Salicylic acid is a systemic signal and an inducer of pathogenesis-related proteins in virus-infected tobacco

Interconnection between flowering time control and activation of systemic acquired resistance

The ability to avoid or neutralize pathogens is inherent to all higher organisms including plants. Plants recognize pathogens through receptors, and mount resistance against the intruders, with the help of well-elaborated defense arsenal. In response to some localinfections, plants develop systemic acquired resistance (SAR), which provides heightened resistance during subsequent infections. Infected tissues generate mobile signaling molecules that travel to the systemic tissues, where they epigenetically modify expression o a set of genes to initiate the manifestation of SAR in distant tissues. Immune responses are largely regulated at transcriptional level. Flowering is a developmental transition that occurs as a result of the coordinated action of large numbers of transcription factors that respond to intrinsic signals and environmental conditions. The plant hormone salicylic acid (SA) which is required for SAR activation positively regulates flowering. Certain components of chromatin remodeling complexes that are recruited for suppression of precocious flowering are also involved in suppression of SAR in healthy plants. FLOWERING LOCUS D, a putative histone demethylase positively regulates SAR manifestation and flowering transition in Arabidopsis. Similarly, incorporation of histone variant H2A.Z in nucleosomes mediated by PHOTOPERIOD-INDEPENDENT EARLY FLOWERING 1, an ortholog of yeast chromatin remodeling complex SWR1, concomitantly influences SAR and flowering time. SUMO conjugation and deconjugation mechanisms also similarly affect SAR and flowering in an SA-dependent manner. The evidences suggest a common underlying regulatory mechanism for activation of SAR and flowering in plants.

Different mechanisms for Arabidopsis thaliana hybrid necrosis cases inferred from temperature responses

Temperature is a major determinant of plant growth, development and success. Understanding how plants respond to temperature is particularly relevant in a warming climate. Plant immune responses are often suppressed above species-specific critical temperatures. This is also true for intraspecific hybrids of Arabidopsis thaliana that express hybrid necrosis due to inappropriate activation of the immune system caused by epistatic interactions between alleles from different genomes. The relationship between temperature and defence is unclear, largely due to a lack of studies that assess immune activation over a wide range of temperatures. To test whether the temperature-based suppression of ectopic immune activation in hybrids exhibits a linear or non-linear relationship, we characterised the molecular and morphological phenotypes of two different necrotic A. thaliana hybrids over a range of ecologically relevant temperatures. We found both linear and non-linear responses for expression of immunity markers and for morphological defects depending on the underlying genetic cause. This suggests that the influence of temperature on the trade-off between immunity and growth depends on the specific defence components involved.

Community profiling of culturable fluorescent pseudomonads in the rhizosphere of green gram (Vigna radiata L.)

Study on microbial diversity in the unexplored rhizosphere is important to understand their community structure, biology and ecological interaction with the host plant. This research assessed the genetic and functional diversity of fluorescent pseudomonads [FP] in the green gram rhizophere. One hundred and twenty types of morphologically distinct fluorescent pseudomonads were isolated during vegetative as well as reproductive growth phase of green gram. Rep PCR, ARDRA and RISA revealed two distinct clusters in each case at 75, 61 and 70% similarity coefficient index respectively. 16S rRNA partial sequencing analysis of 85 distantly related fluorescent pseudomonads depicted Pseudomonas aeruginosa as the dominant group. Out of 120 isolates, 23 (19%) showed antagonistic activity towards phytopathogenic fungi. These bacterial isolates showed varied production of salicylic acid, HCN and chitinase, 2, 4-diacetylphloroglucinol (DAPG), phenazine-1-carboxylic acid (PCA) and pyoluteorin (PLT). Production efficiency of inherent level of plant growth promoting (PGP) traits among the 120 isolates demonstrated that 10 (8%) solubilised inorganic phosphates, 25 (20%) produced indoles and 5 (4%) retained ACC deaminase activity. Pseudomonas aeruginosa GGRJ21 showed the highest production of all antagonistic and plant growth promoting (PGP) traits. In a greenhouse experiment, GGRJ21 suppressed root rot disease of green gram by 28-93% (p = 0.05). Consistent up regulation of three important stress responsive genes, i.e., acdS, KatA and gbsA and elevated production efficiency of different PGP traits could promote GGRJ21 as a potent plant growth regulator.

The Arabidopsis thaliana At4g13040 gene, a unique member of the AP2/EREBP family, is a positive regulator for salicylic acid accumulation and basal defense against bacterial pathogens

The Arabidopsis genome contains a large number of putative transcription factors, containing a DNA binding domain similar to APETALA2/ethylene response element binding protein (AP2/EREBP), for most of which a function is not known. Phylogenetic analysis divides the Apetala 2 (AP2) super-family into 5 major groups: AP2, RAV, ethylene response factor (ERF), dehydration response element binding protein (DREB) and At4g13040. Similar to ERF and DREB, the At4g13040 protein contains only one AP2 domain; however, its structural uniqueness places it into a distinct group. In this article, we report that At4g13040 (referred herein as Apetala 2 family protein involved in SA mediated disease defense 1 - APD1) is an important regulator for SA mediated plant defense. The APD1 gene is upregulated upon pathogen inoculation, exogenous SA application and in the mutant that constitutively activates SA signaling. The T-DNA insertion lines (inserted in the APD1 promoter), which fail to induce expression upon pathogen inoculation, are compromised for resistance against virulent bacterial pathogens and show reduced induction of pathogenesis related 1 gene. Our results suggest that APD1 functions downstream of PAD4 in Arabidopsis and promotes pathogen-induced SA accumulation. Exogenous SA application completely restores the loss-of-resistance phenotype of the apd1 mutant. Thus, APD1 is a positive regulator of disease defense that functions upstream of SA accumulation.

Salicylic acid and jasmonic acid are essential for systemic resistance against tobacco mosaic virus in Nicotiana benthamiana

Systemic resistance is induced by pathogens and confers protection against a broad range of pathogens. Recent studies have indicated that salicylic acid (SA) derivative methyl salicylate (MeSA) serves as a long-distance phloem-mobile systemic resistance signal in tobacco, Arabidopsis, and potato. However, other experiments indicate that jasmonic acid (JA) is a critical mobile signal. Here, we present evidence suggesting both MeSA and methyl jasmonate (MeJA) are essential for systemic resistance against Tobacco mosaic virus (TMV), possibly acting as the initiating signals for systemic resistance. Foliar application of JA followed by SA triggered the strongest systemic resistance against TMV. Furthermore, we use a virus-induced gene-silencing-based genetics approach to investigate the function of JA and SA biosynthesis or signaling genes in systemic response against TMV infection. Silencing of SA or JA biosynthetic and signaling genes in Nicotiana benthamiana plants increased susceptibility to TMV. Genetic experiments also proved the irreplaceable roles of MeSA and MeJA in systemic resistance response. Systemic resistance was compromised when SA methyl transferase or JA carboxyl methyltransferase, which are required for MeSA and MeJA formation, respectively, were silenced. Moreover, high-performance liquid chromatography-mass spectrometry analysis indicated that JA and MeJA accumulated in phloem exudates of leaves at early stages and SA and MeSA accumulated at later stages, after TMV infection. Our data also indicated that JA and MeJA could regulate MeSA and SA production. Taken together, our results demonstrate that (Me)JA and (Me)SA are required for systemic resistance response against TMV.

Influence of salicylic acid on rubisco and rubisco activase in tobacco plant grown under sodium chloride in vitro

The present study was designed to evaluate the influence of salicylic acid (SA) on the growth of salt stress (sodium chloride) induced in tobacco plants. In addition, quantification of rubisco and rubisco activase contents of the plants was also determined in treatments with the control, 10(-4) mM SA, 50 mM NaCl, 100 mM NaCl, 150 mM NaCl, SA + 50 mM NaCl, SA + 100 mM NaCl and SA + 150 mM NaCl, respectively after in vitro culture for 5 weeks. The growth of the tobacco plant decreased in 50 mM and 100 mM NaCl when not treated with SA. However, the growth was accelerated by SA, and the growth retardation caused by NaCl was improved by SA. The content of rubisco was improved by SA only in plants treated with 50 mM NaCl, and the activity of rubisco was increased by SA resulting in the decreased effect of NaCl, but only in 50 mM NaCl treated plants. The content of rubisco activase decreased due to NaCl, and SA did not improve the effect caused by NaCl. The activity of rubisco activase was increased by SA resulting in decreased activity caused by NaCl, but increased effect by SA was not recovered to the level of NaCl untreated plants. The activity of rubisco and rubisco activase, which decreased due to denaturing agents, did not demonstrate significant improvement when compared to the control.

The root hair assay facilitates the use of genetic and pharmacological tools in order to dissect multiple signalling pathways that lead to programmed cell death

The activation of programmed cell death (PCD) is often a result of complex signalling pathways whose relationship and intersection are not well understood. We recently described a PCD root hair assay and proposed that it could be used to rapidly screen genetic or pharmacological modulators of PCD. To further assess the applicability of the root hair assay for studying multiple signalling pathways leading to PCD activation we have investigated the crosstalk between salicylic acid, autophagy and apoptosis-like PCD (AL-PCD) in Arabidopsis thaliana. The root hair assay was used to determine rates of AL-PCD induced by a panel of cell death inducing treatments in wild type plants treated with chemical modulators of salicylic acid synthesis or autophagy, and in genetic lines defective in autophagy or salicylic acid signalling. The assay demonstrated that PCD induced by exogenous salicylic acid or fumonisin B1 displayed a requirement for salicylic acid signalling and was partially dependent on the salicylic acid signal transducer NPR1. Autophagy deficiency resulted in an increase in the rates of AL-PCD induced by salicylic acid and fumonisin B1, but not by gibberellic acid or abiotic stress. The phenylalanine ammonia lyase-dependent salicylic acid synthesis pathway contributed only to death induced by salicylic acid and fumonisin B1. 3-Methyladenine, which is commonly used as an inhibitor of autophagy, appeared to influence PCD induction in all treatments suggesting a possible secondary, non-autophagic, effect on a core component of the plant PCD pathway. The results suggest that salicylic acid signalling is negatively regulated by autophagy during salicylic acid and mycotoxin-induced AL-PCD. However, this crosstalk does not appear to be directly involved in PCD induced by gibberellic acid or abiotic stress. This study demonstrates that the root hair assay is an effective tool for relatively rapid investigation of complex signalling pathways leading to the activation of PCD.

Chemical inducers of systemic immunity in plants

Systemic acquired resistance (SAR) is a highly desirable form of resistance that protects against a broad-spectrum of related or unrelated pathogens. SAR involves the generation of multiple signals at the site of primary infection, which arms distal portions against subsequent secondary infections. The last decade has witnessed considerable progress, and a number of chemical signals contributing to SAR have been isolated and characterized. The diverse chemical nature of these chemicals had led to the growing belief that SAR might involve interplay of multiple diverse and independent signals. However, recent results suggest that coordinated signalling from diverse signalling components facilitates SAR in plants. This review mainly discusses organized signalling by two such chemicals, glycerol-3-phoshphate and azelaic acid, and the role of basal salicylic acid levels in G3P-conferred SAR.

The hypervariable amino-terminus of P1 protease modulates potyviral replication and host defense responses

The replication of many RNA viruses involves the translation of polyproteins, whose processing by endopeptidases is a critical step for the release of functional subunits. P1 is the first protease encoded in plant potyvirus genomes; once activated by an as-yet-unknown host factor, it acts in cis on its own C-terminal end, hydrolyzing the P1-HCPro junction. Earlier research suggests that P1 cooperates with HCPro to inhibit host RNA silencing defenses. Using Plum pox virus as a model, we show that although P1 does not have a major direct role in RNA silencing suppression, it can indeed modulate HCPro function by its self-cleavage activity. To study P1 protease regulation, we used bioinformatic analysis and in vitro activity experiments to map the core C-terminal catalytic domain. We present evidence that the hypervariable region that precedes the protease domain is predicted as intrinsically disordered, and that it behaves as a negative regulator of P1 proteolytic activity in in vitro cleavage assays. In viral infections, removal of the P1 protease antagonistic regulator is associated with greater symptom severity, induction of salicylate-dependent pathogenesis-related proteins, and reduced viral loads. We suggest that fine modulation of a viral protease activity has evolved to keep viral amplification below host-detrimental levels, and thus to maintain higher long-term replicative capacity.

Current overview of allergens of plant pathogenesis related protein families

Pathogenesis related (PR) proteins are one of the major sources of plant derived allergens. These proteins are induced by the plants as a defense response system in stress conditions like microbial and insect infections, wounding, exposure to harsh chemicals, and atmospheric conditions. However, some plant tissues that are more exposed to environmental conditions like UV irradiation and insect or fungal attacks express these proteins constitutively. These proteins are mostly resistant to proteases and most of them show considerable stability at low pH. Many of these plant pathogenesis related proteins are found to act as food allergens, latex allergens, and pollen allergens. Proteins having similar amino acid sequences among the members of PR proteins may be responsible for cross-reactivity among allergens from diverse plants. This review analyzes the different pathogenesis related protein families that have been reported as allergens. Proteins of these families have been characterized in regard to their biological functions, amino acid sequence, and cross-reactivity. The three-dimensional structures of some of these allergens have also been evaluated to elucidate the antigenic determinants of these molecules and to explain the cross-reactivity among the various allergens.

Uptake and metabolic effects of salicylic acid on the pulvinar motor cells of Mimosa pudica L

In this paper, the salicylic acid (o-hydroxy benzoic acid) (SA) uptake by the pulvinar tissues of Mimosa pudica L. pulvini was shown to be strongly pH-dependent, increasing with acidity of the assay medium. This uptake was performed according to a unique affinity system (K(m) = 5.9 mM, V(m) = 526 pmol mgDW(-1)) in the concentration range of 0.1-5 mM. The uptake rate increased with increasing temperature (5-35 °C) and was inhibited following treatment with sodium azide (NaN3) and carbonyl cyanide m-chlorophenylhydrazone (CCCP), suggesting the involvement of an active component. Treatment with p-chloromercuribenzenesulfonic acid (PCMBS) did not modify the uptake, indicating that external thiol groups were not necessary. KCl, which induced membrane depolarization had no significant effect, and fusicoccin (FC), which hyperpolarized cell membrane, stimulated the uptake, suggesting that the pH component of the proton motive force was likely a driving force. These data suggest that the SA uptake by the pulvinar tissues may be driven by two components: an ion-trap mechanism playing a pivotal role and a putative carrier-mediated mechanism. Unlike other benzoic acid derivatives acting as classical respiration inhibitors (NaN3 and KCN), SA modified the pulvinar cell metabolism by increasing the respiration rate similar to CCCP and 2,4-dinitrophenol (DNP). Furthermore, SA inhibited the osmoregulated seismonastic reaction in a pH dependent manner and induced characteristic damage to the ultrastructural features of the pulvinar motor cells, particularly at the mitochondrial level.

Regulation of water, salinity, and cold stress responses by salicylic acid

Salicylic acid (SA) is a naturally occurring phenolic compound. SA plays an important role in the regulation of plant growth, development, ripening, and defense responses. The role of SA in the plant-pathogen relationship has been extensively investigated. In addition to defense responses, SA plays an important role in the response to abiotic stresses, including drought, low temperature, and salinity stresses. It has been suggested that SA has great agronomic potential to improve the stress tolerance of agriculturally important crops. However, the utility of SA is dependent on the concentration of the applied SA, the mode of application, and the state of the plants (e.g., developmental stage and acclimation). Generally, low concentrations of applied SA alleviate the sensitivity to abiotic stresses, and high concentrations of applied induce high levels of oxidative stress, leading to a decreased tolerance to abiotic stresses. In this article, the effects of SA on the water stress responses and regulation of stomatal closure are reviewed.

Profiling of genes related to cross protection and competition for NbTOM1 by HLSV and TMV

Cross protection is the phenomenon through which a mild strain virus suppresses symptoms induced by a closely related severe strain virus in infected plants. Hibiscus latent Singapore virus (HLSV) and Tobacco mosaic virus (TMV) are species within the genus tobamovirus. HLSV can protect Nicotianabenthamiana against TMV-U1 strain, resulting in mild symptoms instead of severe systemic necrosis. The mechanism of cross protection between HLSV and TMV is unknown. In the past, some researchers suggest that the protecting virus strain might occupy virus-specific replication sites within a cell leaving no room for the challenge virus. Quantitative real-time RT-PCR was performed to detect viral RNA levels during cross protection. HLSV accumulation increased in cross protected plants compared with that of single HLSV infected plants, while TMV decreased in cross protected plants. This suggests that there is a competition for host factors between HLSV and TMV for replication. To investigate the mechanism under the cross protection between HLSV and TMV, microarray analysis was conducted to examine the transcriptional levels of global host genes during cross protection, using Tobacco Gene Expression Microarray, 4 x 44 k slides. The transcriptional level of some host genes corresponded to accumulation level of TMV. Some host genes were up-regulated only by HLSV. Tobamovirus multiplication gene 1 (TOM1), essential for tobamovirus multiplication, was involved in competition for replication by HLSV and TMV during cross protection. Both HLSV and TMV accumulation decreased when NbTOM1 was silenced. A large quantity of HLSV resulted in decreased TMV accumulation in HLSV+TMV (100:1) co-infection. These results indicate that host genes involved in the plant defense response and virus multiplication are up-regulated by challenge virus TMV but not by protecting virus HLSV during cross protection.

Crosstalk between intracellular and extracellular salicylic acid signaling events leading to long-distance spread of signals

It is well recognized that salicylic acid (SA) acts as a natural signaling molecule involved in both local and systemic plant defense responses upon attacks by pathogens. Recently, cellular SA receptors and a number of SA-related phloem-mobile signals were identified. Here, we compare the old and up-to-date concepts of plant defense signaling events involving SA. Finally, the crosstalk between intracellular and extracellular SA signaling events leading to long-distance spread of signals was outlined by focusing on the modes of both the short- and long-distance signaling events involving the actions of SA. For the above purpose, two distinct conceptual models for local SA perception and signaling mechanisms in the intracellular and extracellular paths (referred to as models i and ii, respectively) were proposed. In addition to two local SA perception models, we propose that the long-distance SA action could be attributed to three different modes, namely, (iii) local increase in SA followed by transport of SA and SA intermediates, (iv) systemic propagation of SA-derived signals with both chemical and electrical natures without direct movement of SA, and (v) integrated crosstalk allowing alternately repeated secondary signal propagation and biosynthesis of SA and/or conversion of inert SA intermediates to free SA finally contributing to the systemic spread of SA-derived signals. We review here that the long-distance SA signaling events (models iii-v), inevitably involve the mechanisms described in the local signaling models (models i and ii) as the key pieces of the crosstalk.

Aspects of pathogen genomics, diversity, epidemiology, vector dynamics, and disease management for a newly emerged disease of potato: zebra chip

An overview is provided for the aspects of history, biology, genomics, genetics, and epidemiology of zebra chip (ZC), a destructive disease of potato (Solanum tuberosum) that represents a major threat to the potato industries in the United States as well as other potato-production regions in the world. The disease is associated with a gram-negative, phloem-limited, insect-vectored, unculturable prokaryote, 'Candidatus Liberibacter solanacearum', that belongs to the Rhizobiaceae family of α-Proteobacteria. The closest cultivated relatives of 'Ca. L. solanacearum' are members of the group of bacteria known as the α-2 subgroup. In spite of the fact that Koch's postulates sensu stricto have not been fulfilled, a great deal of progress has been made in understanding the ZC disease complex since discovery of the disease. Nevertheless, more research is needed to better understand vector biology, disease mechanisms, host response, and epidemiology in the context of vector-pathogen-plant interactions. Current ZC management strategies focus primarily on psyllid control. The ultimate control of ZC likely relies on host resistance. Unfortunately, all commercial potato cultivars are susceptible to ZC. Elucidation of the 'Ca. L. solanacearum' genome sequence has provided insights into the genetic basis of virulence and physiological and metabolic capability of this organism. Finally, the most effective, sustainable management of ZC is likely to be based on integrated strategies, including removal or reduction of vectors or inocula, improvement of host resistance to the presumptive pathogen and psyllid vectors, and novel gene-based therapeutic treatment.

Enzyme action in the regulation of plant hormone responses

Plants synthesize a chemically diverse range of hormones that regulate growth, development, and responses to environmental stresses. The major classes of plant hormones are specialized metabolites with exquisitely tailored perception and signaling systems, but equally important are the enzymes that control the dose and exposure to the bioactive forms of these molecules. Here, we review new insights into the role of enzyme families, including the SABATH methyltransferases, the methylesterases, the GH3 acyl acid-amido synthetases, and the hormone peptidyl hydrolases, in controlling the biosynthesis and modifications of plant hormones and how these enzymes contribute to the network of chemical signals responsible for plant growth, development, and environmental adaptation.

A mutation in a coproporphyrinogen III oxidase gene confers growth inhibition, enhanced powdery mildew resistance and powdery mildew-induced cell death in Arabidopsis

A gene encoding a coproporphyrinogen III oxidase mediates disease resistance in plants by the salicylic acid pathway. A number of genes that regulate powdery mildew resistance have been identified in Arabidopsis, such as ENHANCED DISEASE RESISTANCE 1 to 3 (EDR1 to 3). To further study the molecular interactions between the powdery mildew pathogen and Arabidopsis, we isolated and characterized a mutant that exhibited enhanced resistance to powdery mildew. The mutant also showed dramatic powdery mildew-induced cell death as well as growth defects and early senescence in the absence of pathogens. We identified the affected gene by map-based cloning and found that the gene encodes a coproporphyrinogen III oxidase, a key enzyme in the tetrapyrrole biosynthesis pathway, previously known as LESION INITIATION 2 (LIN2). Therefore, we designated the mutant lin2-2. Further studies revealed that the lin2-2 mutant also displayed enhanced resistance to Hyaloperonospora arabidopsidis (H.a.) Noco2. Genetic analysis showed that the lin2-2-mediated disease resistance and spontaneous cell death were dependent on PHYTOALEXIN DEFICIENT 4 (PAD4), SALICYLIC ACID INDUCTION-DEFICIENT 2 (SID2), and NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1), which are all involved in salicylic acid signaling. Furthermore, the relative expression levels of defense-related genes were induced after powdery mildew infection in the lin2-2 mutant. These data indicated that LIN2 plays an important role in cell death control and defense responses in plants.

Temporal accumulation of salicylic acid activates the defense response against Colletotrichum in strawberry

Many authors have reported interactions between strawberry cultivars and pathogenic microorganisms, yet little is known about the mechanisms triggered in the plant. In this paper we examine the participation of the salicylic acid (SA) signaling pathway involved in the response of Fragaria x ananassa cv. Pájaro plants to pathogens. Strawberry plants were challenged with the virulent strain M11 of Colletotrichum acutatum, or with the avirulent strain M23 of Colletotrichum fragariae which confers resistance to the former. Our study showed that the isolate M23 induced a temporal SA accumulation that was accompanied with the induction of PR-1 gene expression in strawberry plants. Such events occured after the oxidative burst, evaluated as the accumulation of hydrogen peroxide and superoxide anion, and many hours before the protection could be detected. Similar results were obtained with exogenously applied SA. Results obtained supports the hypothesis that strawberry plants activate a SA mediated defense mechanisms that is effective against a causal agent of anthracnose. In contrast, plants inoculated with M11 did not show oxidative burst, SA accumulation or PR1 gene induction. This is the first report about a defense response signaling pathway studied in strawberry plants.

Lichen Depsides and Depsidones Reduce Symptoms of Diseases Caused by Tobacco Mosaic Virus (TMV) in Tobacco Leaves

Two depsides and five depsidones, isolated from lichens, were tested to determine their in vivo protective effects on tobacco leaves challenged with Tobacco Mosaic Virus (TMV). The results indicate that most of these compounds are able to reduce either the number and/or the size of necrotic lesions following virus infection. Pannarin, 1′-chloro-pannarin and stictic acid provided the more effective protective results, reducing by at least 45% the number and size of lesions. Real Time PCR assays were used to explore the target of action against TMV by examining the response behavior of genes involved in the plant defense mechanism. The application of the lichen substances did not lead to changes in the transcriptional levels of pathogen-related ( PR1a), allene oxide synthase 2 ( AOS2) or oxophytodienoate reductase ( OPR3) genes. Thus, the protection observed in the tobacco leaves treated with the lichen compounds may be mediated by a mechanism which does not involved the SA- or JA-mediated defensive plant response. A possible structure-activity relationship is presented.

How do plants achieve immunity? Defence without specialized immune cells

Vertebrates have evolved a sophisticated adaptive immune system that relies on an almost infinite diversity of antigen receptors that are clonally expressed by specialized immune cells that roam the circulatory system. These immune cells provide vertebrates with extraordinary antigen-specific immune capacity and memory, while minimizing self-reactivity. Plants, however, lack specialized mobile immune cells. Instead, every plant cell is thought to be capable of launching an effective immune response. So how do plants achieve specific, self-tolerant immunity and establish immune memory? Recent developments point towards a multilayered plant innate immune system comprised of self-surveillance, systemic signalling and chromosomal changes that together establish effective immunity.

Structural and Biochemical Changes in Salicylic-Acid-Treated Date Palm Roots Challenged with Fusarium oxysporum f. sp. albedinis

Histochemical and ultrastructural analyses were carried out to assess structural and biochemical changes in date palm roots pretreated with salicylic acid (SA) then inoculated with Fusarium oxysporum f. sp. albedinis (Foa). Flavonoids, induced proteins, and peroxidase activity were revealed in root tissues of SA-treated plants after challenge by Foa. These reactions were closely associated with plant resistance to Foa. Host reactions induced after inoculation of SA-treated plants with Foa included the plugging of intercellular spaces, the deposition of electron-dense materials at the sites of pathogen penetration, and several damages to fungal cells. On the other hand, untreated inoculated plants showed marked cell wall degradation and total cytoplasm disorganization, indicating the protective effects provided by salicylic acid in treated plants.

Localization of DIR1 at the tissue, cellular and subcellular levels during Systemic Acquired Resistance in Arabidopsis using DIR1:GUS and DIR1:EGFP reporters

BACKGROUND

Systemic Acquired Resistance (SAR) is an induced resistance response to pathogens, characterized by the translocation of a long-distance signal from induced leaves to distant tissues to prime them for increased resistance to future infection. DEFECTIVE in INDUCED RESISTANCE 1 (DIR1) has been hypothesized to chaperone a small signaling molecule to distant tissues during SAR in Arabidopsis.

RESULTS

DIR1 promoter:DIR1-GUS/dir1-1 lines were constructed to examine DIR1 expression. DIR1 is expressed in seedlings, flowers and ubiquitously in untreated or mock-inoculated mature leaf cells, including phloem sieve elements and companion cells. Inoculation of leaves with SAR-inducing avirulent or virulent Pseudomonas syringae pv tomato (Pst) resulted in Type III Secretion System-dependent suppression of DIR1 expression in leaf cells. Transient expression of fluorescent fusion proteins in tobacco and intercellular washing fluid experiments indicated that DIR1's ER signal sequence targets it for secretion to the cell wall. However, DIR1 expressed without a signal sequence rescued the dir1-1 SAR defect, suggesting that a cytosolic pool of DIR1 is important for the SAR response.

CONCLUSIONS

Although expression of DIR1 decreases during SAR induction, the protein localizes to all living cell types of the vasculature, including companion cells and sieve elements, and therefore DIR1 is well situated to participate in long-distance signaling during SAR.

Free Radicals, Salicylic Acid and Mycotoxins in Asparagus After Inoculation with Fusarium proliferatum and F. oxysporum

Electron paramagnetic resonance was used to monitor free radicals and paramagnetic species like Fe, Mn, Cu generation, stability and status in Asparagus officinalis infected by common pathogens Fusarium proliferatum and F. oxysporum. Occurrence of F. proliferatum and F. oxysporum, level of free radicals and other paramagnetic species, as well as salicylic acid and mycotoxins content in roots and stems of seedlings were estimated on the second and fourth week after inoculation. In the first term free and total salicylic acid contents were related to free radicals level in stem (P = 0.010 and P = 0.033, respectively). Concentration of Fe(3+) ions in porphyrin complexes (g = 2.3, g = 2.9) was related to the species of pathogen. There was no significant difference between Mn(2+) concentrations in stem samples; however, the level of free radicals in samples inoculated with F. proliferatum was significantly higher when compared to F. oxysporum.

Salicylic acid beyond defence: its role in plant growth and development

In recent years salicylic acid (SA) has been the focus of intensive research due to its function as an endogenous signal mediating local and systemic plant defence responses against pathogens. It has also been found that SA plays a role during the plant response to abiotic stresses such as drought, chilling, heavy metal toxicity, heat, and osmotic stress. In this sense, SA appears to be, just like in mammals, an 'effective therapeutic agent' for plants. Besides this function during biotic and abiotic stress, SA plays a crucial role in the regulation of physiological and biochemical processes during the entire lifespan of the plant. The discovery of its targets and the understanding of its molecular modes of action in physiological processes could help in the dissection of the complex SA signalling network, confirming its important role in both plant health and disease. Here, the evidence that supports the role of SA during plant growth and development is reviewed by comparing experiments performed by exogenous application of SA with analysis of genotypes affected by SA levels and/or perception.

Metabolic changes of Brassica rapa transformed with a bacterial isochorismate synthase gene

Metabolome analysis by 1-dimensional proton nuclear magnetic resonance (¹H NMR) coupled with multivariate data analysis was carried out in Brassica rapa plants transformed with a gene encoding bacterial isochorismate synthase (ICS). Partial least square-discrimination analysis (PLS-DA) on selected signals suggested that the resonances that were dominant in the transgenic plants corresponded to a glucosinolate (neoglucobrassicin), phenylpropanoids (sinapoyl malate, feruloyl malate, caffeoyl malate), organic acids (succinic acid and fumaric acid) and sugars (α- and β-glucose). In contrast, amino acids alanine threonine, valine, leucine were dominant in the untransformed controls. In addition, HPLC data showed that the transgenic plant accumulated salicylic acid (SA) at significantly higher levels than the control plants, whereas the phylloquinone levels were not affected. The results suggest that the expression of the bacterial isochorismate synthase gene in B. rapa does not affect fluxes into pathways to other groups of secondary metabolites through competition for the same precursor. On the contrary, the biosynthesis of isochorismate-derived products (SA) seems to induce the competitive pathways via phenylalanine (phenylpropanoids) and tryptophan (IAA and indole glucosinolates).

Age-related resistance of Nicotiana benthamiana against hemibiotrophic pathogen Phytophthora infestans requires both ethylene- and salicylic acid-mediated signaling pathways

Phytophthora infestans, the agent of late blight disease of potato, is a hemibiotrophic pathogen with biotrophic action during early infection and necrotrophic in the later stage of colonization. Mature Nicotiana benthamiana was resistant to P. infestans, whereas relatively young plants were susceptible to this pathogen. Young plants became resistant following a pretreatment with acibenzolar-S-methyl, a functional analog of salicylic acid (SA), indicating that susceptibility of young plants is due to a lack of induction of SA signaling. Further analysis with virus-induced gene silencing indicated that NbICS1 and NbEIN2, the genes for SA biosynthesis and ethylene (ET) signaling, respectively, are required for the resistance of mature N. benthamiana against P. infestans. Furthermore, these genes are required for the production of reactive oxygen species (ROS) induced by treatment of the INF1 elicitor. In NbICS1-silenced plants, cell death induced by either INF1 or necrosis-inducing protein NPP1.1 was significantly accelerated. Expression of genes for phytoalexin (capsidiol) biosynthesis, NbEAS and NbEAH, were regulated by ET, and gene silencing of either of them compromised resistance of N. benthamiana to P. infestans. Together, these results suggest that resistance of N. benthamiana against hemibiotrophic P. infestans requires both SA-regulated appropriate induction of cell death and ET-induced production of phytoalexin.

Genetic and functional diversity among the antagonistic potential fluorescent pseudomonads isolated from tea rhizosphere

Twenty-five fluorescent pseudomonads from rhizospheric soil of six tea gardens in four district of Upper Assam, India were isolated and screened for antagonistic activity against fungal pathogens such as Fusarium oxysporum f. sp. raphani (For), Fusarium oxysporum f. sp. ciceri (Foc), Fusarium semitectum (Fs), and Rhizoctonia solani (Rs); and bacterial pathogens-Staphylococcus aureus (Sa), Escherichia coli (Ec), and Klebsiella pneumoniae (Kp). Most of the isolates exhibited strong antagonistic activity against the fungal pathogens and gram-positive bacterium i.e. Staphylococcus aureus. Productions of siderophore, salicylic acid (SA), hydrogen cyanide (HCN), and cell wall-degrading enzyme (chitinase) were studied to observe the possible mechanisms of antagonistic activity of the isolates. Correlation between the antagonistic potentiality of some isolates and their levels of production of siderophore, salicylic acid, and hydrogen cyanide was observed. Out of the 25 isolates, antibiotic-coding genes, 2,4-diacetylphloroglucinol (DAPG) and pyoluteorin (PLT) were detected in the isolates, Pf12 and Pf373, respectively. Genetic diversity of these fluorescent pseudomonads were analyzed with reference to four strains of Pseudomonas fluorescens NICM 2099(T), P. aeruginosa MTCC 2582(T), P. aureofaciens NICM 2026(T), and P. syringae MTCC 673(T). 16S rDNA-RFLP analysis of these isolates using three tetra cutter restriction enzymes (HaeIII, AluI and MspI) revealed two distinct clusters. Cluster A comprised only two isolates Pf141 and 24-PfM3, and cluster B comprised 23 isolates along with four reference strains.

Characterization of a pathogenesis-related protein 4 (PR-4) induced in Capsicum chinense L3 plants with dual RNase and DNase activities

Resistance conferred by the L(3) gene is active against most of the tobamoviruses, including the Spanish strain (PMMoV-S), a P(1),(2) pathotype, but not against certain strains of pepper mild mottle virus (PMMoV), termed as P(1),(2),(3) pathotype, such as the Italian strain (PMMoV-I). PMMoV-S induces a hypersensitive reaction (HR) in C. chinense PI159236 plant leaves with the formation of necrotic local lesions and restriction of the virus at the primary infection sites. In this paper, a C. chinense PR-4 protein induced during both the compatible and the incompatible interactions has been identified. It was strongly associated with HR induction and to a lesser extent with the compatible interaction, but only in the later stages of infection. Moreover, it was found to accumulate during the necrogenic reaction induced by Potato virus X. The C. chinense PR-4 protein belongs to the PR-4 protein subgroup II, based on the absence of a hevein domain. Furthermore, it is shown that the purified protein does not have chitinase activity, as previously proposed for PR-4 proteins. Instead, it has both RNase and DNase activity, although its contribution to the bulk activity of nucleases in infected plants is very low.

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.

Aphid feeding activates expression of a transcriptome of oxylipin-based defense signals in wheat involved in resistance to herbivory

Damage by the Russian wheat aphid (RWA), Diuraphis noxia, significantly reduces wheat and barley yields worldwide. In compatible interactions, virulent RWA populations flourish and susceptible plants suffer extensive leaf chlorophyll loss. In incompatible interactions, RWA reproduction and population growth are significantly reduced and RWA-related chlorophyll loss in resistant plants is minor. The objectives of this study were to develop an understanding of the molecular and phytochemical bases of RWA resistance in plants containing the Dnx resistance gene. Microarray, real-time polymerase chain reaction, and phytohormone assays were conducted to identify transcriptome components unique to RWA-infested Dnx plants and susceptible (Dn0) plants, and to identify and characterize putative genes involved in Dnx plant defense responses. We found that RWA-infested Dnx plants upregulated >180 genes related to reactive oxygen species, signaling, pathogen defense, and arthropod allelochemical and physical defense. The expression of several of these genes in RWA-infested Dnx plants increased significantly from 6- to 24-h post infestation (hpi), but their expression in Dn0 plants, when present, was delayed until 48- to 96 hpi. Concentrations of 16- and 18-carbon fatty acids, trans-methyl-12-oxophytodienoic acid, and abscisic acid were significantly greater in Dnx foliage than in Dn0 foliage after RWA infestation, suggesting that Dnx RWA defense and resistance genes may be regulated via the oxylipin pathway. These findings provide a foundation for the elucidation of the molecular basis for compatible- and incompatible plant-aphid interactions.

An F-box gene, CPR30, functions as a negative regulator of the defense response in Arabidopsis

Arabidopsis gain-of-resistance mutants, which show HR-like lesion formation and SAR-like constitutive defense responses, were used well as tools to unravel the plant defense mechanisms. We have identified a novel mutant, designated constitutive expresser of PR genes 30 (cpr30), that exhibited dwarf morphology, constitutive resistance to the bacterial pathogen Pseudomonas syringae and the dramatic induction of defense-response gene expression. The cpr30-conferred growth defect morphology and defense responses are dependent on ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1), PHYTOALEXIN DEFICIENT 4 (PAD4), and NONRACE-SPECIFIC DISEASE RESISTANCE 1 (NDR1). Further studies demonstrated that salicylic acid (SA) could partially account for the cpr30-conferred constitutive PR1 gene expression, but not for the growth defect, and that the cpr30-conferred defense responses were NPR1 independent. We observed a widespread expression of CPR30 throughout the plant, and a localization of CPR30-GFP fusion protein in the cytoplasm and nucleus. As an F-box protein, CPR30 could interact with multiple Arabidopsis-SKP1-like (ASK) proteins in vivo. Co-localization of CPR30 and ASK1 or ASK2 was observed in Arabidopsis protoplasts. Based on these results, we conclude that CPR30, a novel negative regulator, regulates both SA-dependent and SA-independent defense signaling, most likely through the ubiquitin-proteasome pathway in Arabidopsis.

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.

Forward and reverse genetics to identify genes involved in the age-related resistance response in Arabidopsis thaliana

SUMMARY Age-related resistance (ARR) occurs in numerous plant species, often resulting in increased disease resistance as plants mature. ARR in Arabidopsis to Pseudomonas syringae pv. tomato is associated with intercellular salicylic acid (SA) accumulation and the transition to flowering. Forward and reverse genetic screens were performed to identify genes required for ARR and to investigate the mechanism of the ARR response. Infiltration of SA into the intercellular space of the ARR-defective mutant iap1-1 (important for the ARR pathway) partially restored ARR function. Inter- and intracellular SA accumulation was reduced in the mutant iap1-1 compared with the wild-type, and the SA regulatory gene EDS1 was also required for ARR. Combining microarray analysis with reverse genetics using T-DNA insertion lines, four additional ARR genes were identified as contributing to ARR: two plant-specific transcription factors of the NAC family [ANAC055 (At3g15500) and ANAC092 (At5g39610)], a UDP-glucose glucosyltransferase [UGT85A1 (At1g22400)] and a cytidine deaminase [CDA1 (At2g19570)]. These four genes and IAP1 are also required for ARR to Hyaloperonospora parasitica. IAP1 encodes a key component of ARR that acts upstream of SA accumulation and possibly downstream of UGT85A1, CDA1 and the two NAC transcription factors (ANAC055, ANAC092).

Salicylic acid and salicylic acid glucoside in xylem sap of Brassica napus infected with Verticillium longisporum

Salicylic acid (SA) and its glucoside (SAG) were detected in xylem sap of Brassica napus by HPLC-MS. Concentrations of SA and SAG in xylem sap from the root and hypocotyl of the plant, and in extracts of shoots above the hypocotyl, increased after infection with the vascular pathogen Verticillium longisporum. Both concentrations were correlated with disease severity assessed as the reduction in shoot length. Furthermore, SAG levels in shoot extracts were correlated with the amount of V. longisporum DNA in the hypocotyls. Although the concentration of SAG (but not SA) in xylem sap of infected plants gradually declined from 14 to 35 days post infection, SAG levels remained significantly higher than in uninfected plants during the whole experiment. Jasmonic acid (JA) and abscisic acid (ABA) levels in xylem sap were not affected by infection with V. longisporum. SA and SAG extend the list of phytohormones potentially transported from root to shoot with the transpiration stream. The physiological relevance of this transport and its contribution to the distribution of SA in plants remain to be elucidated.

The Arabidopsis RESURRECTION1 gene regulates a novel antagonistic interaction in plant defense to biotrophs and necrotrophs

We report a role for the Arabidopsis (Arabidopsis thaliana) RESURRECTION1 (RST1) gene in plant defense. The rst1 mutant exhibits enhanced susceptibility to the biotrophic fungal pathogen Erysiphe cichoracearum but enhanced resistance to the necrotrophic fungal pathogens Botrytis cinerea and Alternaria brassicicola. RST1 encodes a novel protein that localizes to the plasma membrane and is predicted to contain 11 transmembrane domains. Disease responses in rst1 correlate with higher levels of jasmonic acid (JA) and increased basal and B. cinerea-induced expression of the plant defensin PDF1.2 gene but reduced E. cichoracearum-inducible salicylic acid levels and expression of pathogenesis-related genes PR1 and PR2. These results are consistent with rst1's varied resistance and susceptibility to pathogens of different life styles. Cuticular lipids, both cutin monomers and cuticular waxes, on rst1 leaves were significantly elevated, indicating a role for RST1 in the suppression of leaf cuticle lipid synthesis. The rst1 cuticle exhibits normal permeability, however, indicating that the disease responses of rst1 are not due to changes in this cuticle property. Double mutant analysis revealed that the coi1 mutation (causing defective JA signaling) is completely epistatic to rst1, whereas the ein2 mutation (causing defective ethylene signaling) is partially epistatic to rst1, for resistance to B. cinerea. The rst1 mutation thus defines a unique combination of disease responses to biotrophic and necrotrophic fungi in that it antagonizes salicylic acid-dependent defense and enhances JA-mediated defense through a mechanism that also controls cuticle synthesis.

Salicylic acid transport in Ricinus communis involves a pH-dependent carrier system in addition to diffusion

Despite its important functions in plant physiology and defense, the membrane transport mechanism of salicylic acid (SA) is poorly documented due to the general assumption that SA is taken up by plant cells via the ion trap mechanism. Using Ricinus communis seedlings and modeling tools (ACD LogD and Vega ZZ softwares), we show that phloem accumulation of SA and hydroxylated analogs is completely uncorrelated with the physicochemical parameters suitable for diffusion (number of hydrogen bond donors, polar surface area, and, especially, LogD values at apoplastic pHs and Delta LogD between apoplast and phloem sap pH values). These and other data (such as accumulation in phloem sap of the poorly permeant dissociated form of monohalogen derivatives from apoplast and inhibition of SA transport by the thiol reagent p-chloromercuribenzenesulfonic acid [pCMBS]) lead to the following conclusions. As in intestinal cells, SA transport in Ricinus involves a pH-dependent carrier system sensitive to pCMBS; this carrier can translocate monohalogen analogs in the anionic form; the efficiency of phloem transport of hydroxylated benzoic acid derivatives is tightly dependent on the position of the hydroxyl group on the aromatic ring (SA corresponds to the optimal position) but moderately affected by halogen addition in position 5, which is known to increase plant defense. Furthermore, combining time-course experiments and pCMBS used as a tool, we give information about the localization of the SA carrier. SA uptake by epidermal cells (i.e. the step preceding the symplastic transport to veins) insensitive to pCMBS occurs via the ion-trap mechanism, whereas apoplastic vein loading involves a carrier-mediated mechanism (which is targeted by pCMBS) in addition to diffusion.

Biosynthesis of salicylic acid in plants

Salicylic acid (SA) is an important signal molecule in plants. Two pathways of SA biosynthesis have been proposed in plants. Biochemical studies using isotope feeding have suggested that plants synthesize SA from cinnamate produced by the activity of phenylalanine ammonia lyase (PAL). Silencing of PAL genes in tobacco or chemical inhibition of PAL activity in Arabidopsis, cucumber and potato reduces pathogen-induced SA accumulation. Genetic studies, on the other hand, indicate that the bulk of SA is produced from isochorismate. In bacteria, SA is synthesized from chorismate through two reactions catalyzed by isochorismate synthase (ICS) and isochorismate pyruvate lyase (IPL). Arabidopsis contains two ICS genes but has no gene encoding proteins similar to the bacterial IPL. Thus, how SA is synthesized in plants is not fully elucidated. Two recently identified Arabidopsis genes, PBS3 and EPS1, are important for pathogen-induced SA accumulation. PBS3 encodes a member of the acyl-adenylate/thioester-forming enzyme family and EPS1 encodes a member of the BAHD acyltransferase superfamily. PBS3 and EPS1 may be directly involved in the synthesis of an important precursor or regulatory molecule for SA biosynthesis. The pathways and regulation of SA biosynthesis in plants may be more complicated than previously thought.

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).

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.

Salicylic Acid, a multifaceted hormone to combat disease

For more than 200 years, the plant hormone salicylic acid (SA) has been studied for its medicinal use in humans. However, its extensive signaling role in plants, particularly in defense against pathogens, has only become evident during the past 20 years. This review surveys how SA in plants regulates both local disease resistance mechanisms, including host cell death and defense gene expression, and systemic acquired resistance (SAR). Genetic studies reveal an increasingly complex network of proteins required for SA-mediated defense signaling, and this process is amplified by several regulatory feedback loops. The interaction between the SA signaling pathway and those regulated by other plant hormones and/or defense signals is also discussed.

Characterization of Arabidopsis mur3 mutations that result in constitutive activation of defence in petioles, but not leaves

A screen was established for mutants in which the plant defence response is de-repressed. The pathogen-inducible isochorismate synthase (ICS1) promoter was fused to firefly luciferase (luc) and a homozygous transgenic line generated in which the ICS1:luc fusion is co-regulated with ICS1. This line was mutagenized and M(2) seedlings screened for constitutive ICS1:luc expression (cie). The cie mutants fall into distinct phenotypic classes based on tissue-specific localization of luciferase activity. One mutant, cie1, that shows constitutive luciferase activity specifically in petioles, was chosen for further analysis. In addition to ICS1, PR and other defence-related genes are constitutively expressed in cie1 plants. The cie1 mutant is also characterized by an increased production of conjugated salicylic acid and reactive oxygen intermediates, as well as spontaneous lesion formation, all confined to petiole tissue. Significantly, defences activated in cie1 are sufficient to prevent infection by a virulent isolate of Hyaloperonospora parasitica, and this enhanced resistance response protects petiole tissue alone. Furthermore, cie1-mediated resistance, along with PR gene expression, is abolished in a sid2-1 mutant background, consistent with a requirement for salicylic acid. A positional cloning approach was used to identify cie1, which carries two point mutations in a gene required for cell wall biosynthesis and actin organization, MUR3. A mur3 knockout mutant also resists infection by H. parasitica in its petioles and this phenotype is complemented by transformation with wild-type MUR3. We propose that perturbed cell wall biosynthesis may activate plant defence and provide a rationale for the cie1 and the mur3 knockout phenotypes.

Root-fed salicylic acid in grape involves the response caused by aboveground high temperature

In order to investigate the transportation and distribution of salicylic acid (SA) from root to aboveground tissues in response to high temperature, the roots of grape plant were fed with (14)C-SA before high temperature treatment. Radioactivity results showed that progressive increase in SA transportation from root to aboveground as compared with the control varied exactly with the heat treatment time. Radioactivity results of leaves at different stem heights indicated that the increase in SA amount at the top and middle leaves during the early period was most significant in comparison with the bottom leaves. The up-transportation of SA from root to aboveground tissues was dependent on xylem rather than phloem. Auto-radiographs of whole grape plants strongly approved the conclusions drawn above. Root-derived SA was believed to be a fundamental source in response to aboveground high temperature.

[Phloem, transport between organs and long-distance signalling]

Phloem plays a major role in carbohydrate partitioning in the plant. It also controls the redistribution of various metabolites such as amino acids, vitamins, hormones, and ions. The molecular mechanisms responsible for phloem loading and unloading have been particularly well characterised, with the identification of sucrose and polyol transporters. The discovery of the role of phloem in the long-distance translocation of macromolecules, proteins, mRNA and small RNA has modified our understanding of the regulation of the coordination of some developmental and adaptation processes. This review details recent results concerning the transport and long-distance signalling that take place in the phloem.

3-Acetonyl-3-hydroxyoxindole: a new inducer of systemic acquired resistance in plants

Systemic acquired resistance (SAR) is an inducible defence mechanism which plays a central role in protecting plants from microbial pathogen attack. Guided by bioassays, a new chemical inducer of SAR was isolated from the extracts of Strobilanthes cusia and identified to be 3-acetonyl-3-hydroxyoxindole (AHO), a derivative of isatin. Tobacco plants treated with AHO exhibited enhanced resistance to tobacco mosaic virus (TMV) and to the fungal pathogen Erysiphe cichoracearum (powdery mildew), accompanied by increased levels of pathogenesis-related gene 1 (PR-1) expression, salicylic acid (SA) accumulation and phenylalanine ammonia-lyase activity. To study the mode of action of AHO, its ability to induce PR-1 expression and TMV resistance in nahG transgenic plants expressing salicylate hydroxylase, which prevents the accumulation of SA, was analysed. AHO treatment did not induce TMV resistance or PR-1 expression in nahG transgenic plants, suggesting that AHO acts upstream of SA in the SAR signalling pathway. In addition, using two-dimensional gel electrophoresis combined with mass spectrometry, five AHO-induced plant proteins were identified which were homologous to the effector proteins with which SA interacts. Our data suggest that AHO may represent a novel class of inducer that stimulates SA-mediated defence responses.