Systemic Acquired Resistance In Plants Research Articles

Bacillus subtilis NBRI-W9 simultaneously activates SAR and ISR against Fusarium chlamydosporum NBRI-FOL7 to increase wilt resistance in tomato.

The study aimed to determine the pathogenicity of Fusarium species currently prevalent in tomato fields having history of chemical fungicide applications and determine the bio-efficacy of Bacillus subtilis NBRI-W9 as a potent biological control agent. Fusarium was isolated from surface-sterilized infected tomato plants collected from fields. Pathogenicity of 30 Fusarium isolates was determined by in vitro and in vivo assays. Following Koch's postulates, F. chlamydosporum (FOL7) was identified as a virulent pathogen. The biological control of FOL 7 by B. subtilis NBRI-W9 (W9) and the colonization potential of W9 were established using spontaneous rifampicin-resistant mutants. W9 showed 82% inhibition of FOL7 on a dual-culture plate and colonization levels in tomato plants of ∼5.5, ∼3.3, and ∼2.2 log10 CFU/g in root, stem, and leaf tissue, respectively. Antagonistic activity was shown by scanning electron microscopy (SEM) and cell-wall-degradative enzymes. W9 reduced FOL7 infection in net-house and field experiments by 60% and 41%, respectively. Biochemical investigation, defence enzymes, defence gene expression analysis, SEM, and field studies provide evidence of hyperparasitism and induced resistance as the mode of biological control. The study also demonstrates that the potent biocontrol agent W9, isolated from Piper, can colonize tomato plants, control fungal disease by inducing induced systemic resistance (ISR) and systemic acquired resistance (SAR) simultaneously, and increase crop yield by 21.58% under field conditions. This study concludes that F. chlamydosporum (NBRI-FOL7) is a potent, fungicide-resistant pathogen causing wilt in tomatoes. NBRI-W9 controlled FOL7 through mycoparasitism and simultaneously activated ISR and SAR in plants, providing an attractive tool for disease control that acts at multiple levels.

Plant Defense Responses to a Novel Plant Elicitor Candidate LY5-24-2.

Plant elicitors enhance plant defense against pathogen attacks by inducing systemic acquired resistance (SAR) with no or low direct fungicidal activity. Here we report the synthesis of a novel plant elicitor candidate LY5-24-2 [3,4-dichloro-N-(3-chloro-5-(trifluoromethyl)pyridin-2-yl)isothiazole-5-carboxamide] and evaluation of its SAR inducing activity. Bioassays indicated that LY5-24-2 did not show significant anti-fungal activity but provided long-lasting resistance in Arabidopsis thaliana (A. thaliana) through promoting the accumulation of lignin, cellulose and pectin by 60.1%, 82.4% and 305.6%, respectively, at a concentration of 100 µM. LY5-24-2 also facilitated the closure of leaf stomata and increased the intracellular free Ca2+ by 47.8%, induced reactive oxygen species (ROS) accumulation, and inhibited the activity of ascorbate peroxidase (APX, EC 1.11.1.11) and catalase (CAT, EC 1.11.1.6) by 38.9% and 34.0%, respectively, as compared with the control at a concentration of 100 µM. LY5-24-2 induced SAR in plants and was dependent on the NPR1-mediated SA pathway by up-regulating expression of 2273 genes in A. thaliana. Meanwhile, LY5-24-2 also improved cucumber (Cucumis sativus L.) defense against Pseudoperonospora cubensis (P. cubensis) through promoting ROS accumulation and inhibiting activity of APX and CAT by 30.7% and 23.1%, respectively. Its expression of SA signaling genes CsNPR1, CsPR4 and CsPR5 was enhanced by 10.8, 5.8 and 6.6 times, respectively. These results demonstrated that LY5-24-2 is a novel elicitor candidate for plant protection via inducing SAR.

Recombinant Protein Foliar Application Activates Systemic Acquired Resistance and Increases Tolerance against Papaya Dieback Disease

Similar to animals, plants possess ‘immune memory’ in response to invading pathogens that lead to enhanced defense reaction following pathogen exposure. Systemic acquired resistance (SAR) is a well-characterized type of plant immunity and is associated with coordinated expression of a set of pathogenesis-related (PR) genes and proteins also known as SAR markers. Induction of SAR in plants was shown to be initiated by group of chemicals and biological compounds known as SAR inducers that can be used for the management of important plant diseases. Elucidation and characterization of potential SAR inducers as potential elicitors that can protect papaya from the papaya dieback disease pathogen were carried out using HRPX protein, which was produced as a recombinant protein in an Escherichia coli system. Disease severity analysis in a glasshouse experiment indicated lower disease infection rates in the HRPX-treated plants than in water-treated plants. Selected SAR-associated defense gene expression was also shown to increase in treated plants, via quantitative real-time PCR analysis, confirming enhanced disease response through SAR activation. In this report, the selected recombinant protein was shown to activate the SAR mechanism in papaya for increased tolerance against papaya dieback disease, which was proven via physiological and molecular analysis.

Editorial: Salicylic Acid Signaling Networks

Editorial: Salicylic Acid Signaling Networks

Systemic acquired resistance in moss: further evidence for conserved defense mechanisms in plants.

Vascular plants possess multiple mechanisms for defending themselves against pathogens. One well-characterized defense mechanism is systemic acquired resistance (SAR). In SAR, a plant detects the presence of a pathogen and transmits a signal throughout the plant, inducing changes in the expression of various pathogenesis-related (PR) genes. Once SAR is established, the plant is capable of mounting rapid responses to subsequent pathogen attacks. SAR has been characterized in numerous angiosperm and gymnosperm species; however, despite several pieces of evidence suggesting SAR may also exist in non-vascular plants6–8, its presence in non-vascular plants has not been conclusively demonstrated, in part due to the lack of an appropriate culture system. Here, we describe and use a novel culture system to demonstrate that the moss species Amblystegium serpens does initiate a SAR-like reaction upon inoculation with Pythium irregulare, a common soil-borne oomycete. Infection of A. serpens gametophores by P. irregulare is characterized by localized cytoplasmic shrinkage within 34 h and chlorosis and necrosis within 7 d of inoculation. Within 24 h of a primary inoculation (induction), moss gametophores grown in culture became highly resistant to infection following subsequent inoculation (challenge) by the same pathogen. This increased resistance was a response to the pathogen itself and not to physical wounding. Treatment with β-1,3 glucan, a structural component of oomycete cell walls, was equally effective at triggering SAR. Our results demonstrate, for the first time, that this important defense mechanism exists in a non-vascular plant, and, together with previous studies, suggest that SAR arose prior to the divergence of vascular and non-vascular plants. In addition, this novel moss – pathogen culture system will be valuable for future characterization of the mechanism of SAR in moss, which is necessary for a better understanding of the evolutionary history of SAR in plants.

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.

Glycerol-3-phosphate and systemic immunity

Glycerol-3-phosphate (G3P), a conserved three-carbon sugar, is an obligatory component of energy-producing reactions including glycolysis and glycerolipid biosynthesis. G3P can be derived via the glycerol kinase-mediated phosphorylation of glycerol or G3P dehydrogenase (G3Pdh)-mediated reduction of dihydroxyacetone phosphate. Previously, we showed G3P levels contribute to basal resistance against the hemibiotrophic pathogen, Colletotrichum higginsianum. Inoculation of Arabidopsis with C. higginsianum correlated with an increase in G3P levels and a concomitant decrease in glycerol levels in the host. Plants impaired in GLY1 encoded G3Pdh accumulated reduced levels of G3P after pathogen inoculation and showed enhanced susceptibility to C. higginsianum. Recently, we showed that G3P is also a potent inducer of systemic acquired resistance (SAR) in plants. SAR is initiated after a localized infection and confers whole-plant immunity to secondary infections. SAR involves generation of a signal at the site of primary infection, which travels throughout the plants and alerts the un-infected distal portions of the plant against secondary infections. Plants unable to synthesize G3P are defective in SAR and exogenous G3P complements this defect. Exogenous G3P also induces SAR in the absence of a primary pathogen. Radioactive tracer experiments show that a G3P derivative is translocated to distal tissues and this requires the lipid transfer protein, DIR1. Conversely, G3P is required for the translocation of DIR1 to distal tissues. Together, these observations suggest that the cooperative interaction of DIR1 and G3P mediates the induction of SAR in plants.

Glycerol-3-phosphate is a critical mobile inducer of systemic immunity in plants

Glycerol-3-phosphate (G3P) is an important metabolite that contributes to the growth and disease-related physiologies of prokaryotes, plants, animals and humans alike. Here we show that G3P serves as the inducer of an important form of broad-spectrum immunity in plants, termed systemic acquired resistance (SAR). SAR is induced upon primary infection and protects distal tissues from secondary infections. Genetic mutants defective in G3P biosynthesis cannot induce SAR but can be rescued when G3P is supplied exogenously. Radioactive tracer experiments show that a G3P derivative is translocated to distal tissues, and this requires the lipid transfer protein, DIR1. Conversely, G3P is required for the translocation of DIR1 to distal tissues, which occurs through the symplast. These observations, along with the fact that dir1 plants accumulate reduced levels of G3P in their petiole exudates, suggest that the cooperative interaction of DIR1 and G3P orchestrates the induction of SAR in plants.

Induction of Flowering by Inducers of Systemic Acquired Resistance in theLemnaPlant

In plants, salicylic acid induces both systemic acquired resistance (SAR) and flowering. We found that flower induction in the Lemna species was promoted by reactive oxygen species-generating conditions and SAR inducers. The present paper suggests that SAR in plants and flowering in Lemna share common inducers.

Vitamin B1 functions as an activator of plant disease resistance.

Vitamin B(1) (thiamine) is an essential nutrient for humans. Vitamin B(1) deficiency causes beriberi, which disturbs the central nervous and circulatory systems. In countries in which rice (Oryza sativa) is a major food, thiamine deficiency is prevalent because polishing of rice removes most of the thiamine in the grain. We demonstrate here that thiamine, in addition to its nutritional value, induces systemic acquired resistance (SAR) in plants. Thiamine-treated rice, Arabidopsis (Arabidopsis thaliana), and vegetable crop plants showed resistance to fungal, bacterial, and viral infections. Thiamine treatment induces the transient expression of pathogenesis-related (PR) genes in rice and other plants. In addition, thiamine treatment potentiates stronger and more rapid PR gene expression and the up-regulation of protein kinase C activity. The effects of thiamine on disease resistance and defense-related gene expression mobilize systemically throughout the plant and last for more than 15 d after treatment. Treatment of Arabidopsis ecotype Columbia-0 plants with thiamine resulted in the activation of PR-1 but not PDF1.2. Furthermore, thiamine prevented bacterial infection in Arabidopsis mutants insensitive to jasmonic acid or ethylene but not in mutants impaired in the SAR transduction pathway. These results clearly demonstrate that thiamine induces SAR in plants through the salicylic acid and Ca(2+)-related signaling pathways. The findings provide a novel paradigm for developing alternative strategies for the control of plant diseases.

Abnormal callose response phenotype and hypersusceptibility to Peronospoara parasitica in defence-compromised arabidopsis nim1-1 and salicylate hydroxylase-expressing plants.

To investigate the impact of induced host defenses on the virulence of a compatible Peronospora parasitica strain on Arabidopsis thaliana, we examined growth and development of this pathogen in nim1-1 mutants and transgenic salicylate hydroxylase plants. These plants are unable to respond to or accumulate salicylic acid (SA), respectively, are defective in expression of systemic acquired resistance (SAR), and permit partial growth of some normally avirulent pathogens. We dissected the P. parasitica life cycle into nine stages and compared its progression through these stages in the defense-compromised hosts and in wild-type plants. NahG plants supported the greatest accumulation of pathogen biomass and conidiophore production, followed by nim1-1 and then wild-type plants. Unlike the wild type, NahG and nim1-1 plants showed little induction of the SAR gene PR-1 after colonization with P parasitica, which is similar to our previous observations. We examined the frequency and morphology of callose deposits around parasite haustoria and found significant differences between the three hosts. NahG plants showed a lower fraction of haustoria surrounded by thick callose encasements and a much higher fraction of haustoria with callose limited to thin collars around haustorial necks compared to wild type, whereas nim1-1 plants were intermediate between NahG and wild type. Chemical induction of SAR in plants colonized by P. parasitica converted the extrahaustorial callose phenotype in NahG to resemble closely the wild-type pattern, but had no effect on nim1-1 plants. These results suggest that extrahaustorial callose deposition is influenced by the presence or lack of SA and that this response may be sensitive to the NIM1/NPR1 pathway. Additionally, the enhanced susceptibility displayed by nim1-1 and NahG plants shows that even wild-type susceptible hosts exert defense functions that reduce disease severity and pathogen fitness.

Cultured parsley cells, a model system for the rapid testing of abiotic and natural substances as inducers of systemic acquired resistance

In cultured parsley (Petroselinum crispumL.) cells, chemical and natural agents that are known to induce systemic acquired resistance (SAR) in different plant species were checked for their ability to condition the cells for an enhanced response to low doses of an elicitor from the cell wall of the fungal pathogenPhytophthora sojae. Use of the fluorescence properties of elicitor-induced furanocoumarin phytoalexins in the culture fluids of parsley cells allows rapid testing of many substances over a broad range of concentrations for their SAR-inducing capacity. Except for probenazole, a number of agents which have been described as effective inducers of disease resistance in plants were able to activate cells for an enhanced elicitor-mediated furanocoumarin accumulation. Among the compounds tested, benzo[1,2,3]thiadiazole-7-carbothioic acid-S-methyl ester, the active ingredient of the commercially available SAR activator Bion®, was found to have the highest potential for conditioning parsley cells. On the other hand, compounds which are known to be inactive as inducers of disease resistance failed to enhance the elicitor response. The results suggest that cultured parsley cells provide a suitable system for the identification of SAR-inducing agents.Using the parsley test system, the artificial sweetener saccharin, a substance not previously reported to induce SAR in plants, was found to be very effective in conditioning the cells for an enhanced accumulation of coumarin derivatives. Furthermore, when applied to cucumber, tobacco and bean plants, saccharin was found to induce resistance against both fungal and viral diseases.

Intermediates of salicylic acid biosynthesis in tobacco

Salicylic acid (SA) is an important component of systemic-acquired resistance in plants. It is synthesized from benzoic acid (BA) as part of the phenylpropanoid pathway. Benzaldehyde (BD), a potential intermediate of this pathway, was found in healthy and tobacco mosaic virus (TMV)-inoculated tobacco (Nicotiana tabacum L. cv Xanthi-nc) leaf tissue at 100 ng/g fresh weight concentrations as measured by gas chromatography-mass spectrometry. BD was also emitted as a volatile organic compound from tobacco tissues. Application of gaseous BD to plants enclosed in jars caused a 13-fold increase in SA concentration, induced the accumulation of the pathogenesis-related transcript PR-1, and increased the resistance of tobacco to TMV inoculation. [13C6]BD and [2H5]benzyl alcohol were converted to BA and SA. Labeling experiments using [13C1]Phe in temperature-shifted plants inoculated with the TMV showed high enrichment of cinnamic acids (72%), BA (34%), and SA (55%). The endogenous BD, however, contained nondetectable enrichment, suggesting that BD was not the intermediate between cinnamic acid and BA. These results show that BD and benzyl alcohol promote SA accumulation and expression of defense responses in tobacco, and provide insight into the early steps of SA biosynthesis.

Signalling molecules and the synthesis of alkaloids in Catharanthus roseus seedlings

The Madagascar periwinkle, Catharanthus roseus, produces numerous alkaloids, several of which have important pharmaceutical uses. Catharanthus seedlings rapidly accumulate the monomeric alkaloids, vindoline, catharanthine and tabersonine, during germination. Various plant signalling molecules were tested for their ability to enhance alkaloid synthesis in Catharanthus seedlings. The compounds tested included plant hormones, fatty acid-derived messengers and agents that can induce systemic-acquired resistance in plants. Of these compounds, only methyl jasmonate (MeJa) enhanced the synthesis of monomeric alkaloids. However, feeding of a MeJa biosynthetic precursor, or inhibition of the lipoxygenase pathway, had no effect on alkaloid production in the seedlings. We conclude that, although MeJa is able to enhance alkaloid synthesis when supplied exogenously, the lipoxygenase pathway probably does not play a role in the regulation of alkaloid synthesis during normal germination of Catharanthus. Furthermore, it was found that Catharanthus seedlings accumulate small quantities of the dimeric alkaloid, vinblastine, a valuable antitumour drug. Contrary to its effect on the accumulation of monomeric alkaloids, MeJa did not influence the accumulation of vinblastine.