Induction of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> Resistance to Pathogenic Bacteria by Lipopolysaccharide and Salicylic Acid

The effects of combined treatment with an elicitor (lipopolysaccharide) and a signaling molecule (salicylic acid) on the disease resistance of wild-type (Col-0) and mutant Arabidopsis thaliana L. plants have been compared. The mutant lines used were jin1 (with impaired jasmonate signaling), npr1 (lacking expression of pathogen-dependent PR genes), and NahG (expressing an active bacterial salicylate hydroxylase transgene). The lipopolysaccharide was isolated from a saprophytic strain (8614) of Pseudomonas aeruginosa bacteria. Treatment of A. thaliana seeds with a composite preparation (lipopolysaccharide and salicylic acid–SA) increased the resistance of seedlings to a subsequent infection by the pathogenic 9096 strain of P. aeruginosa bacteria. The protective effect was more pronounced in jin1 mutant seedlings, which was indicative of the possible compensation of jasmonate signaling impairment due to activation of the SA-dependent signaling pathway. We concluded that a preparation composed of an elicitor and a signaling molecule could affect regulatory mechanism functioning in a plant cell and, in particular, compensate for the absence of a certain signaling pathway by activating another.     

The Role of MPK6 as Mediator of Ethylene/Jasmonic Acid Signaling in <Emphasis Type="Italic">Serendipita indica</Emphasis>-Colonized Arabidopsis Roots

Serendipita indica is an axenically cultivable fungus, which colonizes a broad range of plant species including the model plant Arabidopsis thaliana. Root colonization by this endophyte leads to enhanced plant fitness and performance and promotes resistance against different biotic and abiotic stresses. The involvement of MPK6 in this mutualistic interaction had been previously shown with an mpk6 A. thaliana mutant, which failed to respond to S. indica colonization. Here, we demonstrate that mpk6 roots are significantly less colonized by S. indica compared to wild-type roots and the foliar application of plant hormones, ethylene, or jasmonic acid, restores the colonization rate at least to the wild-type level. Further, hormone-treated mpk6 plants show typical S. indica-induced growth promotion effects. Moreover, expression levels of several genes related to plant defense and hormone signaling are significantly changed at different colonization phases. Our results demonstrate that the successful root colonization by S. indica depends on efficient suppression of plant immune responses. In A. thaliana, this process relies on intact hormone signaling in which MPK6 seems to play a pivotal role.     

Abnormal Glycosphingolipid Mannosylation Triggers Salicylic Acid–Mediated Responses in Arabidopsis

The Arabidopsis thaliana protein GOLGI-LOCALIZED NUCLEOTIDE SUGAR TRANSPORTER (GONST1) has been previously identified as a GDP-d-mannose transporter. It has been hypothesized that GONST1 provides precursors for the synthesis of cell wall polysaccharides, such as glucomannan. Here, we show that in vitro GONST1 can transport all four plant GDP-sugars. However, gonst1 mutants have no reduction in glucomannan quantity and show no detectable alterations in other cell wall polysaccharides. By contrast, we show that a class of glycosylated sphingolipids (glycosylinositol phosphoceramides [GIPCs]) contains Man and that this mannosylation is affected in gonst1. GONST1 therefore is a Golgi GDP-sugar transporter that specifically supplies GDP-Man to the Golgi lumen for GIPC synthesis. gonst1 plants have a dwarfed phenotype and a constitutive hypersensitive response with elevated salicylic acid levels. This suggests an unexpected role for GIPC sugar decorations in sphingolipid function and plant defense signaling. Additionally, we discuss these data in the context of substrate channeling within the Golgi.     

Trichoderma-induced plant immunity likely involves both hormonal- and camalexin-dependent mechanisms in Arabidopsis thaliana and confers resistance against necrotrophic fungus Botrytis cinerea

Filamentous fungi belonging to the genus Trichoderma have long been recognized as agents for the biocontrol of plant diseases. In this work, we investigated the mechanisms involved in the defense responses of Arabidopsis thaliana seedlings elicited by co-culture with Trichoderma virens and Trichoderma atroviride. Interaction of plant roots with fungal mycelium induced growth and defense responses, indicating that both processes are not inherently antagonist. Expression studies of the pathogenesis-related reporter markers pPr1a:uidA and pLox2:uidA in response to T. virens or T. atroviride provided evidence that the defense signaling pathway activated by these fungi involves salicylic acid (SA) and/or jasmonic acid (JA) depending on the amount of conidia inoculated. Moreover, we found that Arabidopsis seedlings colonized by Trichoderma accumulated hydrogen peroxide and camalexin in leaves. When grown under axenic conditions, T. virens produced indole-3-carboxaldehyde (ICAld) a tryptophan-derived compound with activity in plant development. In Arabidopsis seedlings whose roots are in contact with T. virens or T. atroviride, and challenged with Botrytis cinerea in leaves, disease severity was significantly reduced compared with axenically grown seedlings. Our results indicate that the defense responses elicited by Trichoderma in Arabidopsis are complex and involve the canonical defense hormones SA and JA as well as camalexin, which may be important factors in boosting plant immunity.Key words: Arabidopsis, Trichoderma, phytostimulation, defense responses, jasmonic acid, salicylic acid, camalexin     

Quantitation of ascorbic acid in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> reveals distinct differences between organs and growth phases

Optimal plant growth is the result of the interaction of a complex network of plant hormones and environmental signals. Ascorbic acid (AsA) is a crucial antioxidant in plants and is involved in the regulation of cell division, cell expansion, photosynthesis and hormone biosynthesis. Quantitative analysis of AsA in Arabidopsis thaliana organs was conducted using HPLC with d-isoascorbic acid (Iso-AsA) as an internal standard. Analysis revealed fluctuations in the levels of AsA in different organs and growth phases when plants were grown under standard conditions. AsA concentrations increased in leaves in direct proportion to leaf size and age. Young siliques (seed set stage) and flowering buds (open and unopened) showed the highest levels of AsA. A relationship was found between the level of AsA and indole acetic acid (IAA) in leaves, stems, flowers, and siliques and the highest level of IAA and AsA were found in the flowers. In contrast, the lowest level of the plant hormone, salicylic acid, was found in the flowers, and the highest quantity measured in the leaves. Consequently, AsA has been found to be a multifunctional molecule that is involved as a key regulator of plant growth and development.     

A comparative study of the early osmotic,ionic, redox and hormonal signaling response in leaves and roots of two halophytes and a glycophyte to salinity

Salt stress is one of the most important abiotic stress factors affecting plant growth and productivity in natural ecosystems. In this study, we aimed at determining possible differences between salt tolerant and salt sensitive species in early (within 72 h) salt stress response in leaves and roots. To this purpose, we subjected three Brassicaceae species, namely two halophytes—Cakile maritima and Thellungiella salsuginea—and a glycophyte—Arabidopsis thaliana— to short-term salt stress (400 mM NaCl). The results indicate that the halophytes showed a differential osmotic and ionic response together with an early and transient oxidative burst, which was characterized by enhanced hydrogen peroxide levels and subsequent activation of antioxidant defenses in both leaves and roots. In addition, the halophytes displayed enhanced accumulation of abscisic acid, jasmonic acid (JA) and ACC (aminocyclopropane-1-carboxylic acid, the precursor of ethylene) in leaves and roots, as compared to A. thaliana under salt stress. Moreover, the halophytes showed enhanced expression of ethylene response factor1 (ERF1), the convergence node of the JA and ethylene signaling pathways in both leaves and roots upon exposure to salt stress. In conclusion, we show that the halophytes C. maritima and T. salsuginea experience an early oxidative burst, improved antioxidant defenses and hormonal response not only in leaves but also in roots, in comparison to the glycophyte A. thaliana. This differential signaling response converging, at least in part, into increased ERF1 expression in both above- and underground tissues seems to underlay, at least in part, the enhanced tolerance of the two studied halophytes to salt stress.     

Influence of protein kinase KIN10 gene expression on root phenotype of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> root system under condition of energy stress

The phenotypic changes in the root system of Arabidopsis thaliana seedlings in transgenic lines with overexpression and suppressed gene expression of serine-threonine protein kinase KIN10, under conditions of energy shortage and under normal conditions, were shown. The normal growth and development of KIN10 overexpressing plants under energy deficiency conditions were detected. The significant inhibition of the development of these plant lines was observed under normal conditions. The levels of KIN10 gene expression under normal conditions in different organs of Arabidopsis thaliana, particularly in the roots, stems, leaves and flowers were analyzed. The highest-level expression of the gene was found in the leaves.     

Cross-talk between gibberellins and salicylic acid in early stress responses in Arabidopsis thaliana seeds

Salicylic acid (SA) is a plant hormone mainly associated with the induction of defense mechanism in plants, although in the last years there is increasing evidence on the role of SA in plant responses to abiotic stress. We recently reported that an increase in endogenous SA levels are able to counteract the inhibitory effects of several abiotic stress conditions during germination and seedling establishment of Arabidopsis thaliana and that this effect is modulated by gibberellins (GAs) probably through a member of the GASA (Giberellic Acid Stimulated in Arabidopsis) gene family, clearly showing the existence of a cross talk between these two plant hormones in Arabidopsis.Key words: abiotic stress responses, Arabidopsis thaliana, gibberellins, hormone cross-talk, salicylic acidGAs and SA play important roles in many processes of plant growth and development, and despite the recent papers reporting the existence of a complex network of hormone interactions, evidences of a cross talk between these two plant hormones have been very scarce.^1,2 These authors indicate that GAs are able to regulate SA biosynthesis during plant responses to pathogens. Interestingly, ABA has recently been proved to negative regulate SA-mediated defenses by downregulating SA biosynthesis.³ These data are consistent with the well known ABA/GAs antagonistic regulation of many aspects of plant development, such as seed dormancy or germination.^4,5 Thus, it seems clear that ABA and GAs are able to control plant immune responses by modulating the levels of salicylic acid and/or jasmonic acid.^1–3 In addition to the role of GAs in the regulation of plant responses to biotic stress, we have recently documented a role of GAs in early plant abiotic stress responses in Arabidopsis through modulation of SA levels,⁶ hormone that been involved in responses to abiotic stress conditions.⁷ For instance, it has been proved that SA has an important role in heat stress responses⁸ or in the improved germination of Arabidopsis thaliana seeds under salt stress conditions.⁹We showed that GAs and the overexpression of a GA-responsive gene were able to increase not only endogenous levels of SA, but also the expression of ics1 and npr1 genes, involved in SA biosynthesis and action, respectively.⁶ In addition, we have also analyzed expression levels of other genes that have been reported as SA-regulated. For instance, isocitrate lyase, a key enzyme involved in lipid metabolism during seed germination¹⁰ and a good marker of seed vigor under stress conditions,¹¹ was found to be induced by SA in germinated seeds of Arabidopsis thaliana.⁹ Thus, we proved that the expression of isocitrate lyase was upregulated in GASA4 overexpressing lines, and after exogenous application of GA₃ (Fig. 1), both situations increasing endogenous SA levels.⁶ We have documented that SA may have a role in some of the physiological processes associated with GAs, since exogenous application of SA was able to both revert the inhibitory effect of PCB on seed germination and improve germination of the GA-deficient mutant ga1–3.⁶ Thus, we can hypothesize that the GA-mediated induction of isocitrate lyase gene observed in Arabidopsis thaliana is the result of the increased levels of SA detected either after overexpression of the GA-induced GASA4 gene in Arabidopsis or after exogenous application of gibberellic acid. In other words, GAs are able to induce the expression of isocitrate lyase gene in a SA-dependent manner, producing the establishment of a vigorous seedling.⁹ These data support the idea that GAs may have an important role in SA biosynthesis and action, and that some of the physiological effects of this hormone may be mediate by SA. In summary, our results clearly show the existence of a cross talk between these two plant hormones during Arabidopsis thaliana seeds germination and early seedling growth under abiotic stress conditions, showing another junction in the complex mechanism of hormone interactions.Open in a separate windowFigure 1(A) Expression of the isocitrate lyase gene in FsGASA-overexpressing plants (G1 to G3) compared to Col-0. (B) Expression of the isocitrate lyase gene in Arabidopsis seedlings treated or not with 100 µM GA₃. mRNA levels were determined by northern blot analysis using total RNAs (10 µg/line) isolated from 7 d-old seedlings. Bottom, ethidium bromide stained gels showing rRNAs. Top: quantification of hybridization signals obtained by using a phosphoimage scanner. Data were normalized to the rRNA value. Blots were repeated twice and yielded similar results.     

Root growth is affected differently by mechanical wounding in seedlings of the ecological model species Nicotiana attenuata and the molecular model species Arabidopsis thaliana

In the ecological model plant Nicotiana attenuata, leaf wounding or herbivory lead to a reduction of root growth via jasmonic acid (JA) signaling. A single wounding treatment is sufficient to induce this response; multiple wounding does not increase the plant growth reaction. in a recent study, in which JA bursts were elicited in leaves of the molecular model species Arabidopsis thaliana in different ways,¹ we tested whether JA induces the same response there. Root growth reduction was neither induced by foliar application of herbivore oral secretions nor by direct application of methyl jasmonate to leaves. Root growth reduction was observed when leaves were infected with the pathogen Pseudomonas syringae pv. tomato, which persistently induces the JA signaling pathway. Yet, growth analyses of this effect in wild type and JA-signaling mutants showed that it was elicited by the bacterial toxin coronatine which suggests ethylene—but not JA-induced root growth reduction in A. thaliana. Moreover, the growth effects were somewhat masked by a light-induced diurnal decrease of root growth. Overall, we conclude that the reaction of root growth to herbivore-induced JA signaling differs among species, which is related to different ecological defence strategies that have evolved in different species.Key words: coronatine, ethylene, image analysis, phytohormones, Pseudomonas syringae pv. tomato, woundingUpon pathogen or herbivore attack, plants have to meet the decision how much of their resources are invested in growth processes and how much into defense. The ecological model species Nicotiana attenuata increases defence measures and decreases root, but not leaf growth immediately after a single simulated herbivory event.² This reaction is elucidated via jasmonic acid (JA) signaling.³ The intensity of root growth reduction is not amplified when multiple wounding events occur (Fig. 1A). This clearly demonstrates that wounding acts as a signal for the reduction of root growth and that root growth is not reduced due to a lack of growth resources as a consequence of a resource-based trade-off between growth and defence. This hypothesis is further supported by the finding that a surplus of carbohydrates is stored in the root system,⁴ which thereby acts as a safe retreat for future re-growth of the plant after herbivore damage.Open in a separate windowFigure 1Root growth in Nicotiana attenuata and Arabidopsis thaliana seedlings. (A) Root growth dynamics of Nicotiana attenuata seedlings after single and multiple wounding treatments as well as multiple wounding treatments followed by application of oral secretions of Manduca sexta (OS). Wounding treatments were applied at time points 0 h (single treatments) or at the time points 0 h, 2 h and 4 h (multiple treatments). Controls were not treated. (B) Normalized values of velocity of the root tip (v_Tip) of Arabidopsis thaliana seedlings whose roots were exposed to light (control and wounded) and seedlings whose roots were darkened by wrapping aluminium foil around the Petri dish throughout the growth period. Shaded areas indicate the night period. Mean ± SE. N = 4–8.We asked ourselves whether this is a general reaction pattern that is followed in more plant species. To test this, we performed a suite of experiments on the molecular model species Arabidopsis thaliana.¹ Several studies showed that direct application of JA or methyl jasmonate (MeJA), which is commonly used to mimick herbivory-induced signaling, to the cultivation medium decrease root growth of A. thaliana. Yet, in contrast to the situation in N. attenuata, the application of MeJA to leaves did not lead to a decrease in root growth. To exclude the possibility that the MeJA applied to the leaf was not taken up by the plants, we induced plant-internal JA bursts by mechanical wounding and/or application of bacteria. The treatments were performed on Col-0 and Col-6 wild type plants. Additionally, two mutants defective in the JA signaling pathway were used to select for JA-induced effects. coi1-1 (coronatine-insensitive) is known to lack the F-box protein COI1 and shows decreased sensitivity to JA application compared to wild type plants.⁵ The aos mutant, in contrast, is unable to produce JA following mechanical wounding as the biosynthesis of the rate-limiting enzyme allene oxide synthase is blocked.⁶Upon mechanical wounding of two leaves with sterile tweezers, JA concentration in the seedlings increased and root growth decreased rapidly, but only very transiently in all four investigated A. thaliana lines. In contrast to the situation in N. attenuata, root growth in A. thaliana recovered to pre-treatment levels within a few hours (Fig. 1B) and growth was not further decreased upon addition of oral secretions of Spodoptera littoralis larvae. This suggests that the observed short-term growth reduction was caused by hydraulic decrease of the plant growth potential. A slight, but continuous decrease of root growth during the day was noted both in wounded and in control plants that were not completely protected from ambient light in the transparent Petri dishes. When root systems were completely protected from ambient light by shading, root growth was almost steady throughout 24 h (Fig. 1B).In another experimental approach to clarify the connection between JA signaling and root growth reduction, we infected leaves with the avirulent Pseudomonas syringae pv. tomato (Pst) DC3000 avrRpt2 strain. Upon mechanical wounding and application of bacterial suspension in order to facilitate infection, root growth decreased more rapidly than upon mere wounding. In the course of two days after infection, v_Tip was lower in the wild types and the aos mutant suggesting that JA was not the major reason of the decrease of root growth. With Pst DC3000 deficient in coronatine biosynthesis, it was verified that the bacterial toxin was the major reason of the root growth reduction following Pst infection. Using the ethylene reception blocker 1-methyl cyclopropene (1-MCP), ethylene was also figured out to be involved in coronatine-mediated root growth impairment in Arabidopsis. Thus, root growth of Arabidopsis is more sensitive to ethylene than to JA which is very different to observations on N. attenuata.The conclusion has to be drawn that elicitation of JA-bursts in the leaves of A. thaliana does not induce the same root growth reactions as in N. attenuata, although roots of both species react towards MeJA externally applied to the cultivation medium. This in turn demonstrates clearly that the interpretation of the JA signal differs between species. Possibly, this reflects different survival strategies to which the two investigated annual rosette species have evolved. While N. attenuata uses the root as a safe retreat for resources allowing later re-growth after the herbivore threat has passed by, A. thaliana is more successful in its ecological niche if it does not slow down growth in response to herbivory but continues its development as rapidly as possible.     

Cloning,characterization and functional analysis of a flavonol synthase from <Emphasis Type="Italic">Vaccinium corymbosum</Emphasis>

Key message

VcFLS from Vaccinium corymbosum promoted myricetin biosynthesis in Arabidopsis thaliana and VcFLS expression was induced by salicylic acid.

Abstract

Flavonoids are polyphenols with important functions in pigmentation, UV filtration, and symbiotic nitrogen fixation. Flavonols are a class of flavonoids that are produced by the desaturation of dihydroflavanols in a reaction that is catalyzed by flavonol synthase (FLS). In the study reported here, we cloned the full-length cDNA of FLS (designated as VcFLS) from Vaccinium corymbosum (blueberry) using rapid amplification of cDNA ends (RACE). The cDNA contained a 1005-bp open reading frame that encoded a 334-amino acid protein. Phylogenetic analysis showed that VcFLS was closely related to FaFLS, a flavonol synthase that catalyzed the formation of kaempferol and had little effect on the formation of quercetin. Quantitative RT-PCR analysis demonstrated that VcFLS was expressed in all of the tissues tested, with particularly high expression in the petals and young leaves (both green and red). The flavanols myricetin and quercetin also occurred in all of these tested tissues, with the highest levels detected in mature leaves. The expression of VcFLS was not consistent with the accumulation of quercetin and myricetin in different tissues, nor were the expressions of VcFLS, VcPAL, VcCHS, VcF3H, and VcF3′5′H consistent with the accumulation of the quercetin during fruit development. However, the change in the trend of VcCHS and VcF3H expression was similar with myricetin accumulation during fruit development. Expression profiling analysis revealed that VcFLS expression was induced by salicylic acid, a phytohormone involved in plant defense against pathogens, and was suppressed by gibberellic acid, a phytohormone involved in various aspects of plant development. Heterologous expression of VcFLS in Arabidopsis thaliana increased the content of myricetin, but did not affect quercetin content. Thus, we conclude that VcFLS is a key enzyme in the flavonol biosynthetic pathway and would appear to be involved in the plant defense response.

     

Pipecolic Acid,an Endogenous Mediator of Defense Amplification and Priming,Is a Critical Regulator of Inducible Plant Immunity

Metabolic signals orchestrate plant defenses against microbial pathogen invasion. Here, we report the identification of the non-protein amino acid pipecolic acid (Pip), a common Lys catabolite in plants and animals, as a critical regulator of inducible plant immunity. Following pathogen recognition, Pip accumulates in inoculated Arabidopsis thaliana leaves, in leaves distal from the site of inoculation, and, most specifically, in petiole exudates from inoculated leaves. Defects of mutants in AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1) in systemic acquired resistance (SAR) and in basal, specific, and β-aminobutyric acid–induced resistance to bacterial infection are associated with a lack of Pip production. Exogenous Pip complements these resistance defects and increases pathogen resistance of wild-type plants. We conclude that Pip accumulation is critical for SAR and local resistance to bacterial pathogens. Our data indicate that biologically induced SAR conditions plants to more effectively synthesize the phytoalexin camalexin, Pip, and salicylic acid and primes plants for early defense gene expression. Biological priming is absent in the pipecolate-deficient ald1 mutants. Exogenous pipecolate induces SAR-related defense priming and partly restores priming responses in ald1. We conclude that Pip orchestrates defense amplification, positive regulation of salicylic acid biosynthesis, and priming to guarantee effective local resistance induction and the establishment of SAR.     

<Emphasis Type="Italic">CKB1</Emphasis> is involved in abscisic acid and gibberellic acid signaling to regulate stress responses in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Casein kinase II (CK2), an evolutionarily well-conserved Ser/Thr kinase, plays critical roles in all higher organisms including plants. CKB1 is a regulatory subunit beta of CK2. In this study, homozygous T-DNA mutants (ckb1-1 and ckb1-2) and over-expression plants (35S:CKB1-1, 35S:CKB1-2) of Arabidopsis thaliana were studied to understand the role of CKB1 in abiotic stress and gibberellic acid (GA) signaling. Histochemical staining showed that although CKB1 was expressed in all organs, it had a relatively higher expression in conducting tissues. The ckb1 mutants showed reduced sensitivity to abscisic acid (ABA) during seed germination and seedling growth. The increased stomatal aperture, leaf water loss and proline accumulation were observed in ckb1 mutants. In contrast, the ckb1 mutant had increased sensitivity to polyaluminum chloride during seed germination and hypocotyl elongation. We obtained opposite results in over-expression plants. The expression levels of a number of genes in the ABA and GA regulatory network had changed. This study demonstrates that CKB1 is an ABA signaling-related gene, which subsequently influences GA metabolism, and may play a positive role in ABA signaling.     

Hydrogen peroxide-induced salt tolerance in the <Emphasis Type="Italic">Arabidopsis</Emphasis> salicylate-deficient transformants <Emphasis Type="Italic">NahG</Emphasis>

The effect of hydrogen peroxide treatment on the salt tolerance of wild-type Arabidopsis thaliana L. plants (Col-0) and plants transformed with the bacterial salicylate hydroxylase gene (NahG) was studied. The base tolerance to salt stress caused by 200 mM of NaCl in solution culture was higher in plants with the NahG genotype in comparison with the wild-type plants. Growth inhibition was observed for wild-type plants under the action of exogenous hydrogen peroxide, which was not observed for the NahG transformants; salt tolerance increased in the both types of plants after treatment, which was assessed based on the growth indicators and the ability to preserve the chlorophyll pool following NaCl treatment. The content of endogenous Н2О2 in the leaves of wild-type plants increased significantly following exogenous hydrogen peroxide treatment and salt stress, while it practically did not change in the leaves of the NahG genotype. The SOD activity increased in both genotypes after treatment with exogenous hydrogen peroxide, and remained at an elevated level after salt stress in comparison with the nontreated plants. Furthermore, the catalase activity increased in leaves of the salicylate-deficient genotype but not in the Col-0 genotype. The guaiacol peroxidase activity increased in plants of both genotypes under the action of hydrogen peroxide and salt stress, with the NahG plants demonstrating a higher degree of increase. The Н₂О₂ treatment facilitated the increase of the proline content in leaves of the plants of both genotypes under conditions of salt stress. It was concluded that there were hydrogen peroxide signal transduction pathways in Arabidopsis plants that were salicylic acid independent and that the antioxidant system functioned more effectively in salicylate-deficient Arabidopsis plants.