Molecular cloning and expression analysis of PDR1-like gene in ginseng subjected to salt and cold stresses or hormonal treatment

The plant pleiotropic drug resistance (PDR) family of ATP-binding cassette (ABC) transporters is potentially involved in diverse biological processes. Currently, little is known about their actual physiological functions. A Panax ginseng PDR transporter gene (PgPDR1) was cloned and the cDNA has an open reading frame of 4344 bp. The deduced amino acid sequence contained the characteristic domains of PDR transporters: Walker A, Walker B, and ABC signature. Genomic DNA hybridization analysis indicated that one copy of PgPDR1 gene was present in P. ginseng. Subcellular localization showed that PgPDR1-GFP fusion protein was specifically localized in the cell membrane. Promoter region analysis revealed the presence of cis-acting elements, some of which are putatively involved in response to hormone, light and stress. To understand the functional roles of PgPDR1, we investigated the expression patterns of PgPDR1 in different tissues and under various conditions. Quantitative real-time PCR (qRT-PCR) and Western blotting analysis showed that PgPDR1 was expressed at a high level in the roots and leaves compared to seeds and stems. The expression of PgPDR1 was up-regulated by salicylic acid (SA) or chilling, down-regulated by ABA, and regulated differently at transcript and protein levels by MeJA. These results suggest that PgPDR1 might be involved in responding to environmental stresses and hormones.

Systemic Induction of the Small Antibacterial Compound in the Leaf Exudate During Benzothiadiazole-elicited Systemic Acquired Resistance in Pepper

Plants protect themselves from diverse potential pathogens by induction of the immune systems such as systemic acquired resistance (SAR). Most bacterial plant pathogens thrive in the intercellular space (apoplast) of plant tissues and cause symptoms. The apoplastic leaf exudate (LE) is believed to contain nutrients to provide food resource for phytopathogenic bacteria to survive and to bring harmful phytocompounds to protect plants against bacterial pathogens. In this study, we employed the pepper-Xanthomonas axonopodis system to assess whether apoplastic fluid from LE in pepper affects the fitness of X. axonopodis during the induction of SAR. The LE was extracted from pepper leaves 7 days after soil drench-application of a chemical trigger, benzothiadiazole (BTH). Elicitation of plant immunity was confirmed by significant up-regulation of four genes, CaPR1, CaPR4, CaPR9, and CaCHI2, by BTH treatment. Bacterial fitness was evaluated by measuring growth rate during cultivation with LE from BTH- or water-treated leaves. LE from BTH-treatment significantly inhibited bacterial growth when compared to that from the water-treated control. The antibacterial activity of LE from BTH-treated samples was not affected by heating at 100°C for 30 min. Although the antibacterial molecules were not precisely identified, the data suggest that small (less than 5 kDa), heat-stable compound(s) that are present in BTH-induced LE directly attenuate bacterial growth during the elicitation of plant immunity.

Analysis of transcript and metabolite levels in Italian rice (Oryza sativa L.) cultivars subjected to osmotic stress or benzothiadiazole treatment

One of the major objectives of rice (Oryza sativa L.) breeding programs is the development of new varieties with higher tolerance/resistance to both abiotic and biotic stresses. In this study, Italian rice cultivars were subjected to osmotic stress or benzothiadiazole (BTH) treatments. An analysis of the expression of selected genes known to be involved in the stress response and (1)H nuclear magnetic resonance ((1)H NMR) metabolic profiling were combined with multivariate statistical analyses to elucidate potential correlations between gene expression or metabolite content and observed tolerant/resistant phenotypes. We observed that the expression of three chosen genes (two WRKY genes and one peroxidase encoding gene) differed between susceptible and resistant cultivars in response to BTH treatments. Moreover, the analysis of metabolite content, in particular in the osmotic stress experiment, enabled discrimination between selected cultivars based on differences in the accumulation of some primary metabolites, primarily sugars. This research highlights the potential usefulness of this approach to characterise rice varieties based on transcriptional or metabolic changes due to adverse environmental conditions.

Lipopolysaccharides elicit an oxidative burst as a component of the innate immune system in the seagrass Thalassia testudinum

This study represents the first report characterizing the biological effects of a lipopolysaccharide (LPS) immune modulator on a marine vascular plant. LPS was shown to serve as a strong elicitor of the early defense response in the subtropical seagrass Thalassia testudinum Banks ex König and was capable of inducing an oxidative burst identified at the single cell level. The formation of reactive oxygen species (ROS), detected by a redox-sensitive fluorescent probe and luminol-based chemiluminescence, included a diphenyleneiodonium sensitive response, suggesting the involvement of an NADPH oxidase. A 900 bp cDNA fragment coding for this enzyme was sequenced and found to encode a NAD binding pocket domain with extensive homology to the Arabidopsis thaliana rbohF (respiratory burst oxidase homolog) gene. The triggered release of ROS occurred at 20 min post-elicitation and was dose-dependent, requiring a minimal threshold of 50 μg/mL LPS. Pharmacological dissection of the early events preceding ROS emission indicated that the signal transduction chain of events involved extracellular alkalinization, G-proteins, phospholipase A2, as well as K(+), Ca(2+), and anion channels. Despite exclusively thriving in a marine environment, seagrasses contain ROS-generating machinery and signal transduction components that appear to be evolutionarily conserved with the well-characterized defense response systems found in terrestrial plants.

The Arabidopsis Mediator complex subunits MED14/SWP and MED16/SFR6/IEN1 differentially regulate defense gene expression in plant immune responses

Pathogen infection in plants triggers large-scale transcriptional changes, both locally and systemically. Emerging evidence suggests that the Arabidopsis Mediator complex plays a crucial role in these transcriptional changes. Mediator is highly conserved in eukaryotes, and its core comprises more than 20 subunits organized into three modules named head, middle and tail. The head and middle modules interact with general transcription factors and RNA polymerase II, whereas the tail module associates with activators, and signals through the head and middle modules to the basal transcription machinery. In Arabidopsis, three tail module subunits, MED14, MED15 and MED16, have been identified. Both MED15 and MED16 have been implicated in plant immunity, but the role of MED14 has not been established. Here, we report the characterization of an Arabidopsis T-DNA insertion mutant of the MED14 gene. Similarly to the med15 and/or med16 mutations, the med14 mutation significantly suppresses salicylic acid-induced defense responses, alters transcriptional changes induced by the avirulent bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000/avrRpt2, and renders plants susceptible to both Pst DC3000/avrRpt2 and Pst DC3000. The med14 mutation also completely compromises biological induction of systemic acquired resistance (SAR), indicating that the tail module as a whole is essential for SAR. Interestingly, unlike the med16 mutation, which differentially affects expression of several SAR positive and negative regulators, med14 inhibits induction of a large group of defense genes, including both SAR positive and negative regulators, suggesting that individual subunits of the Mediator tail module employ distinct mechanisms to regulate plant immune responses.

The folate precursor para-aminobenzoic acid elicits induced resistance against Cucumber mosaic virus and Xanthomonas axonopodis

BACKGROUND AND AIMS

The use of vitamins including vitamin B1, B2 and K3 for the induction of systemic acquired resistance (SAR) to protect crops against plant pathogens has been evaluated previously. The use of vitamins is beneficial because it is cost effective and safe for the environment. The use of folate precursors, including ortho-aminobenzoic acid, to induce SAR against a soft-rot pathogen in tobacco has been reported previously.

METHODS

In the present study, para-aminobenzoic acid (PABA, also referred to as vitamin Bx) was selected owing to its effect on the induction of SAR against Xanthomonas axonopodis pv. vesicatoria in pepper plants through greenhouse screening.

KEY RESULTS

Dipping of pepper seedlings in a 1 mm PABA solution in field trials induced SAR against artificially infiltrated X. axonopodis pv. vesicatoria and naturally occurring cucumber mosaic virus. Expression of the Capsicum annuum pathogenesis-related 4 gene was primed in response to pathogen infection as assessed by quantitative real-time PCR. The accumulation of cucumber mosaic virus RNA was reduced in PABA-treated pepper plants at 40 and 105 d post-treatment. Unexpectedly, fruit yield was increased in PABA-treated plants, indicating that PABA-mediated SAR successfully protected pepper plants from infection by bacterial and viral pathogens without significant fitness allocation costs.

CONCLUSIONS

The present study is the first to demonstrate the effective elicitation of SAR by a folate precursor under field conditions.

Proteomics analysis suggests broad functional changes in potato leaves triggered by phosphites and a complex indirect mode of action against Phytophthora infestans

Phosphite (salts of phosphorous acid; Phi)-based fungicides are increasingly used in controlling oomycete pathogens, such as the late blight agent Phytophthora infestans. In plants, low amounts of Phi induce pathogen resistance through an indirect mode of action. We used iTRAQ-based quantitative proteomics to investigate the effects of phosphite on potato plants before and after infection with P. infestans. Ninety-three (62 up-regulated and 31 down-regulated) differentially regulated proteins, from a total of 1172 reproducibly identified proteins, were identified in the leaf proteome of Phi-treated potato plants. Four days post-inoculation with P. infestans, 16 of the 31 down-regulated proteins remained down-regulated and 42 of the 62 up-regulated proteins remained up-regulated, including 90% of the defense proteins. This group includes pathogenesis-related, stress-responsive, and detoxification-related proteins. Callose deposition and ultrastructural analyses of leaf tissues after infection were used to complement the proteomics approach. This study represents the first comprehensive proteomics analysis of the indirect mode of action of Phi, demonstrating broad effects on plant defense and plant metabolism. The proteomics data and the microscopy study suggest that Phi triggers a hypersensitive response that is responsible for induced resistance of potato leaves against P. infestans.

BIOLOGICAL SIGNIFICANCE

Phosphie triggers complex functional changes in potato leaves that are responsible for the induced resistance against Phytophthora infestans. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Long distance movement of DIR1 and investigation of the role of DIR1-like during systemic acquired resistance in Arabidopsis

DIR1 is a lipid transfer protein (LTP) postulated to complex with and/or chaperone a signal(s) to distant leaves during Systemic Acquired Resistance (SAR) in Arabidopsis. DIR1 was detected in phloem sap-enriched petiole exudates collected from wild-type leaves induced for SAR, suggesting that DIR1 gains access to the phloem for movement from the induced leaf. Occasionally the defective in induced resistance1 (dir1-1) mutant displayed a partially SAR-competent phenotype and a DIR1-sized band in protein gel blots was detected in dir1-1 exudates suggesting that a highly similar protein, DIR1-like (At5g48490), may contribute to SAR. Recombinant protein studies demonstrated that DIR1 polyclonal antibodies recognize DIR1 and DIR1-like. Homology modeling of DIR1-like using the DIR1-phospholipid crystal structure as template, provides clues as to why the dir1-1 mutant is rarely SAR-competent. The contribution of DIR1 and DIR1-like during SAR was examined using an Agrobacterium-mediated transient expression-SAR assay and an estrogen-inducible DIR1-EGFP/dir1-1 line. We provide evidence that upon SAR induction, DIR1 moves down the leaf petiole to distant leaves. Our data also suggests that DIR1-like displays a reduced capacity to move to distant leaves during SAR and this may explain why dir1-1 is occasionally SAR-competent.

Reprogramming of plants during systemic acquired resistance

Genome-wide microarray analyses revealed that during biological activation of systemic acquired resistance (SAR) in Arabidopsis, the transcript levels of several hundred plant genes were consistently up- (SAR(+) genes) or down-regulated (SAR(-) genes) in systemic, non-inoculated leaf tissue. This transcriptional reprogramming fully depended on the SAR regulator FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1). Functional gene categorization showed that genes associated with salicylic acid (SA)-associated defenses, signal transduction, transport, and the secretory machinery are overrepresented in the group of SAR(+) genes, and that the group of SAR(-) genes is enriched in genes activated via the jasmonate (JA)/ethylene (ET)-defense pathway, as well as in genes associated with cell wall remodeling and biosynthesis of constitutively produced secondary metabolites. This suggests that SAR-induced plants reallocate part of their physiological activity from vegetative growth towards SA-related defense activation. Alignment of the SAR expression data with other microarray information allowed us to define three clusters of SAR(+) genes. Cluster I consists of genes tightly regulated by SA. Cluster II genes can be expressed independently of SA, and this group is moderately enriched in H2O2- and abscisic acid (ABA)-responsive genes. The expression of the cluster III SAR(+) genes is partly SA-dependent. We propose that SA-independent signaling events in early stages of SAR activation enable the biosynthesis of SA and thus initiate SA-dependent SAR signaling. Both SA-independent and SA-dependent events tightly co-operate to realize SAR. SAR(+) genes function in the establishment of diverse resistance layers, in the direct execution of resistance against different (hemi-)biotrophic pathogen types, in suppression of the JA- and ABA-signaling pathways, in redox homeostasis, and in the containment of defense response activation. Our data further indicated that SAR-associated defense priming can be realized by partial pre-activation of particular defense pathways.

Acyl CoA Binding Proteins are Required for Cuticle Formation and Plant Responses to Microbes

Fatty acids (FA) and lipids are well known regulators of plant defense. Our previous studies have shown that components of prokaryotic (plastidal) FA biosynthesis pathway regulate various aspects of plant defense. Here, we investigated the defense related roles of the soluble acyl CoA binding proteins (ACBPs), which are thought to facilitate the intracellular transport of FA/lipids. We show that ACBP3 and 4 are required for maintaining normal lipid levels and that ACBP3 contributes to the lipid flux between the prokaryotic and eukaryotic pathways. We also show that loss of ACBP3, 4, or 6 impair normal development of the cuticle and affect both basal and resistance protein-mediated defense against bacterial and fungal pathogens. Loss of ACBP3, 4, or 6 also inhibits the induction of systemic acquired resistance (SAR) due to the plants inability to generate SAR inducing signal(s). Together, these data show that ACBP3, ACBP4, and ACBP6 are required for cuticle development as well as defense against microbial pathogens.

Biochemical and Molecular-Genetic Characterization of SFD1's Involvement in Lipid Metabolism and Defense Signaling

The Arabidopsis thaliana SFD1 (suppressor of fatty acid desaturase deficiency1) gene (also known as GLY1) is required for accumulation of 34:6 (i.e., 18:3-16:3) monogalactosyldiacylglycerol (MGDG) and for the activation of systemic acquired resistance (SAR), an inducible defense mechanism that confers resistance against a broad spectrum of pathogens. SFD1, which has been suggested to be involved in lipid-based signaling in SAR, contains a putative chloroplast transit peptide and has glycerol-3-phosphate synthesizing dihydroxyacetone phosphate (DHAP) reductase (also referred as glycerol-3-phosphate dehydrogenase) activity. The goals of this study were to determine if the DHAP reductase activity and chloroplast localization are required for SFD1's involvement in galactolipid metabolism and SAR signaling. The crystal structure of a Leishmania mexicana glycerol-3-phosphate dehydrogenase was used to model SFD1 structure and identify Lys194, Lys279, and Asp332 as potential catalytic site residues in SFD1. Mutational analysis of SFD1 confirmed that Lys194, Lys279, and Asp332 are critical for SFD1's DHAP reductase activity, and its involvement in SAR. SFD1 proteins with these residues individually substituted by Ala lacked DHAP reductase activity and were unable to complement the SAR defect of the sfd1 mutant. The SFD1-Ala279 protein was also unable to restore 34:6-MGDG content when expressed in the sfd1 mutant. In vivo imaging of a green fluorescent protein-tagged SFD1 protein demonstrated that SFD1 is targeted to the chloroplast. The N-terminal 43 amino acids, which are required for proper targeting of SFD1 to the chloroplast, are also required for SFD1's function in lipid metabolism and SAR. Taken together, these results demonstrate that SFD1's DHAP reductase activity is required in the chloroplast for lipid metabolism and defense signaling.

Salicylic acids: local, systemic or inter-systemic regulators?

Salicylic acid is well known phytohormone, emerging recently as a new paradigm of an array of manifestations of growth regulators. The area unleashed yet encompassed the applied agriculture sector to find the roles to strengthen the crops against plethora of abiotic and biotic stresses. The skipped part of integrated picture, however, was the evolutionary insight of salicylic acid to either allow or discard the microbial invasion depending upon various internal factors of two interactants under the prevailing external conditions. The metabolic status that allows the host invasion either as pathogenesis or symbiosis with possible intermediary stages in close systems has been tried to underpin here.

Induction of Systemic Acquired Resistance by Rotylenchulus reniformis and Meloidogyne incognita in Cotton Following Separate and Concomitant Inoculations

Systemic acquired resistance (SAR) can be elicited by virulent and avirulent pathogenic strains and SAR against plant-parasitic nematodes has been documented. Our objective was to determine whether co-infection of cotton by Meloidogyne incognita and Rotylenchulus reniformis affects the population level of either nematode compared to infection by each species individually. Split-root trials were conducted in which plants were inoculated with i) R. reniformis only, ii) M. incognita only, iii) both R. reniformis and M. incognita, or iv) no nematodes. Half of the root system was inoculated with R. reniformis or M. incognita on day 0 and the other half with M. incognita or R. reniformis on day 0 or day 14 depending on the experiment. Experiments were conducted on cotton cultivar DP 0935 B2RF (susceptible to both nematodes), LONREN-1 (germplasm line resistant to R. reniformis), and M-120 RNR (germplasm line resistant to M. incognita), and tests were terminated 8 wk after the last inoculation. Both soil (vermiform) and roots (egg) extracted from each half of the root system to determine the total nematode population levels, and root galling was rated on a 0 to 10 scale. Mixed models analysis and comparison of least squares means indicated no differences in root galling (except on LONREN-1) or population levels when the two nematode species were introduced on the same day. When M. incognita was introduced 14 d after R. reniformis, reduction in galling (36% on DP 0935 and 33% on LONREN-1) and M. incognita population levels (35% on DP 0935 and 45% on LONREN-1) were significant (P ≤ 0.05). When R. reniformis was inoculated 14 d after M. incognita, reduction in R. reniformis population levels (18% on DP 0935 and 26% on M-120) were significant. This study documents for the first time that infection of cotton by a nematode can elicit SAR to another nematode species.

Influence of infection of cotton by rotylenchulus reniformis and meloidogyne incognita on the production of enzymes involved in systemic acquired resistance

Systemic acquired resistance (SAR), which results in enhanced defense mechanisms in plants, can be elicited by virulent and avirulent strains of pathogens including nematodes. Recent studies of nematode reproduction strongly suggest that Meloidogyne incognita and Rotylenchulus reniformis induce SAR in cotton, but biochemical evidence of SAR was lacking. Our objective was to determine whether infection of cotton by M. incognita and R. reniformis increases the levels of P-peroxidase, G-peroxidase, and catalase enzymes which are involved in induced resistance. A series of greenhouse trials was conducted; each trial included six replications of four treatments applied to one of three cotton genotypes in a randomized complete block design. The four treatments were cotton plants inoculated with i) R. reniformis, ii) M. incognita, iii) BTH (Actigard), and iv) a nontreated control. Experiments were conducted on cotton genotypes DP 0935 B2RF (susceptible to both nematodes), LONREN-1 (resistant to R. reniformis), and M-120 RNR (resistant to M. incognita), and the level of P-peroxidase, G-peroxidase, and catalase activity was measured before and 2, 4, 6, 10, and 14 d after treatment application. In all cotton genotypes, activities of all three enzymes were higher (P ≤ 0.05) in leaves of plants infected with M. incognita and R. reniformis than in the leaves of control plants, except that M. incognita did not increase catalase activity on LONREN-1. Increased enzyme activity was usually apparent 6 d after treatment. This study documents that infection of cotton by M. incognita or R. reniformis increases the activity of the enzymes involved in systemic acquired resistance; thereby providing biochemical evidence to substantiate previous reports of nematode-induced SAR in cotton.

Effect of Transplant Age, Tobacco Cultivar, Acibenzolar-S-Methyl, and Imidacloprid on Tomato Spotted Wilt Infection in Flue-Cured Tobacco

Tomato spotted wilt virus (TSWV) has become the most serious problem in flue-cured tobacco in Georgia and is a growing problem in other tobacco-growing areas in the United States. The effects of transplant age (6 to 10 weeks), tobacco cultivar (K-326 and NC-71), and preplant applications of acibenzolar-S-methyl (ASM) and the insecticide imidacloprid (IMD) were evaluated on levels of TSWV infection, number of symptomatic plants, and yield in field trials over 4 years. In all 4 years and in four of five trials, treatment of transplants with ASM and IMD resulted in fewer symptomatic plants, smaller areas under the disease progress curve (AUDPC), and higher yields compared with the nontreated controls. There were no consistent effects of transplant age or cultivar on number of symptomatic plants or systemic infections, AUDPC, or yield. Treatment of transplants with ASM and IMD can significantly reduce the number of symptomatic plants in the field and substantially increase yields and value per hectare.

The search for the salicylic acid receptor led to discovery of the SAR signal receptor

Systemic acquired resistance (SAR) is a state of heightened defense which is induced throughout a plant by an initial infection; it provides long-lasting, broad-spectrum resistance to subsequent pathogen challenge. Recently we identified a phloem-mobile signal for SAR which has been elusive for almost 30 years. It is methyl salicylate (MeSA), an inactive derivative of the defense hormone, salicylic acid (SA). This discovery resulted from extensive characterization of SA-binding protein 2 (SABP2), a protein whose high affinity for SA and extremely low abundance suggested that it might be the SA receptor. Instead we discovered that SABP2 is a MeSA esterase whose function is to convert biologically inactive MeSA in the systemic tissue to active SA. The accumulated SA then activates or primes defenses leading to SAR. SABP2's esterase activity is inhibited in the initially/primary infected tissue by SA binding in its active site; this facilitates accumulation of MeSA, which is then translocated through the phloem to systemic tissue for perception and processing by SABP2 to SA. Thus, while SABP2 is not the SA receptor, it can be considered the receptor for the SAR signal. This study of SABPs not only illustrates the unexpected nature of scientific discoveries, but also underscores the need to use biochemical approaches in addition to genetics to address complex biological processes, such as disease resistance.

Management of Xanthomonas Leaf Blight of Onion with Bacteriophages and a Plant Activator

Xanthomonas leaf blight of onion (Allium cepa), caused by Xanthomonas axonopodis pv. allii, continues to be a challenging and yield-threatening disease in Colorado and other regions of onion production worldwide. Studies were conducted to develop management strategies for this disease that are equally effective and more sustainable than the current practices of making multiple applications of copper bactericides. Mixtures of bacteriophages and the plant defense activator, acibenzolar-S-methyl, were evaluated under field and greenhouse conditions for their abilities to reduce Xanthomonas leaf blight severity. Bacteriophage populations in the phyllosphere of onion were monitored over time. Bacteriophage populations persisted on onion leaves for at least 72 to 96 h under field and greenhouse conditions, respectively. Under field conditions at one location, biweekly or weekly applications of bacteriophages reduced disease severity by 26 to 50%, which was equal to or better than weekly applications of copper hydroxide plus mancozeb. Acibenzolar-S-methyl also successfully reduced disease severity by up to 50% when used alone preventatively or followed by biweekly bacteriophage applications. Reductions in disease severity generally were not associated with improvements in onion bulb size or yield. Integration of bacteriophage mixtures with acibenzolar-S-methyl appears to be a promising strategy for managing Xanthomonas leaf blight of onion, and could reduce grower reliance on conventional copper bactericide applied with ethylenebisdithiocarbamate fungicides.

Virulence of Meloidogyne spp. and Induced Resistance in Grape Rootstocks

Harmony grape rootstock displays resistance to several Meloidogyne spp. but that resistance is not durable in commercial vineyard settings. A 2-year experiment in a microplot setting revealed host specificities of two virulent populations of Meloidogyne arenaria and an avirulent population of Meloidogyne incognita. In a subsequent split-root experiment, the avirulent nematode population was demonstrated to induce resistance to the virulent nematode population. To quantify the level of resistance, reproduction of the virulent nematode population was determined 63 days after being challenged by an avirulent nematode population using a range of inoculum densities and timeframes. Induction of resistance became apparent when the virulent nematode population was inoculated 7 days after the avirulent nematode population and increased thereafter. The level of induced resistance increased with increased inoculum levels of the avirulent nematode population. Root systems of perennial crops are commonly fed upon simultaneously by multiple nematode species. These two studies indicate that field populations can become preferentially virulent upon one or multiple rootstocks and that co-inhabiting populations may induce existing resistance mechanisms. In perennial crops, it is common for numerous nematode species besides Meloidogyne spp. to be present, including some that feed without causing apparent damage.

Effects of Inducers of Systemic Acquired Resistance on Reproduction of Meloidogyne javanica and Rotylenchulus reniformis in Pineapple

The potency of the inducers of systemic acquired resistance (SAR), acibenzolar-s-methyl, DL-alpha-amino-n-butyric acid (AABA), DL-beta-amino-n-butyric acid (BABA), gamma-amino-n-butyric acid (GABA), p-aminobenzoic acid (PABA), riboflavin, and salicylic acid (SA), in reducing reproduction of Meloidogyne javanica and Rotylenchulus reniformis in pineapple was investigated. All inducers were applied as foliar sprays to 1-mon-old pineapple plants (20 ml/plant) grown in 22-cm-diam. pots in the greenhouse. Two days after application, 10,000 eggs of M. javanica or R. reniformis were inoculated onto the plants. Six months after inoculation, nematode reproduction was measured. Acibenzolar decreased R. reniformis egg production by 58% compared to the nontreated control (P </= 0.05). Acibenzolar, BABA, and riboflavin reduced M. javanica egg production by 60% to 64% compared to the nontreated control (P </= 0.05). The point in the pineapple SAR pathway that each compound activates may explain the differing results between M. javanica and its giant cells and R. reniformis and its syncytia. Foliar application of acibenzolar at 100 and 200 mg/liter decreased by 30% and 60%, respectively, the number of M. javanica eggs as compared to the nontreated control. Fresh shoot weight of pineapple treated with 50, 100, 200, and 400 mg/liter acibenzolar was reduced by 1.2%, 3.3%, 9.9%, and 33% compared to the nontreated pineapple, respectively (P </= 0.05). Foliar application of acibenzolar may activate intrinsic resistance of pineapple to M. javanica and R. reniformis and may have a role in the sustainable management of nematodes in pineapple.

Structural and biochemical studies identify tobacco SABP2 as a methyl salicylate esterase and implicate it in plant innate immunity

Salicylic acid (SA) is a critical signal for the activation of plant defense responses against pathogen infections. We recently identified SA-binding protein 2 (SABP2) from tobacco as a protein that displays high affinity for SA and plays a crucial role in the activation of systemic acquired resistance to plant pathogens. Here we report the crystal structures of SABP2, alone and in complex with SA at up to 2.1-A resolution. The structures confirm that SABP2 is a member of the alpha/beta hydrolase superfamily of enzymes, with Ser-81, His-238, and Asp-210 as the catalytic triad. SA is bound in the active site and is completely shielded from the solvent, consistent with the high affinity of this compound for SABP2. Our biochemical studies reveal that SABP2 has strong esterase activity with methyl salicylate as the substrate, and that SA is a potent product inhibitor of this catalysis. Modeling of SABP2 with MeSA in the active site is consistent with all these biochemical observations. Our results suggest that SABP2 may be required to convert MeSA to SA as part of the signal transduction pathways that activate systemic acquired resistance and perhaps local defense responses as well.

Systemic Resistance Induced by Trichoderma hamatum 382 in Cucumber Against Phytophthora Crown Rot and Leaf Blight

Phytophthora root rot, crown rot, leaf and stem blight, and fruit rot of cucumber can cause serious losses, and are difficult to control. Although composts can be used successfully for control of Phytophthora root rots, little is known about their effects on Phytophthora diseases of aboveground plant parts. This research shows that the severity of Phytophthora root and crown rot of cucumber caused by Phytophthora capsici was suppressed significantly in cucumber transplants produced in a composted cow manure-amended mix compared with those in a dark sphagnum peat mix. In split root bioassays, Trichoderma hamatum 382 (T₃₈₂) inoculated into the compost-amended potting mix significantly reduced the severity of Phytophthora root and crown rot on paired roots in the peat mix. This effect did not differ significantly from that provided by a drench with benzothiadiazole (BTH) or mefenoxam (Subdue MAXX). Based on area under disease progress curves, T₃₈₂ also significantly reduced the severity of Phytophthora leaf blight in transplants produced in the compost mix compared with controls not inoculated with T₃₈₂. Efficacy of T₃₈₂ did not differ significantly from that provided by a drench with BTH. T₃₈₂ re-mained spatially separated from the pathogen in plants in both the split root and leaf blight bioassays, suggesting that these effects were systemic in nature.

Effects of Acibenzolar-S-Methyl Application to Rotylenchulus reniformis and Meloidogyne javanica

Effects of acibenzolar-s-methyl, an inducer of systemic acquired resistance in plants, on Rotylenchulus reniformis and Meloidogyne javanica in vitro and in vivo were determined. A single foliar application of acibenzolar at 50 mg/liter (5 ml of solution per plant) to 7-day-old cowpea or soybean seedlings decreased R. reniformis and M. javanica egg production by 50% 30 days after inoculation. The mechanism of acibenzolar on plant-parasitic nematodes was then investigated. Acibenzolar at 50 to 200 mg/liter did not affect movement of R. reniformis and M. javanica or penetration of second-stage juveniles (J2) of M. javanica on cowpea. However, M. javanica development was slowed and fecundity was reduced in plants treated with acibenzolar. On average, 50% of J2 that penetrated acibenzolar-treated cowpeas developed into mature females with eggs, whereas the other 50% exhibited arrested development. The number of eggs per egg mass was 450 in water-treated cowpeas, whereas the number declined to 250 in acibenzolar-treated plants. Acibenzolar may be responsible for stimulating the plants to express some resistance to the nematodes.

Induced systemic resistance (ISR) against pathogens in the context of induced plant defences

Induced systemic resistance (ISR) of plants against pathogens is a widespread phenomenon that has been intensively investigated with respect to the underlying signalling pathways as well as to its potential use in plant protection. Elicited by a local infection, plants respond with a salicylic-dependent signalling cascade that leads to the systemic expression of a broad spectrum and long-lasting disease resistance that is efficient against fungi, bacteria and viruses. Changes in cell wall composition, de novo production of pathogenesis-related-proteins such as chitinases and glucanases, and synthesis of phytoalexins are associated with resistance, although further defensive compounds are likely to exist but remain to be identified. In this Botanical Briefing we focus on interactions between ISR and induced resistance against herbivores that is mediated by jasmonic acid as a central signalling molecule. While many studies report cross-resistance, others have found trade-offs, i.e. inhibition of one resistance pathway by the other. Here we propose a framework that explains many of the thus far contradictory results. We regard elicitation separately from signalling and from production, i.e. the synthesis of defensive compounds. Interactions on all three levels can act independently from each other.

Biocontrol of Cucumber Diseases in the Field by Plant Growth-Promoting Rhizobacteria With and Without Methyl Bromide Fumigation

Field trials were conducted in 1996 and 1997 to determine the effect of plant growth-promoting rhizobacteria (PGPR) strains, which previously were found to induce systemic resistance in cucumber, on cucumber plant growth and on naturally occurring cucumber diseases with and without methyl bromide fumigation. Seven PGPR seed treatments included single-strain treatments and mixtures of Bacillus pumilus strain INR7, Curtobacterium flaccumfaciens strain ME1, and Bacillus subtilis strain GB03. In both years, in the absence of methyl bromide, all seven PGPR treatments significantly promoted plant growth, compared to the non-treated control, while with methyl bromide fumigation, only 3 and 1 of the same PGPR treatments promoted growth significantly in 1996 and 1997, respectively. In 1996, main runner length of plants in all seven PGPR treatments without fumigation was statistically equivalent to the main runner length of the nontreated control with methyl bromide fumigation. Naturally occurring foliar diseases were angular leaf spot, caused by Pseudomonas syringae pv. lachrymans in 1996, and a mixed infestation of angular leaf spot and anthracnose, caused by Colletotrichum orbiculare in 1997. In both years, all PGPR treatments significantly reduced severity of foliar disease, compared to the nontreated control, with and without methyl bromide. Mixtures of PGPR strains showed a higher level of disease protection in both years with and without methyl bromide. The results indicate that attempts to develop PGPR-mediated induced systemic resistance into components of vegetable integrated pest management should not be negatively impacted by the planned withdraw of MeBr from standard vegetable production and that PGPR may help compensate for reduced plant growth often seen without methyl bromide fumigation.