Salicylic Acid In Resistance Research Articles

Investigating the mechanisms of isochorismate synthase: An approach to improve salicylic acid synthesis and increase resistance to Fusarium head blight in wheat

Salicylic acid (SA), a vital endogenous hormone, plays a crucial role in plant growth and the response to abiotic and biotic stress. Isochorismate synthase (ICS) and phenylalanine ammonia lyase (PAL) are critical rate-limiting enzymes for SA synthesis. Fusarium head blight (FHB) seriously threatens the safety of wheat production, but increasing the content of SA can enhance FHB resistance. However, the pathway of SA synthesis and regulation in wheat remains unknown. In this study, three wheat ICS (TaICSA, TaICSB, and TaICSD) were identified, and their functions were validated in vitro for isomerizing chorismate to isochorismate. The mutation of one or two homoeoalleles of TaICSA, TaICSB, and TaICSD in the wheat variety ‘Cadenza’ reduced SA levels under ultraviolet treatment and Fusarium graminearum infection, further enhancing sensitivity to FHB. Overexpression of TaICSA can significantly enhance SA levels and resistance to FHB. To further study SA synthesis pathways in wheat and avoid interference with pathogenicity related genes, the leaves of wild-type Cadenza and different TaICS mutant lines were subjected to ultraviolet treatment for transcriptomic analysis. The results showed that 37 PALs might be involved in endogenous SA synthesis, and 82 WRKY and MYB family transcription factors may regulate the expression of ICS and PAL. These results were further confirmed by RT-PCR. In conclusion, this study expands our knowledge of SA biosynthesis and identifies TaICSA, as well as several additional candidate genes that encode transcription factors for regulating endogenous SA levels, as part of an efficient strategy for enhancing FHB resistance in wheat.

Transformation of Salicylic Acid and Its Distribution in Tea Plants (Camellia sinensis) at the Tissue and Subcellular Levels.

Salicylic acid (SA) is a well-known immune-related hormone that has been well studied in model plants. However, less attention has been paid to the presence of SA and its derivatives in economic plants, such as tea plants (Camellia sinensis). This study showed that tea plants were rich in SA and responded differently to different pathogens. Feeding experiments in tea tissues further confirmed the transformation of SA into salicylic acid 2-O-β-glucoside (SAG) and methyl salicylate. Nonaqueous fractionation techniques confirmed that SA and SAG were mostly distributed in the cytosol of tea leaves, consistent with distributions in other plant species. Furthermore, the stem epidermis contained more SA than the stem core both in C. sinensis cv. “Jinxuan” (small-leaf species) and “Yinghong No. 9” (large-leaf species). Compared with cv. “Yinghong No. 9”, cv. “Jinxuan” contained more SAG in the stem epidermis, which might explain its lower incidence rate of wilt disease. This information will improve understanding of SA occurrence in tea plants and provide a basis for investigating the relationship between SA and disease resistance in tea plants.

Hormone response of Lycium barbarum to gall mite Aceria pallida and effects of exogenous salicylic acid on the infestation and development of gall mite

To investigate the hormone responses of Lycium barbarum to Aceria pallida infestation and the effects of exogenous salicylic acid on the infestation and development of gall mite, four phytohormones (salicylic acid (SA), jasmonic acid (JA), Auxin, and abscisic acid (ABA)) of L. barbarum leaves infested with A. pallida were determined by LC-MS/MS. Moreover, the effects of exogenous SA on the growth of gall and mite population were investigated. The results showed that contents of SA and JA in leaves infested with A. pallida were significantly higher (4.0 and 13.0 folds, respectively) than that in control, but that of Auxin and ABA did not change. The growth of gall was inhibited significantly by exogenous SA, with the inhibition increasing with the prolongation of treatment time. Meanwhile, mite population in galls treated with SA was significantly lower than that in control. Our results suggested that A. pallida could induce SA and JA resistance of L. barbarum, and that the exogenous SA could inhibit the growth of gall and mite population. Exogenous SA could be potentially used in A. pallida prevention.

Salicylic Acid-Producing Endophytic Bacteria Increase Nicotine Accumulation and Resistance against Wildfire Disease in Tobacco Plants.

Endophytic bacteria (EB) are both a novel source of bioactive compounds that confer phytopathogen resistance and inducers of secondary metabolites in host plants. Twenty-seven EB isolated from various parts of Metasequoia glyptostroboides, Ginkgo biloba, Taxus brevifolia, Pinus densiflora, Salix babylonica, and S. chaenomeloides could produce salicylic acid (SA). The highest producers were isolates EB-44 and EB-47, identified as Pseudomonas tremae and Curtobacterium herbarum, respectively. Nicotiana benthamiana grown from EB-44-soaked seeds exhibited a 2.3-fold higher endogenous SA concentration and increased resistance against P. syringae pv. tabaci, the causative agent of tobacco wildfire disease, than plants grown from water-soaked seeds. N benthamiana and N. tabacum grown from EB-44-treated seeds developed 33% and 54% disease lesions, respectively, when infected with P. syringae pv. tabaci, and showed increased height and weight, in addition to 4.6 and 1.4-fold increases in nicotine accumulation, respectively. The results suggest that SA-producing EB-44 can successfully colonize Nicotiana spp., leading to increased endogenous SA production and resistance to tobacco wildfire disease. The newly isolated EB can offer an efficient and eco-friendly solution for controlling wildfire disease and nicotine accumulation in Nicotiana, with additional application for other important crops to increase both productivity and the generation of bioactive compounds.

Salicylic acid activates poplar defense against the biotrophic rust fungus Melampsora larici-populina via increased biosynthesis of catechin and proanthocyanidins.

Summary Poplar trees synthesize flavan‐3‐ols (catechin and proanthocyanidins) as a defense against foliar rust fungi, but the regulation of this defense response is poorly understood. Here, we investigated the role of hormones in regulating flavan‐3‐ol accumulation in poplar during rust infection.We profiled levels of defense hormones, signaling genes, and flavan‐3‐ol metabolites in black poplar leaves at different stages of rust infection. Hormone levels were manipulated by external sprays, genetic engineering, and drought to reveal their role in rust fungal defenses.Levels of salicylic acid (SA), jasmonic acid, and abscisic acid increased in rust‐infected leaves and activated downstream signaling, with SA levels correlating closely with those of flavan‐3‐ols. Pretreatment with the SA analog benzothiadiazole increased flavan‐3‐ol accumulation by activating the MYB–bHLH–WD40 complex and reduced rust proliferation. Furthermore, transgenic poplar lines overproducing SA exhibited higher amounts of flavan‐3‐ols constitutively via the same transcriptional activation mechanism. These findings suggest a strong association among SA, flavan‐3‐ol biosynthesis, and rust resistance in poplars. Abscisic acid also promoted poplar defense against rust infection, but likely through stomatal immunity independent of flavan‐3‐ols. Jasmonic acid did not confer any apparent defense responses to the fungal pathogen.We conclude that SA activates flavan‐3‐ol biosynthesis in poplar against rust infection.

Temporal Transcriptional Changes in SAR and Sugar Transport-Related Genes During Wheat and Leaf Rust Pathogen Interactions

Leaf rust (Puccinia triticina Erikss.) is one of the most damaging pathogens of wheat (Triticum aestivum L.). With the rapid evolution of new races, worldwide distribution, and high genetic diversity, P. triticina has the ability to cause severe epidemics in wheat growing areas. In plants, salicylic acid (SA) and sugar-mediated defense pathways are expected to provide durable and broad-spectrum resistance. To understand the role of SA and sugar-mediated resistance mechanisms in wheat during early leaf rust infection, expression profiles of the key regulators of SA (TaEDS1, TaPAD4, TaNDR1, TaRAR1, TaSGT1, TaHSP90, TaEDS5, TaPAL, and TaNPR1) and sugar (TaHTP, TaSTP13A) pathways were analyzed in time-course experiments between two wheat near-isogenic lines (NILs) differing in the leaf rust resistance gene, Lr24. The quantification of candidate gene expression using reverse transcription quantitative real-time PCR at different time points post inoculation showed stage-specific transcriptional reprogramming between compatible and incompatible interactions. Interestingly, two distinct expression patterns were observed between two types of interactions. The genes acting upstream of SA in the SA pathway (TaEDS1, TaPAD4, TaNDR1, TaRAR1, TaSGT1, TaHSP90, TaEDS5) showed strong expressions at a later stage [48 h post inoculation (hpi)] of leaf rust infection in the compatible interaction compared to unchanged or slightly changed expressions in the incompatible interaction. Further, these genes showed similar expression patterns in either of the interactions, suggesting their cooperative or coordinated functions. On the other hand, the genes involved in SA biosynthesis (TaPAL), SA downstream signaling (TaNPR1), and sugar transportation (TaHTP, TaSTP13A) showed a strong expression at mid phase of infection between 6 and 24 hpi in the incompatible interaction compared to the compatible interaction. These expression patterns suggest that TaPAL and TaNPR1 play a positive regulatory role in the SA-mediated resistance pathway whereas TaHTP (Lr67) plays an important role in the sugar-mediated resistance pathway activated by the leaf rust resistance gene, Lr24.

Central role of salicylic acid in resistance of safflower (Carthamus tinctorius L.) against salinity

ABSTRACTThe effects of salicylic acid (SA) on growth parameters and enzyme activities were investigated in salt-stressed safflower (Carthamus tinctorius L.). Twenty-five days after sowing, seedlings were treated with NaCl (0, 100, and 200 mM) and SA (1 mM), and were harvested at 21 days after treatments. Results showed that some growth parameters decreased under salinity, while malondialdehyde (MDA) and hydrogen peroxide (H2O2) content, phenolic compounds, and some enzyme activities increased. SA application increased some growth parameters, MDA and H2O2 content, and enzyme activities except catalase (CAT), which was different from the other enzymes and SA significantly reduced CAT activity in plants. These results suggest that SA-induced tolerance to salinity may be related to regulation of antioxidative responses and H2O2 level. Our study suggested that the resistant safflower can direct reactive oxygen species from a threat to an opportunity by using SA. Therefore, exogenous application of SA played this role through regulation of the antioxidant system.

Role of salicylic acid in resistance to cadmium stress in plants.

We review and introduce the importance of salicylic acid in plants under cadmium stress, and provide insights into potential regulatory mechanisms for alleviating cadmium toxicity. Cadmium (Cd) is a widespread and potentially toxic environmental pollutant, originating mainly from rapid industrial processes, the application of fertilizers, manures and sewage sludge, and urban activities. It is easily taken up by plants, resulting in obvious toxicity symptoms, including growth retardation, leaf chlorosis, leaf and root necrosis, altered structures and ultrastructures, inhibition of photosynthesis, and cell death. Therefore, alleviating Cd toxicity in plants is a major aim of plant research. Salicylic acid (SA) is a ubiquitous plant phenolic compound that has been used in many plant species to alleviate Cd toxicity by regulating plant growth, reducing Cd uptake and distribution in plants, protecting membrane integrity and stability, scavenging reactive oxygen species and enhancing antioxidant defense system, improving photosynthetic capacity. Furthermore, SA functions as a signaling molecule involved in the expression of several important genes. Significant amounts of research have focused on understanding SA functions and signaling in plants under Cd stress, but several questions still remain unanswered. In this article, the influence of SA on Cd-induced stress in plants and the potential regulation mechanism for alleviating Cd toxicity are reviewed.

Upregulated pleiotropic drug resistance genes in Saccharomyces cerevisiae yrr1–52

Gain-of-function mutations of Saccharomyces cerevisiae pleiotropic drug resistance (PDR) genes PDR1, PDR3, and YRR1 cause constitutive high expression of membrane-associated drug efflux pumps, resulting in the excretion of antifungal drugs. We previously identified a novel mutation of YRR1 that conferred salicylic acid (SA) resistance to S. cerevisiae BY4741 cells randomly mutagenized with ethyl methane sulfonate. Yeast cells carrying the yrr1–52 allele activated several drug efflux pumps and exhibited resistance to 4-nitroquinoline-N-oxide and cycloheximide. However, the activated genes and other factors that are directly involved in SA resistance remain unclear. Therefore, microarray analyses of total RNA extracted from yrr1–52 and wild-type cells during the logarithmic phase were performed. Twenty genes demonstrating >2-fold higher expression than the wild-type were selected. Among the upregulated genes, YOR1, SNQ2, AZR1, and FLR1 encode transport proteins previously associated with drug efflux pumps. YOR1 and SNQ2, which encode an ATP-binding cassette (ABC) transporter, were investigated as candidate genes involved in SA resistance. Spot assay results showed no changes in SA susceptibility after disruption of YOR1 or SNQ2 on a background of YRR1 and yrr1–52, indicating that Yor1p and Snq2p are not involved in SA resistance.

Central Role of Salicylic Acid in Resistance of Wheat Against Fusarium graminearum

Head blight caused by Fusarium graminearum (F. graminearum) is one of the major threats to wheat and barley around the world. The importance of this disease is due to a reduction in both grain yield and quality in infected plants. Currently, there is limited knowledge about the physiological mechanisms involved in plant resistance against this pathogen. To reveal the physiological mechanisms underlying the resistance to F. graminearum, spikes of resistant (Sumai3) and susceptible (Falat) wheat cultivars were analyzed 4 days after inoculation, as the first symptoms of pathogen infection appeared. F. graminearum inoculation resulted in a greater induction level and activity of salicylic acid (SA), callose, phenolic compounds, peroxidase, phenylalanine ammonia lyase (PAL), and polyphenol oxidase in resistant versus susceptible cultivars. Soil drench application to spikes of SA, 24 h before inoculation with F. graminearum alleviated Fusarium head blight symptoms in both resistant and susceptible cultivars. SA treated plants showed a significant increment in hydrogen peroxide (H2O2) production, lipid peroxidation, SA, and callose content. SA-induced H2O2 level seems to be related to increased superoxide dismutase and decreased catalase activities. In addition, real-time quantitative PCR analysis showed that SA pretreatment induced expression of PAL genes in both infected and non-infected head tissues of the susceptible and resistant cultivars. Our data showed that soil drench application of SA activates antioxidant defense responses and may subsequently induce systemic acquired resistance, which may contribute to the resistance against F. graminearum. These results provide novel insights about the physiological and molecular role of SA in plant resistance against hemi-biotrophic pathogen infection.

Effect of salicylic acid and methyl jasmonate on phenylalanine ammonia-lyase activity and total phenol in wheat infected by Pratylenchus thornei

The effect of biochemical responses in wheat seedlings cultivar Falat against lesion nematode (Pratylenchus thornei) was investigated by two factors inducer resistance salicylic acid (SA) and methyl jasmonate (MeJA). Foliar sprays with 1 ml SA 1 mM and 1 ml MeJA 100 μM was conducted on 20-day-old wheat (cv. Falat) seedlings. About 24 h after foliar sprays, plants were inoculated by root lesion nematode. Phenylalanine ammonia-lyase (PAL) activity was increased significantly in all treatments two days after inoculation over control. However, PAL activity was decreased in all treatments five days after inoculation in compared to control. What is more, in both time points, total phenol increased in all treatments over control. The comparison of both inducers SA and MeJA on total phenol in healthy and infected wheat seedlings in both days two and five after inoculation of nematode indicated that total phenol has decreased significantly in all treatments in the fifth day.

Salicylic acid resistance is conferred by a novel YRR1 mutation in Saccharomyces cerevisiae

Yeast cells can extrude intracellular drugs through membrane-associated efflux pumps, such as ATP-binding cassette (ABC) transporters and members of the major facilitator superfamily. Gene expression of drug efflux pumps is regulated by several transcription factors involved in pleiotropic drug resistance (PDR). Salicylic acid (SA) possesses weak antifungal activity. Although the excretion mechanisms of some antifungal drugs have been revealed, the mechanism of SA extrusion remains unclear. To elucidate the mechanism of SA excretion, we screened SA-resistant mutants from random mutagenized Saccharomyces cerevisiae BY4741 cells. We successfully isolated 60 SA-resistant clones (KinSal001–060). KinSal052, one of the strongest SA-resistant clones, also exhibited resistance to 4-nitroquinoline-1-oxide and cycloheximide, indicating that it acquired the PDR phenotype. We identified a novel mutation in YRR1 conferring SA resistance to KinSal052. YRR1 encodes a Zn(II)2Cys6-type zinc-finger transcription factor that reportedly activates gene expression involved in PDR. Yeast cells carrying the yrr1 allele (yrr1-52) activated expression of several efflux pump-encoding genes, including YOR1, SNQ2, AZR1, and FLR1. These results suggested that SA resistance in KinSal052 is conferred by the overexpression of efflux pumps constitutively activated by the mutant form of Yrr1p.

The Pseudomonas syringae effector protein HopZ1a suppresses effector‐triggered immunity

*The Pseudomonas syringae pv syringae type III effector HopZ1a is a member of the HopZ effector family of cysteine-proteases that triggers immunity in Arabidopsis. This immunity is dependent on HopZ1a cysteine-protease activity, and independent of known resistance genes. We have previously shown that HopZ1a-triggered immunity is partially additive to that triggered by AvrRpt2. These partially additive effects could be caused by at least two mechanisms: their signalling pathways share a common element(s), or one effector interferes with the response triggered by the other. *Here, we investigate the molecular basis for the partially additive effect displayed by AvrRpt2- and HopZ1a-triggered immunities, by analysing competitive indices, hypersensitive response and symptom induction, PR-1 accumulation, expression of PR genes, and systemic acquired resistance (SAR) induction. *Partially additive effects between these defence responses require HopZ1a cysteine-protease activity, and also take place between HopZ1a and AvrRps4 or AvrRpm1-triggered responses. We establish that HopZ1a-triggered immunity is independent of salicylic acid (SA), EDS1, jasmonic acid (JA) and ethylene (ET)-dependent pathways, and show that HopZ1a suppresses the induction of PR-1 and PR-5 associated with P. syringae pv tomato (Pto)-triggered effector-triggered immunity (ETI)-like defences, AvrRpt2-triggered immunity, and Pto or Pto (avrRpt2) activation of SAR, and that suppression requires HopZ1a cysteine-protease activity. *Our results indicate that HopZ1a triggers an unusual resistance independent of known pathways and suppresses SA and EDS1-dependent resistance.

Salicylic Acid Can Induce Resistance to Plant Virus Movement

Salicylic acid (SA) treatment has recently been reported to inhibit replication of tobacco mosaic virus (TMV) in inoculated tissue. Furthermore, resistance is induced via a novel defensive signal transduction pathway sensitive to inhibition by salicylhydroxamic acid (SHAM; S. Chivasa, A. M. Murphy, M. Naylor, and J. P. Carr, Plant Cell 9: 547–557, 1997). The goals of this study were to determine if replication of viruses other than TMV could be inhibited by SA and, if so, whether the resistance to other viruses could also be prevented by SHAM. Potato virus X (PVX) RNA accumulation in inoculated tobacco leaf tissue was reduced by SA treatment and resistance was dependent on the SHAM-sensitive signaling pathway. However, although symptoms of cucumber mosaic virus (CMV) infection were delayed in SA-treated tobacco plants, this was not due to inhibition of replication but rather to inhibition of systemic movement of the virus.14CO2-feeding experiments indicated that SA-induced interference with long-distance virus movement is not a by-product of disrupted photosynthate translocation. Significantly, SA-induced resistance to CMV was abolished by SHAM. Thus, the SHAM-sensitive signaling pathway activates both resistance mechanisms: inhibition of long-distance CMV movement and inhibition of TMV and PVX replication.

Ensnaring microbes: the components of plant disease resistance

SUMMARYPlants activate an array of co‐ordinated defence responses to restrict microbial attack. Timely perception of foreign molecules appears to be critical for the success of these defences. However, the nature of the molecular events required for resistance is largely unknown. Recent isolation of disease resistance (R) genes, has revealed that R gene products have several features in common. This finding suggests that plants have evolved several common or similar signal transduction pathways to activate resistance to a range of unrelated microbes. R gene isolation and the genetic identification of other loci required for R function permits analysis of the structure and evolution of microbe‐perception mechanisms. Numerous types of activated defence responses are recognized. A requirement for salicylic acid in resistance has been established in some systems. Roles for other events, like reactive oxygen species and the hypersensitive (host cell death) response, remain enigmatic. A thorough understanding of the components leading to pathogen recognition and the expression of resistance should permit the design of novel strategies to engineer broad‐spectrum and durable plant disease control.