Sequence of a wheat cDNA encoding a pathogen-induced thaumatin-like protein

Characterization and expression analysis of a thaumatin-like protein PpTLP1 from ground cherry Physalis pubescens

Ground cherry, Physalis pubescens, is mainly cultivated as a fruit worldwide and popularly used as a food supplement and traditional Chinese medicine. Plants are challenged by external environmental stress and can initiate resistance to the stress through the regulation of pathogenesis-related (PR) proteins. Among PR proteins, PR-5, a thaumatin-like protein (TLP), was identified in many plants and found to be able to enhance stress resistance. However, PR-5 in ground cherry is not characterized and its expression is yet to be understood. In this study, a PR-5 protein PpTLP1 in P. pubescens was firstly identified. Analysis of the amino acid sequences revealed that PpTLP1 was highly similar to PR-NP24 identified in tomato with a difference in only one amino acid. Expression analysis indicated that the PpTLP1 gene was highly expressed in leaf while the PpTLP1 protein was tissue-specifically accumulated in cherry exocarp. Furthermore, the down-regulation of PpTLP1 in ground cherry was induced by NaCl treatment while the up-regulation was promoted by the infection of Sclerotinia sclerotiorum and Botrytis cinerea. This study will provide a new plant resource containing a TLP in Physalis genus and a novel insight for the improvement of postharvest management of ground cherry and other Solanaceae plants.

Proteomics-Based Data Integration of Wheat Cultivars Facing Fusarium graminearum Strains Revealed a Core-Responsive Pattern Controlling Fusarium Head Blight

Fusarium head blight (FHB), mainly occurring upon Fusarium graminearum infection in a wide variety of small-grain cereals, is supposed to be controlled by a range of processes diverted by the fungal pathogen, the so-called susceptibility factors. As a mean to provide relevant information about the molecular events involved in FHB susceptibility in bread wheat, we studied an extensive proteome of more than 7,900 identified wheat proteins in three cultivars of contrasting susceptibilities during their interaction with three F. graminearum strains of different aggressiveness. No cultivar-specific proteins discriminated the three wheat genotypes, demonstrating the establishment of a core proteome regardless of unequivocal FHB susceptibility differences. Quantitative protein analysis revealed that most of the FHB-induced molecular adjustments were shared by wheat cultivars and occurred independently of the F. graminearum strain aggressiveness. Although subtle abundance changes evidenced genotype-dependent responses to FHB, cultivar distinction was found to be mainly due to basal abundance differences, especially regarding the chloroplast functions. Integrating these data with previous proteome mapping of the three F. graminearum strains facing the three same wheat cultivars, we demonstrated strong correlations between the wheat protein abundance changes and the adjustments of fungal proteins supposed to interfere with host molecular functions. Together, these results provide a resourceful dataset that expands our understanding of the specific molecular events taking place during the wheat-F. graminearum interaction.

Prediction, isolation, overexpression and antifungal activity analysis of Medicago truncatula var. truncatula putative thaumatin like proteins (TLP-1, -2, -3, -4 and -5)

Pathogenesis-related proteins (PR-proteins) are induced in response to environmental stresses such as osmotic and drought stress, wounding, microbial infections and treatment with specific plant hormones and elicitors. These proteins are classified into several groups (PR-1 through PR-17) based on their amino acid sequence and biochemical functions. The present study focuses on prediction, isolation, over-expression and analysis of the antifungal activities of the thaumatin-like proteins (i.e. PR-5) in the model legume M. truncatula var. truncatula. Analysis of M. truncatula genome sequence, available freely on the NCBI website, indicated the presence of at least 15 PR-5 Open Reading Frames (ORFs), 5 of them (dubbed TLP-1, -2, -3, -4 and -5) were selected for this study. Using gene-specific primers, the genomic coding sequences were isolated, sequenced and all confirmed to match with those reported in the database. All the fragments were, then, cloned in Escherichia coli isolate BL21 (DE3), using pET-21c(+) plasmids for subsequent overexpression (overexpression). All 5 genes were expressed as inclusion bodies (IBs) with masses, estimated by SDS PAGE, corresponding to the theoretical values. As expected, none of the protein IBs had no detectable effect on the phytopathogenic fungi Rhizoctonia solani, Alternaria alternata, Fusarium graminearum, Fusarium solani, Verticillium sp. and Phytophtora spp. However, when the in vitro refolded IB preparations were applied, all displayed comparable strong antifungal activities against the tested fungi. The current study is the first report of overexpression and evaluation of antifungal activities of PR-5 family of proteins from M. truncatula Var. truncatula, and provides experimental evidence that all investigated proteins have the potential for enhancing resistance against some important fungal pathogens.

Disease Resistance Mechanisms in Plants

Plants have developed a complex defense system against diverse pests and pathogens. Once pathogens overcome mechanical barriers to infection, plant receptors initiate signaling pathways driving the expression of defense response genes. Plant immune systems rely on their ability to recognize enemy molecules, carry out signal transduction, and respond defensively through pathways involving many genes and their products. Pathogens actively attempt to evade and interfere with response pathways, selecting for a decentralized, multicomponent immune system. Recent advances in molecular techniques have greatly expanded our understanding of plant immunity, largely driven by potential application to agricultural systems. Here, we review the major plant immune system components, state of the art knowledge, and future direction of research on plant⁻pathogen interactions. In our review, we will discuss how the decentralization of plant immune systems have provided both increased evolutionary opportunity for pathogen resistance, as well as additional mechanisms for pathogen inhibition of such defense responses. We conclude that the rapid advances in bioinformatics and molecular biology are driving an explosion of information that will advance agricultural production and illustrate how complex molecular interactions evolve.

Proteomic analysis by iTRAQ-MRM of soybean resistance to Lamprosema Indicate

BACKGROUND

Lamprosema indicate is a major leaf feeding insect pest to soybean, which has caused serious yield losses in central and southern China. To explore the defense mechanisms of soybean resistance to Lamprosema indicate, a highly resistant line (Gantai-2-2) and a highly susceptible line (Wan 82-178) were exposed to Lamprosema indicate larval feedings for 0 h and 48 h, and the differential proteomic analyses of these two lines were carried out.

RESULTS

The results showed that 31 differentially expressed proteins (DEPs) were identified in the Gantai-2-2 when comparing 48 h feeding with 0 h feeding, and 53 DEPs were identified in the Wan 82-178. 28 DEPs were identified when comparing Gantai-2-2 with Wan 82-178 at 0 h feeding. The bioinformatic analysis results showed that most of the DEPs were associated with ribosome, linoleic acid metabolism, flavonoid biosynthesis, phenylpropanoid biosynthesis, peroxisome, stilbenoid, diarylheptanoid and gingerol biosynthesis, glutathione metabolism, pant hormone signal transduction, and flavone and flavonol biosynthesis, as well as other resistance related metabolic pathways. The MRM analysis showed that the iTRAQ results were reliable.

CONCLUSIONS

According to the analysis of the DEPs results, the soybean defended or resisted the Lamprosema indicate damage by the induction of a synthesis of anti-digestive proteins which inhibit the growth and development of insects, reactive oxygen species scavenging, signaling pathways, secondary metabolites synthesis, and so on.

Differential gene expression in incompatible interaction between wheat and stripe rust fungus revealed by cDNA-AFLP and comparison to compatible interaction

BACKGROUND

Stripe rust of wheat, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most important diseases of wheat worldwide. Due to special features of hexaploid wheat with large and complex genome and difficulties for transformation, and of Pst without sexual reproduction and hard to culture on media, the use of most genetic and molecular techniques in studying genes involved in the wheat-Pst interactions has been largely limited. The objective of this study was to identify transcriptionally regulated genes during an incompatible interaction between wheat and Pst using cDNA-AFLP technique

RESULTS

A total of 52,992 transcript derived fragments (TDFs) were generated with 64 primer pairs and 2,437 (4.6%) of them displayed altered expression patterns after inoculation with 1,787 up-regulated and 650 down-regulated. We obtained reliable sequences (>100 bp) for 255 selected TDFs, of which 113 (44.3%) had putative functions identified. A large group (17.6%) of these genes shared high homology with genes involved in metabolism and photosynthesis; 13.8% to genes with functions related to disease defense and signal transduction; and those in the remaining groups (12.9%) to genes involved in transcription, transport processes, protein metabolism, and cell structure, respectively. Through comparing TDFs identified in the present study for incompatible interaction and those identified in the previous study for compatible interactions, 161 TDFs were shared by both interactions, 94 were expressed specifically in the incompatible interaction, of which the specificity of 43 selected transcripts were determined using quantitative real-time polymerase chain reaction (qRT-PCR). Based on the analyses of homology to genes known to play a role in defense, signal transduction and protein metabolism, 20 TDFs were chosen and their expression patterns revealed by the cDNA-AFLP technique were confirmed using the qRT-PCR analysis.

CONCLUSION

We uncovered a number of new candidate genes possibly involved in the interactions of wheat and Pst, of which 11 TDFs expressed specifically in the incompatible interaction. Resistance to stripe rust in wheat cv. Suwon11 is executed after penetration has occurred. Moreover, we also found that plant responses in compatible and incompatible interactions are qualitatively similar but quantitatively different soon after stripe rust fungus infection.

TLXI, a novel type of xylanase inhibitor from wheat (Triticum aestivum) belonging to the thaumatin family

Wheat (Triticum aestivum) contains a previously unknown type of xylanase (EC 3.2.1.8) inhibitor, which is described in the present paper for the first time. Based on its >60% similarity to TLPs (thaumatin-like proteins) and the fact that it contains the Prosite PS00316 thaumatin family signature, it is referred to as TLXI (thaumatin-like xylanase inhibitor). TLXI is a basic (pI> or =9.3 in isoelectric focusing) protein with a molecular mass of approx. 18-kDa (determined by SDS/PAGE) and it occurs in wheat with varying extents of glycosylation. The TLXI gene sequence encodes a 26-amino-acid signal sequence followed by a 151-amino-acid mature protein with a calculated molecular mass of 15.6-kDa and pI of 8.38. The mature TLXI protein was expressed successfully in Pichia pastoris, resulting in a 21-kDa (determined by SDS/PAGE) recombinant protein (rTLXI). Polyclonal antibodies raised against TLXI purified from wheat react with epitopes of rTLXI as well as with those of thaumatin, demonstrating high structural similarity between these three proteins. TLXI has a unique inhibition specificity. It is a non-competitive inhibitor of a number of glycoside hydrolase family 11 xylanases, but it is inactive towards glycoside hydrolase family 10 xylanases. Progress curves show that TLXI is a slow tight-binding inhibitor, with a K(i) of approx. 60-nM. Except for zeamatin, an alpha-amylase/trypsin inhibitor from maize (Zea mays), no other enzyme inhibitor is currently known among the TLPs. TLXI thus represents a novel type of inhibitor within this group of proteins.

Phylogenetic and structural relationships of the PR5 gene family reveal an ancient multigene family conserved in plants and select animal taxa

Pathogenesis-related group 5 (PR5) plant proteins include thaumatin, osmotin, and related proteins, many of which have antimicrobial activity. The recent discovery of PR5-like (PR5-L) sequences in nematodes and insects raises questions about their evolutionary relationships. Using complete plant genome data and discovery of multiple insect PR5-L sequences, phylogenetic comparisons among plants and animals were performed. All PR5/PR5-L protein sequences were mined from genome data of a member of each of two main angiosperm groups-the eudicots (Arabidoposis thaliana) and the monocots (Oryza sativa)-and from the Caenorhabditis nematode (C. elegans and C. briggsase). Insect PR5-L sequences were mined from EST databases and GenBank submissions from four insect orders: Coleoptera (Diaprepes abbreviatus and Biphyllus lunatus), Orthoptera (Schistocerca gregaria), Hymenoptera (Lysiphlebus testaceipes), and Hemiptera (Toxoptera citricida). Parsimony and Bayesian phylogenetic analyses showed that the PR5 family is paraphyletic in plants, likely arising from 10 genes in a common ancestor to monocots and eudicots. After evolutionary divergence of monocots and eudicots, PR5 genes increased asymmetrically among the 10 clades. Insects and nematodes contain multiple sequences (seven PR5-Ls in nematodes and at least three in some insects) all related to the same plant clade, with nematode and insect sequences separating as two clades. Protein structural homology modeling showed strong similarity among animal and plant PR5/PR5-Ls, with divergence only in surface-exposed loops. Sequence and structural conservation among PR5/PR5-Ls suggests an important and conserved role throughout the evolutionary divergence of the diverse organisms from which they reside.

Macroarray expression analysis of barley susceptibility and nonhost resistance to Blumeria graminis

Different formae speciales of the grass powdery mildew fungus Blumeria graminis undergo basic-compatible or basic-incompatible (nonhost) interactions with barley. Background resistance in compatible interactions and nonhost resistance require common genetic and mechanistic elements of plant defense. To build resources for differential screening for genes that potentially distinguish a compatible from an incompatible interaction on the level of differential gene expression of the plant, we constructed eight dedicated cDNA libraries, established 13.000 expressed sequence tag (EST) sequences and designed DNA macroarrays. Using macroarrays based on cDNAs derived from epidermal peels of plants pretreated with the chemical resistance activating compound acibenzolar-S-methyl, we compared the expression of barley gene transcripts in the early host interaction with B. graminis f.sp. hordei or the nonhost pathogen B. graminis f.sp. tritici, respectively. We identified 102 spots corresponding to 94 genes on the macroarray that gave significant B. graminis-responsive signals at 12 and/or 24 h after inoculation. In independent expression analyses, we confirmed the macroarray results for 11 selected genes. Although the majority of genes showed a similar expression profile in compatible versus incompatible interactions, about 30 of the 94 genes were expressed on slightly different levels in compatible versus incompatible interactions.

Glyphosate inhibits rust diseases in glyphosate-resistant wheat and soybean

Glyphosate is a broad-spectrum herbicide used for the control of weeds in glyphosate-resistant crops. Glyphosate inhibits 5-enolpyruvyl shikimate 3-phosphate synthase, a key enzyme in the synthesis of aromatic amino acids in plants, fungi, and bacteria. Studies with glyphosate-resistant wheat have shown that glyphosate provided both preventive and curative activities against Puccinia striiformis f. sp. tritici and Puccinia triticina, which cause stripe and leaf rusts, respectively, in wheat. Growth-chamber studies demonstrated wheat rust control at multiple plant growth stages with a glyphosate spray dose typically recommended for weed control. Rust control was absent in formulation controls without glyphosate, dependent on systemic glyphosate concentrations in leaf tissues, and not mediated through induction of four common systemic acquired resistance genes. A field test with endemic stripe rust inoculum confirmed the activities of glyphosate pre- and postinfestation. Preliminary greenhouse studies also demonstrated that application of glyphosate in glyphosate-resistant soybeans suppressed Asian soybean rust, caused by Phakopsora pachyrhizi.

Specific patterns of changes in wheat gene expression after treatment with three antifungal compounds

The two fungicides azoxystrobin and fenpropimorph are used against powdery mildew and rust diseases in wheat (Triticum aestivumL). Azoxystrobin, a strobilurin, inhibits fungal mitochondrial respiration and fenpropimorph, a morpholin, represses biosynthesis of ergosterol, the major sterol of fungal membranes. Although the fungitoxic activity of these compounds is well understood, their effects on plant metabolism remain unclear. In contrast to the fungicides which directly affect pathogen metabolism, benzo(1,2,3) thiadiazole-7-carbothioic acid S-methylester (BTH) induces resistance against wheat pathogens by the activation of systemic acquired resistance in the host plant. In this study, we monitored gene expression in spring wheat after treatment with each of these agrochemicals in a greenhouse trial using a microarray containing 600 barley cDNA clones. Defence-related genes were strongly induced after treatment with BTH, confirming the activation of a similar set of genes as in dicot plants following salicylic acid treatment. A similar gene expression pattern was observed after treatment with fenpropimorph and some defence-related genes were induced by azoxystrobin, demonstrating that these fungicides also activate a defence reaction. However, less intense responses were triggered than with BTH. The same experiments performed under field conditions gave dramatically different results. No gene showed differential expression after treatment and defence genes were already expressed at a high level before application of the agrochemicals. These differences in the expression patterns between the two environments demonstrate the importance of plant growth conditions for testing the impact of agrochemicals on plant metabolism.

New TAXI-type Xylanase Inhibitor Genes are Inducible by Pathogens and Wounding in Hexaploid Wheat

TAXI-I (Triticum aestivum xylanase inhibitor I) is a wheat grain protein that inhibits arabinoxylan fragmentation by microbial endo-beta-1,4-xylanases used in the food industry. Although TAXI was speculated to be involved in counterattack against pathogens, there is actually no evidence to support this hypothesis. We have now demonstrated the presence of TAXI family members with isolation of two mRNA species, Taxi-III and Taxi-IV. At the nucleotide sequence level, Taxi-III and Taxi-IV were 91.7% and 92.0% identical, respectively, to Taxi-I, and Taxi-III and Taxi-IV were 96.8% identical. Accumulation of Taxi-III/IV transcripts was most evident in roots and older leaves where transcripts of Taxi-I were negligible. When challenged by fungal pathogens Fusarium graminearum and Erysiphe graminis, the concentrations of Taxi-III/IV transcripts increased significantly. In contrast, the increases in Taxi-I transcripts in response to these pathogens were rather limited. Both Taxi-I and Taxi-III/IV were strongly expressed in wounded leaves. The upstream region of Taxi-III contained W boxes and GCC boxes, which are sufficient to confer pathogen and wound inducibility on promoters. Recombinant TAXI-III protein inhibited Aspergillus niger and Trichoderma sp. xylanases: it was also active against some spelt xylan-induced xylanases of F. graminearum. These features suggest that some, but not all, TAXI-type xylanase inhibitors have a role in plant defense.

An integrative genetic linkage map of winter wheat (Triticum aestivum L.)

We constructed a genetic linkage map based on a cross between two Swiss winter wheat ( Triticum aestivum L.) varieties, Arina and Forno. Two-hundred and forty F(5) single-seed descent (SSD)-derived lines were analysed with 112 restriction fragment length polymorphism (RFLP) anonymous probes, 18 wheat cDNA clones coding for putative stress or defence-related proteins and 179 simple-sequence repeat (SSR) primer-pairs. The 309 markers revealed 396 segregating loci. Linkage analysis defined 27 linkage groups that could all be assigned to chromosomes or chromosome arms. The resulting genetic map comprises 380 loci and spans 3,086 cM with 1,131 cM for the A genome, 920 cM for the B genome and 1,036 cM for the D genome. Seventeen percent of the loci showed a significant ( P < 0.05) deviation from a 1:1 ratio, most of them in favour of the Arina alleles. This map enabled the mapping of QTLs for resistance against several fungal diseases such as Stagonospora glume blotch, leaf rust and Fusarium head blight. It will also be very useful for wheat genetic mapping, as it combines RFLP and SSR markers that were previously located on separate maps.

A lectin-like wheat gene responds systemically to attempted feeding by avirulent first-instar Hessian fly larvae

Through gene-for-gene interactions, wheat plants respond to specific biotypes of Hessian fly upon the initiation of first-instar larval feeding. Leaves of plants containing the H9 resistance gene responded to avirulent biotype L. larvae with rapid changes in the levels of several mRNA transcripts and initiation of an incompatible interaction. A low-copy gene, Hfr-1 (Hessian fly-response gene 1), responded with increased mRNA levels for two days before returning to preinfestation levels by day five. Hfr-1 mRNA was constitutively expressed in uninfested control plants as well as in plants infested with virulent larvae. The cDNA sequence was similar to a maize gene encoding a beta-glucosidase aggregating factor (BGAF), to jacalin-like mannose-binding lectins, and to several plant genes that respond to microbial infections. The potential roles of Hfr-1 in defending wheat against Hessian fly damage are discussed.

Expression of thaumatin-like permatin PR-5 genes switches from the ovary wall to the aleurone in developing barley and oat seeds

Permatins are antifungal thaumatin-like proteins (TLPs) of the PR-5 family of pathogenesis-related proteins. They occur in many cereals, but little is known of their expression and roles. Permatin cDNA clones were produced and used to study expression in developing barley and oat seeds. Actin and CDC48 mRNAs declined rapidly following inoculation of barley spikes with Fusarium graminearum. Despite this, permatin mRNA levels remained constant or increased slightly. Studies of permatin gene expression in healthy plants revealed that developing barley and oat seeds accumulate permatin mRNA in an unusual bimodal pattern. Permatin mRNA and protein are highly abundant around the time of pollination and then decrease rapidly to near-zero. A second peak occurs in the doughy stage of development. Antibody and DNA probe hybridization studies showed that expression initially occurs in the ovary wall and then switches to the aleurone and ventral furrow of developing seeds, reaching a peak in the doughy stage. Small amounts of permatin mRNAs also occur in certain vegetative tissues. The barley and oat permatin sequences provided sufficient comparisons between cereal TLPs to suggest that deletions or additions in specific elements could have led to the divergence of leaf- and seed-specific TLPs.

Three major somatic embryogenesis related proteins in Cichorium identified as PR proteins

In Cichorium hybrid clone '474' (C. intybus L., var. sativum x C. endivia L., var. latifolia), the direct somatic embryogenesis process in leaf tissues is accompanied by an overall increase in the amount of proteins secreted into the culture medium. Amongst these, three major protein bands of 38 kDa, 32 kDa and 25 kDa were found in the conditioned media. These extracellular protein bands accumulated in the medium of the embryogenic Cichorium hybrid up to 8-fold compared with those in the medium of a nonembryogenic variety. 32 and 25 kDa proteins were purified from the medium and their identities were determined as already described for 38 kDa beta-1,3-glucanases. To investigate their possible function in somatic embryogenesis, peptide sequences, serological relationships or biochemical properties revealed that there were at least two acidic chitinases of 32 kDa and one glycosylated osmotin-like protein of 25 kDa in the embryogenic culture medium. Comparing the amounts of the 38 kDa glucanases, the 32 kDa chitinases, and the 25 kDa osmotin-like protein present in the conditioned media of the embryogenic '474' hybrid and of a non-embryogenic variety, a 2-8-fold higher accumulation of these proteins was observed in the embryogenic hybrid culture medium. This may suggest that part of the accumulation of these three pathogenesis-related (PR) proteins could be correlated with the somatic embryogenesis process. Their possible involvement in this developmental process is discussed.

Fungal development and induction of defense response genes during early infection of wheat spikes by Fusarium graminearum

Fusarium head blight (FHB) of wheat is a crippling disease that causes severe economic losses in many of the wheat-growing regions of the world. Temporal patterns of fungus development and transcript accumulation of defense response genes were studied in Fusarium graminearum-inoculated wheat spikes within the first 48 to 76 h after inoculation (hai). Microscopy of inoculated glumes revealed that the fungus appeared to penetrate through stomata, exhibited subcuticular growth along stomatal rows, colonized glume parenchyma cells, and sporulated within 48 to 76 hai. No major differences in the timing of these events were found between Sumai 3 (resistant) and Wheaton (susceptible) genotypes. In complementary experiments, RNA was extracted from spikes at several time intervals up to 48 hai and temporal expression patterns were determined for defense response genes encoding peroxidase, PR-1, PR-2 (beta-1,3-glucanase), PR-3 (chitinase), PR-4, and PR-5 (thaumatin-like protein). In both genotypes, transcripts for the six defense response genes accumulated as early as 6 to 12 hai during F. graminearum infection and peaked at 36 to 48 hai. Greater and earlier PR-4 and PR-5 transcript accumulation was observed in Sumai 3, compared with Wheaton. Our results show that the timing of defense response gene induction is correlated with F. graminearum infection.

Induced resistance responses in maize

Systemic acquired resistance (SAR) is a widely distributed plant defense system that confers broad-spectrum disease resistance and is accompanied by coordinate expression of the so-called SAR genes. This type of resistance and SAR gene expression can be mimicked with chemical inducers of resistance. Here, we report that chemical inducers of resistance are active in maize. Chemical induction increases resistance to downy mildew and activates expression of the maize PR-1 and PR-5 genes. These genes are also coordinately activated by pathogen infection and function as indicators of the defense reaction. Specifically, after pathogen infection, the PR-1 and PR-5 genes are induced more rapidly and more strongly in an incompatible than in a compatible interaction. In addition, we show that monocot lesion mimic plants also express these defense-related genes and that they have increased levels of salicylic acid after lesions develop, similar to pathogeninfected maize plants. The existence of chemically inducible disease resistance and PR-1 and PR-5 gene expression in maize indicates that maize is similar to dicots in many aspects of induced resistance. This reinforces the notion of an ancient plant-inducible defense pathway against pathogen attack that is shared between monocots and dicots.

Systemic acquired resistance

This paper examines induced resistance (SAR) in plants against various insect and pathogenic invaders. SAR confers quantitative protection against a broad spectrum of microorganisms in a manner comparable to immunization in mammals, although the underlying mechanisms differ. Discussed here are the molecular events underlying SAR: the mechanisms involved in SAR, including lignification and other structural barriers, pathogenesis-related proteins and their expression, and the signals for SAR including salicylic acid. Recent findings on the biological role of systemin, ethylene, jasmonates, and electrical signals are reviewed. Chemical activators of SAR comprise inorganic compounds, natural compounds, and synthetic compounds. Plants known to exhibit SAR and induced systemic resistance are listed.