plant pathways.

Siroheme: an essential component for life on earth

Life on earth is dependent on sulphur (S) and nitrogen (N). In plants, the second step in the reduction of sulphate and nitrate are mediated by the enzymes sulphite and nitrite reductases, which contain the iron (Fe)-containing siroheme as a cofactor. It is synthesized from the tetrapyrrole primogenitor uroporphyrinogen III in the plastids via three enzymatic reactions, methylation, oxidation and ferrochelatation. Without siroheme biosynthesis, there would be no life on earth. Limitations in siroheme should have an enormous effect on the S- and N-metabolism, plant growth, development, fitness and reproduction, biotic and abiotic stresses including growth under S, N and Fe limitations, and the response to pathogens and beneficial interaction partners. Furthermore, the vast majority of redox-reactions in plants depend on S-components, and S-containing compounds are also involved in the detoxification of heavy metals and other chemical toxins. Disturbance of siroheme biosynthesis may cause the accumulation of light-sensitive intermediates and reactive oxygen species, which are harmful, or they can function as signaling molecules and participate in interorganellar signaling processes. This review highlights the role of siroheme in these scenarios.

Overexpression of rice (Oryza sativa L.) OsCDR1 leads to constitutive activation of defense responses in rice and Arabidopsis

Plant aspartic proteases (AP) play key roles in the regulation of biological processes, such as the recognition of pathogens and pests and the induction of effective defense responses. A large number of AP (>400) have been identified in silico in the rice genome. None have previously been isolated and functionally characterized for their involvement in disease resistance. We describe here the isolation and characterization of a gene (OsCDR1) from rice which encodes a predicted aspartate protease. Expression of OsCDR1 was activated upon treatments with benzothiadiazole and salicylic acid, which are signal molecules in plant disease resistance responses. Ectopic expression of OsCDR1 in Arabidopsis and rice conferred enhanced resistance against bacterial and fungal pathogens. The enhanced disease resistance observed in transgenic plants was correlated with induction of pathogenesis-related gene expression and was shown by mutational analysis to be dependent on AP activity of the transgene-encoded product. OsCDR1 accumulates in intercellular fluids (IF) in transgenic plants. Infiltration of IF from transgenic Arabidopsis plants into leaves of wild-type (WT) Arabidopsis induced the systemic defense response. These results demonstrate the conservation of CDR1 function between rice and Arabidopsis during the disease resistance response.

Study of the effects of salicylic acid on soybean mitochondrial lipids and respiratory properties using the alternative oxidase as a stress-reporter protein

Biotic and abiotic stresses can lead to modifications in the lipid composition of cell membranes. Although mitochondria appear to be implicated in stress responses, little is known about the membrane lipid changes that occur in these organelles in plants. Besides cytochrome c oxidase, plant mitochondria have an alternative oxidase (AOX) that accepts electrons directly from ubiquinol, dissipating energy as heat. AOX upregulation occurs under a variety of stresses and its induction by salicylic acid (SA) has been observed in different plant species. AOX was also suggested to be used as a functional marker for cell reprogramming under stress. In the present study, we have used etiolated soybean (Glycine max (L.) Merr. cv Cresir) seedlings to study the effects of SA treatment on the lipid composition and the respiratory properties of hypocotyl mitochondria. AOX expression was studied in detail, as a reporter protein, to evaluate whether modifications in mitochondrial energy metabolism were occurring. In mitochondria extracted from SA-treated seedlings, AOX capacity and protein contents increased. Both AOX1 and AOX2b transcripts accumulated in response to SA, but with different kinetics. A reduction in external NADH oxidation capacity was observed, whereas succinate respiration remained unchanged. The phospholipid composition of mitochondria remained similar in control and SA-treated plants, but a reduction in the relative amount of linolenic acid was observed in phosphatidylcholine, phosphatidylethanolamine and cardiolipin. The possible causes of the fatty acid modifications observed, and the implications for mitochondrial metabolism are discussed.

Satellite panicum mosaic virus coat protein enhances the performance of plant virus gene vectors

The coat protein of satellite panicum mosaic virus (SPCP) is known to effectively protect its cognate RNA from deleterious events, and here, we tested its stabilizing potential for heterologous virus-based gene vectors in planta. In support of this, a Potato virus X (PVX) vector carrying the SPMV capsid protein (PVX-SPCP) gene was stable for at least three serial systemic passages through Nicotiana benthamiana. To test the effect of SPCP in trans, PVX-SPCP was co-inoculated onto N. benthamiana together with a Tomato bushy stunt virus (TBSV) vector carrying a green fluorescent protein (GFP) gene that normally does not support systemic GFP expression. In contrast, co-inoculation of TBSV-GFP plus PVX-SPCP resulted in GFP accumulation and concomitant green fluorescent spots in upper, non-inoculated leaves in a temperature-responsive manner. These results suggest that the multifaceted SPMV CP has intriguing effects on virus-host interactions that surface in heterologous systems.

Arabidopsis 14-3-3 lambda is a positive regulator of RPW8-mediated disease resistance

The RPW8 locus from Arabidopsis thaliana Ms-0 includes two functional paralogous genes (RPW8.1 and RPW8.2) and confers broad-spectrum resistance via the salicylic acid-dependent signaling pathway to the biotrophic fungal pathogens Golovinomyces spp. that cause powdery mildew diseases on multiple plant species. To identify proteins involved in regulation of the RPW8 protein function, a yeast two-hybrid screen was performed using RPW8.2 as bait. The 14-3-3 isoform lambda (designated GF14lambda) was identified as a potential RPW8.2 interactor. The RPW8.2-GF14lambda interaction was specific and engaged the C-terminal domain of RPW8.2, which was confirmed by pulldown assays. The physiological impact of the interaction was revealed by knocking down GF14lambda by T-DNA insertion, which compromised basal and RPW8-mediated resistance to powdery mildew. In addition, over-expression of GF14lambda resulted in hypersensitive response-like cell death and enhanced resistance to powdery mildew via the salicylic acid-dependent signaling pathway. The results from this study suggest that GF14lambda may positively regulate the RPW8.2 resistance function and play a role in enhancing basal resistance in Arabidopsis.

(Homo)glutathione depletion modulates host gene expression during the symbiotic interaction between Medicago truncatula and Sinorhizobium meliloti

Under nitrogen-limiting conditions, legumes interact with symbiotic rhizobia to produce nitrogen-fixing root nodules. We have previously shown that glutathione and homoglutathione [(h)GSH] deficiencies impaired Medicago truncatula symbiosis efficiency, showing the importance of the low M(r) thiols during the nodulation process in the model legume M. truncatula. In this study, the plant transcriptomic response to Sinorhizobium meliloti infection under (h)GSH depletion was investigated using cDNA-amplified fragment length polymorphism analysis. Among 6,149 expression tags monitored, 181 genes displayed significant differential expression between inoculated control and inoculated (h)GSH depleted roots. Quantitative reverse transcription polymerase chain reaction analysis confirmed the changes in mRNA levels. This transcriptomic analysis shows a down-regulation of genes involved in meristem formation and a modulation of the expression of stress-related genes in (h)GSH-depleted plants. Promoter-beta-glucuronidase histochemical analysis showed that the putative MtPIP2 aquaporin might be up-regulated during nodule meristem formation and that this up-regulation is inhibited under (h)GSH depletion. (h)GSH depletion enhances the expression of salicylic acid (SA)-regulated genes after S. meliloti infection and the expression of SA-regulated genes after exogenous SA treatment. Modification of water transport and SA signaling pathway observed under (h)GSH deficiency contribute to explain how (h)GSH depletion alters the proper development of the symbiotic interaction.