Induction of plant defense gene expression by plant activators and Pseudomonas syringae pv. tomato in greenhouse-grown tomatoes

Impacts of a Commercially Available Horticultural Oil Biopesticide (EF-400) on the Tomato-Phytophthora infestans Pathosystem

Biopesticide fungicides are naturally derived compounds that offer protection from plant diseases through various modes of action, including antimicrobial activity and upregulation of defense responses in host plants. These plant protectants provide a sustainable and safe alternative to conventional pesticides in integrated disease management programs and are especially salient in the management of high-risk and economically important diseases such as late blight of tomato and potato, for which host resistance options are limited. In this study, a commercially available biopesticide, EF400 comprised of clove (8.2%), rosemary (8.1%), and peppermint oils (6.7%) (Anjon AG, Corcoran, CA), was investigated for its effects on the Phytophthora infestans-tomato pathosystem. Specifically, we evaluated the impact of EF400 on P. infestans growth in culture, disease symptoms in planta, and activation of host defenses through monitoring transcript accumulation of defense-related genes. The application timing and use rate of EF400 were further investigated for managing tomato late blight. EF400 delayed the onset of tomato late blight symptoms, although not as effectively as the copper hydroxide fungicide Champ WG (Nufarm Americas Inc., Alsip, IL). Pathogen mycelial growth and sporangial quantity on late blight-susceptible tomato leaves were significantly reduced with EF400. The biopesticide also had an enhancing or suppressing effect on the transcript accumulation of three defense-related genes: Pin2, PR1a, and PR1b. Our work in exploring a commercially available horticultural oil biopesticide meaningfully contributed to the essential knowledge base for optimizing recommendations for the management of tomato late blight.

Enhancing Botrytis disease management in tomato plants: insights from a Pseudomonas putida strain with biocontrol activity

AIMS

This study explores the biocontrol potential of Pseudomonas putida Z13 against Botrytis cinerea in tomato plants, addressing challenges posed by the pathogen's fungicide resistance. The aims of the study were to investigate the in vitro and in silico biocontrol traits of Z13, identify its plant-colonizing efficacy, evaluate the efficacy of different application strategies against B. cinerea in planta, and assess the capacity of Z13 to trigger induced systemic resistance (ISR) in plants.

METHODS AND RESULTS

The in vitro experiments revealed that Z13 inhibits the growth of B. cinerea, produces siderophores, and exhibits swimming and swarming activity. Additionally, the Z13 genome harbors genes that encode compounds triggering ISR, such as pyoverdine and pyrroloquinoline quinone. The in planta experiments demonstrated Z13's efficacy in effectively colonizing the rhizosphere and leaves of tomato plants. Therefore, three application strategies of Z13 were evaluated against B. cinerea: root drenching, foliar spray, and the combination of root drenching and foliar spray. It was demonstrated that the most effective treatment of Z13 against B. cinerea was the combination of root drenching and foliar spray. Transcriptomic analysis showed that Z13 upregulates the expression of the plant defense-related genes PR1 and PIN2 upon B. cinerea inoculation.

CONCLUSION

The results of the study demonstrated that Z13 possesses significant biocontrol traits, such as the production of siderophores, resulting in significant plant protection against B. cinerea when applied as a single treatment to the rhizosphere or in combination with leaf spraying. Additionally, it was shown that Z13 root colonization primes plant defenses against the pathogen.

Secreted Peptide SpPIP1 Modulates Disease Resistance and Salt Tolerance in Tomato

Tomato is a globally important horticultural and economic crop, but its productivity is severely affected by various stresses. Plant small secretory peptides have been identified as crucial mediators in plant resistance. Here, we conducted a comparative transcriptome analysis and identified the prePIP1 gene from Solanum pimpinellifolium (SpprePIP1), as an ortholog of Arabidopsis prePIP1 encoding the precursor protein of PAMP-induced SSP 1. The expression level of SpprePIP1 is transcriptionally induced in tomato upon infection with Phytophthora infestans (P. infestans), the pathogen responsible for late blight. Overexpression of SpprePIP1 resulted in enhanced tomato resistance to P. infestans. In addition, exogenous application of SpPIP1, whether through spraying or irrigation, improved tomato resistance by enhancing the transcript accumulations of pathogenesis-related proteins, as well as reactive oxygen species and the jasmonic acid (JA) levels. Integrated analysis of transcriptomics and metabolomics revealed the potential contributions of JA and phenylpropanoid biosynthesis to SpPIP1-induced tomato immunity. Additionally, SpPIP1 may strengthen tomato resistance to salt stress through the ABA signaling pathway. Overall, our findings demonstrate that SpPIP1 positively regulates tomato tolerance to P. infestans and salt stress, making it a potential plant elicitor for crop protection in an environmentally friendly way.

In planta interactions of a novel bacteriophage against Pseudomonas syringae pv. tomato

The biology and biotechnology of bacteriophages have been extensively studied in recent years to explore new and environmentally friendly methods of controlling phytopathogenic bacteria. Pseudomonas syringae pv. tomato (Pst) is responsible for bacterial speck disease in tomato plants, leading to decreased yield. Disease management strategies rely on the use of copper-based pesticides. The biological control of Pst with the use of bacteriophages could be an alternative environmentally friendly approach to diminish the detrimental effects of Pst in tomato cultivations. The lytic efficacy of bacteriophages can be used in biocontrol-based disease management strategies. Here, we report the isolation and complete characterization of a bacteriophage, named Medea1, which was also tested in planta against Pst, under greenhouse conditions. The application of Medea1 as a root drenching inoculum or foliar spraying reduced 2.5- and fourfold on average, respectively, Pst symptoms in tomato plants, compared to a control group. In addition, it was observed that defense-related genes PR1b and Pin2 were upregulated in the phage-treated plants. Our research explores a new genus of Pseudomonas phages and explores its biocontrol potential against Pst, by utilizing its lytic nature and ability to trigger the immune response of plants. KEY POINTS: • Medea1 is a newly reported bacteriophage against Pseudomonas syringae pv. tomato having genomic similarities with the phiPSA1 bacteriophage • Two application strategies were reported, one by root drenching the plants with a phage-based solution and one by foliar spraying, showing up to 60- and 6-fold reduction of Pst population and disease severity in some cases, respectively, compared to control • Bacteriophage Medea1 induced the expression of the plant defense-related genes Pin2 and PR1b.

Microbial Metabolites Beneficial to Plant Hosts Across Ecosystems

Plants are intimately connected with their associated microorganisms. Chemical interactions via natural products between plants and their microbial symbionts form an important aspect in host health and development, both in aquatic and terrestrial ecosystems. These interactions range from negative to beneficial for microbial symbionts as well as their hosts. Symbiotic microbes synchronize their metabolism with their hosts, thus suggesting a possible coevolution among them. Metabolites, synthesized from plants and microbes due to their association and coaction, supplement the already present metabolites, thus promoting plant growth, maintaining physiological status, and countering various biotic and abiotic stress factors. However, environmental changes, such as pollution and temperature variations, as well as anthropogenic-induced monoculture settings, have a significant influence on plant-associated microbial community and its interaction with the host. In this review, we put the prominent microbial metabolites participating in plant-microbe interactions in the natural terrestrial and aquatic ecosystems in a single perspective and have discussed commonalities and differences in these interactions for adaptation to surrounding environment and how environmental changes can alter the same. We also present the status and further possibilities of employing chemical interactions for environment remediation. Our review thus underlines the importance of ecosystem-driven functional adaptations of plant-microbe interactions in natural and anthropogenically influenced ecosystems and their possible applications.

A rapid and non-destructive method for spatial-temporal quantification of colonization by Pseudomonas syringae pv. tomato DC3000 in Arabidopsis and tomato

BACKGROUND

The bacterial leaf pathogen Pseudomonas syringae pv tomato (Pst) is the most popular model pathogen for plant pathology research. Previous methods to study the plant-Pst interactions rely on destructive quantification of Pst colonisation, which can be labour- and time-consuming and does not allow for spatial-temporal monitoring of the bacterial colonisation. Here, we describe a rapid and non-destructive method to quantify and visualise spatial-temporal colonisation by Pst in intact leaves of Arabidopsis and tomato.

RESULTS

The method presented here uses a bioluminescent Pst DC3000 strain that constitutively expresses the luxCDABE operon from Photorhabdus luminescens (Pst::LUX) and requires a common gel documentation (Gel Doc) system with a sensitive CCD/CMOS camera and imaging software (Photoshop or Image J). By capturing bright field and bioluminescence images from Pst::LUX-infected leaves, we imaged the spatiotemporal dynamics of Pst infection. Analysis of bioluminescence from live Pst bacteria over a 5-day time course after spray inoculation of Arabidopsis revealed transition of the bacterial presence from the older leaves to the younger leaves and apical meristem. Colonisation by Pst:LUX bioluminescence was obtained from digital photos by calculating relative bioluminescence values, which is adjusted for bioluminescence intensity and normalised by leaf surface. This method detected statistically significant differences in Pst::LUX colonisation between Arabidopsis genotypes varying in basal resistance, as well as statistically significant reductions in Pst::LUX colonisation by resistance-inducing treatments in both Arabidopsis and tomato. Comparison of relative bioluminescence values to conventional colony counting on selective agar medium revealed a statistically significant correlation, which was reproducible between different Gel Doc systems.

CONCLUSIONS

We present a non-destructive method to quantify colonisation by bioluminescent Pst::LUX in plants. Using a common Gel Doc system and imaging software, our method requires less time and labour than conventional methods that are based on destructive sampling of infected leaf material. Furthermore, in contrast to conventional strategies, our method provides additional information about the spatial-temporal patterns of Pst colonisation.

Secondary Metabolites of Pseudomonas aeruginosa LV Strain Decrease Asian Soybean Rust Severity in Experimentally Infected Plants

Asian Soybean Rust (ASR), a disease caused by Phakopsora pachyrhizi, causing yield losses up to 90%. The control is based on the fungicides which may generate resistant fungi. The activation of the plant defense system, should help on ASR control. In this study, secondary metabolites of Pseudomonas aeruginosa LV strain were applied on spore germination and the expression of defense genes in infected soybean plants. The F4A fraction and the pure metabolites were used. In vitro, 10 µg mL⁻¹ of F4A reduced spore germination by 54%, while 100 µg mL⁻¹ completely inhibited. Overexpression of phenylalanine ammonia lyase (PAL), O-methyltransferase (OMT) and pathogenesis related protein-2 (PR-2; glucanases) defense-related genes were detected 24 and 72 h after soybean sprouts were sprayed with an organocopper antimicrobial compound (OAC). Under greenhouse conditions, the best control was observed in plants treated with 60 µg mL⁻¹ of PCA, which reduced ASR severity and lesion frequency by 75% and 43%, respectively. Plants sprayed with 2 and 20 µg mL⁻¹ of F4A also decreased severity (41%) and lesion frequency (32%). The significant reduction in spore germination ASR in plant suggested that the strain of these metabolites are effective against P. pachyrhizi, and they can be used for ASR control.

A genome-wide association study reveals cytokinin as a major component in the root defense responses against Ralstonia solanacearum

Bacterial wilt caused by the soil-borne pathogen Ralstonia solancearum is economically devastating, with no effective methods to fight the disease. This pathogen invades plants through their roots and colonizes their xylem, clogging the vasculature and causing rapid wilting. Key to preventing colonization are the early defense responses triggered in the host's root upon infection, which remain mostly unknown. Here, we have taken advantage of a high-throughput in vitro infection system to screen natural variability associated with the root growth inhibition phenotype caused by R. solanacearum in Arabidopsis during the first hours of infection. To analyze the genetic determinants of this trait, we have performed a genome-wide association study, identifying allelic variation at several loci related to cytokinin metabolism, including genes responsible for biosynthesis and degradation of cytokinin. Further, our data clearly demonstrate that cytokinin signaling is induced early during the infection process and cytokinin contributes to immunity against R. solanacearum. This study highlights a new role for cytokinin in root immunity, paving the way for future research that will help in understanding the mechanisms underpinning root defenses.

Transcriptional Changes of Plant Defense-Related Genes in Response to Clavibacter Infection in Pepper and Tomato

Pepper and tomato plants infected with two Clavibacter species, C. capsici and C. michiganensis have shown different patterns of disease development depending on their virulence. Here, we investigated how pepper and tomato plants respond to infection by the high-virulent or low-virulent Clavibacter strains. For this, we chose two strains of each Clavibacter species to show different virulence level in the host plants. Although low-virulent strains showed less disease symptoms, they grew almost the same level as the high-virulent strains in both plants. To further examine the response of host plants to Clavibacter infection, we analyzed the expression patterns of plant defense-related genes in the leaves inoculated with different strains of C. capsici and C. michiganensis. Pepper plants infected with high-virulent C. capsici strain highly induced the expression of CaPR1, CaDEF, CaPR4b, CaPR10, and CaLOX1 at 5 days after inoculation (dai), but their expression was much less in low-virulent Clavibacter infection. Expression of CaSAR8.2 was induced at 2 dai, regardless of virulence level. Expression of GluA, Pin2, and PR2 in tomato plants infected with high-virulent C. michiganensis were much higher at 5 dai, compared with mock or low-virulent strain. Expression of PR1a, Osmotin-like, Chitinase, and Chitinase class 2 was increased, regardless of virulence level. Expression of LoxA gene was not affected by Clavibacter inoculation. These results suggested that Clavibacter infection promotes induction of certain defense-related genes in host plants and that differential expression of those genes by low-virulent Clavibacter infection might be affected by their endophytic lifestyle in plants.

Understanding Phytomicrobiome: A Potential Reservoir for Better Crop Management

Recent crop production studies have aimed at an increase in the biotic and abiotic tolerance of plant communities, along with increased nutrient availability and crop yields. This can be achieved in various ways, but one of the emerging approaches is to understand the phytomicrobiome structure and associated chemical communications. The phytomicrobiome was characterized with the advent of high-throughput techniques. Its composition and chemical signaling phenomena have been revealed, leading the way for “rhizosphere engineering”. In addition to the above, phytomicrobiome studies have paved the way to best tackling soil contamination with various anthropogenic activities. Agricultural lands have been found to be unbalanced for crop production. Due to the intense application of agricultural chemicals such as herbicides, fungicides, insecticides, fertilizers, etc., which can only be rejuvenated efficiently through detailed studies on the phytomicrobiome component, the phytomicrobiome has recently emerged as a primary plant trait that affects crop production. The phytomicrobiome also acts as an essential modifying factor in plant root exudation and vice versa, resulting in better plant health and crop yield both in terms of quantity and quality. Not only supporting better plant growth, phytomicrobiome members are involved in the degradation of toxic materials, alleviating the stress conditions that adversely affect plant development. Thus, the present review compiles the progress in understanding phytomicrobiome relationships and their application in achieving the goal of sustainable agriculture.

Tolerance to ozone might impose restrictions to plant disease management in tomato

Tropospheric ozone (O₃ ) is considered a major air pollutant having negative effects on plant growth and productivity. Background concentrations are expected to rise in several regions of the world in the next 50 years, affecting plant responses to diseases, thus requiring new management strategies for food production. The effects of elevated O₃ on the severity of a bacterial disease, and the effectiveness of a chemical defence inducer, were examined in two cultivars of tomato, Roma and Moneymaker, which present different tolerance to this pollutant. The two cultivars differ in their ability to produce and accumulate reactive oxygen species (ROS) in leaf tissues. Tomato plants were challenged with a strain of Xanthomonas vesicatoria, Xv9, which is pathogenic on tomato. Ozone consistently increased severity of the disease by over 40% in both cultivars. In the more tolerant cultivar, O₃ pollution increased disease intensity, even after applying a commercially available product to enhance resistance (acibenzolar-S-methyl, BTH). In the more susceptible cultivar, level of disease attained depended on the oxidative balance that resulted from other stress factors. The antioxidant capacity of the plant at the time of infection was relevant for controlling development of the disease. Our results suggest that development of O₃ tolerance in commercial crops might impose a penalty cost in terms of disease management under projected higher O₃ concentrations.

Integrated Use of Aureobasidium pullulans Strain CG163 and Acibenzolar-S-Methyl for Management of Bacterial Canker in Kiwifruit

An isolate of Aureobasidium pullulans (strain = CG163) and the plant defence elicitor acibenzolar-S-methyl (ASM) were investigated for their ability to control leaf spot in kiwifruit caused by Pseudomonassyringae pv. actinidiae biovar 3 (Psa). Clonal Actinidia chinensis var. deliciosa plantlets (‘Hayward’) were treated with ASM, CG163 or ASM + CG163 at seven and one day before inoculation with Psa. ASM (0.2 g/L) was applied either as a root or foliar treatments and CG163 was applied as a foliar spray containing 2 × 10⁷ CFU/mL. Leaf spot incidence was significantly reduced by all treatments compared with the control. The combination of ASM + CG163 had greater efficacy (75%) than either ASM (55%) or CG163 (40%) alone. Moreover, treatment efficacy correlated positively with the expression of defence-related genes: pathogenesis-related protein 1 (PR1), β-1,3-glucosidase, Glucan endo 1,3-β-glucosidase (Gluc_PrimerH) and Class IV chitinase (ClassIV_Chit), with greater gene upregulation in plants treated with ASM + CG163 than by the individual treatments. Pathogen population studies indicated that CG163 had significant suppressive activity against epiphytic populations of Psa. Endophytic populations were reduced by ASM + CG163 but not by the individual treatments, and by 96–144 h after inoculation were significantly lower than the control. Together these data suggest that ASM + CG163 have complementary modes of action that contribute to greater control of leaf spotting than either treatment alone.

Use of Systemic Acquired Resistance and Whitefly Optical Barriers to Reduce Tomato Yellow Leaf Curl Disease Damage to Tomato Crops

Epidemics of tomato yellow leaf curl disease (TYLCD) caused by tomato yellow leaf curl-like begomoviruses (genus Begomovirus, family Geminiviridae) severely damage open field and protected tomato crops worldwide. Intensive application of insecticides against the whitefly vector Bemisia tabaci is generally used as control strategy to reduce TYLCD impact. This practice, however, is frequently ineffective and has a negative impact on the environment and human health. TYLCD-resistant varieties are commercially available, but cultivation of susceptible traditional tasting ones is also requested if possible. For susceptible tomatoes, here we show that using whitefly optical barriers by means of UV-blocking plastics in protected crops can contribute to reducing TYLCD damage and increasing commercial fruit yield. Moreover, induction of systemic acquired resistance by application of the elicitor of plant defense acibenzolar-S-methyl was effective to reduce yield losses when viral pressure was moderate. Interestingly, combining both practices in protected tomato crops can result in a significant TYLCD control. Therefore, these control practices are proposed to be used commercially as management alternatives to include in integrated management of TYLCD.

The Defense Response of Nicotiana benthamiana to Peanut Stunt Virus Infection in the Presence of Symptom Exacerbating Satellite RNA

Peanut stunt virus (PSV) is a widespread disease infecting legumes. The PSV strains are classified into four subgroups and some are defined by the association of satellite RNAs (satRNAs). In the case of PSV, the presence of satRNAs alters the symptoms of disease in infected plants. In this study, we elucidated the plant response to PSV-G strain, which occurs in natural conditions without satRNA. However, it was found that it might easily acquire satRNA, which exacerbated pathogenesis in Nicotiana benthamiana. To explain the mechanisms underlying PSV infection and symptoms exacerbation caused by satRNA, we carried out transcriptome profiling of N. benthamiana challenged by PSV-G and satRNA using species-specific microarrays. Co-infection of plants with PSV-G + satRNA increased the number of identified differentially expressed genes (DEGs) compared with the number identified in PSV-G-infected plants. In both treatments, the majority of up-regulated DEGs were engaged in translation, ribosome biogenesis, RNA metabolism, and response to stimuli, while the down-regulated DEGs were required for photosynthesis. The presence of satRNA in PSV-G-infected plants caused different trends in expression of DEGs associated with phosphorylation, ATP binding, and plasma membrane.

The Chemistry of Plant-Microbe Interactions in the Rhizosphere and the Potential for Metabolomics to Reveal Signaling Related to Defense Priming and Induced Systemic Resistance

Plant roots communicate with microbes in a sophisticated manner through chemical communication within the rhizosphere, thereby leading to biofilm formation of beneficial microbes and, in the case of plant growth-promoting rhizomicrobes/-bacteria (PGPR), resulting in priming of defense, or induced resistance in the plant host. The knowledge of plant-plant and plant-microbe interactions have been greatly extended over recent years; however, the chemical communication leading to priming is far from being well understood. Furthermore, linkage between below- and above-ground plant physiological processes adds to the complexity. In metabolomics studies, the main aim is to profile and annotate all exo- and endo-metabolites in a biological system that drive and participate in physiological processes. Recent advances in this field has enabled researchers to analyze 100s of compounds in one sample over a short time period. Here, from a metabolomics viewpoint, we review the interactions within the rhizosphere and subsequent above-ground 'signalomics', and emphasize the contributions that mass spectrometric-based metabolomic approaches can bring to the study of plant-beneficial - and priming events.

Plant growth regulator-mediated anti-herbivore responses of cabbage (Brassica oleracea) against cabbage looper Trichoplusia ni Hübner (Lepidoptera: Noctuidae)

Plant elicitors can be biological or chemical-derived stimulators of jasmonic acid (JA) or salicylic acid (SA) pathways shown to prime the defenses in many crops. Examples of chemical elicitors of the JA and SA pathways include methyl-jasmonate and 1,2,3-benzothiadiazole-7-carbothioate (BTH or the commercial plant activator Actigard 50WG, respectively). The use of specific elicitors has been observed to affect the normal interaction between JA and SA pathways causing one to be upregulated and the other to be suppressed, often, but not always, at the expense of the plant's herbivore or pathogen defenses. The objective of this study was to determine whether insects feeding on Brassica crops might be negatively affected by SA inducible defenses combined with an inhibitor of detoxification and anti-oxidant enzymes that regulate the insect response to the plant's defenses. The relative growth rate of cabbage looper Trichoplusia ni Hübner (Lepidoptera: Noctuidae) fed induced cabbage Brassica oleraceae leaves with the inhibitor, quercetin, was significantly less than those fed control cabbage with and without the inhibitor. The reduced growth was related to the reduction of glutathione S-transferases (GSTs) by the combination of quercetin and increased levels of indole glucosinolates in the cabbage treated with BTH at 2.6× the recommended application rate. These findings may offer a novel combination of elicitor and synergist that can provide protection from plant disease and herbivores in cabbage and other Brassica crops.

Independent Effects of a Herbivore's Bacterial Symbionts on Its Performance and Induced Plant Defences

It is well known that microbial pathogens and herbivores elicit defence responses in plants. Moreover, microorganisms associated with herbivores, such as bacteria or viruses, can modulate the plant’s response to herbivores. Herbivorous spider mites can harbour different species of bacterial symbionts and exert a broad range of effects on host-plant defences. Hence, we tested the extent to which such symbionts affect the plant’s defences induced by their mite host and assessed if this translates into changes in plant resistance. We assessed the bacterial communities of two strains of the common mite pest Tetranychus urticae. We found that these strains harboured distinct symbiotic bacteria and removed these using antibiotics. Subsequently, we tested to which extent mites with and without symbiotic bacteria induce plant defences in terms of phytohormone accumulation and defence gene expression, and assessed mite oviposition and survival as a measure for plant resistance. We observed that the absence/presence of these bacteria altered distinct plant defence parameters and affected mite performance but we did not find indications for a causal link between the two. We argue that although bacteria-related effects on host-induced plant defences may occur, these do not necessarily affect plant resistance concomitantly.

Menadione Sodium Bisulphite (MSB) enhances the resistance response of tomato, leading to repel mollusc pests

BACKGROUND

Snails and slugs are terrestrial gastropods representing an important biotic stress that adversely affects crop yields. These pests are typically controlled with molluscicides, which produce pollution and toxicity and further induce the evolution of resistance mechanisms, making pest management even more challenging. In our work, we have assessed the efficacy of two different plant defence activators, menadione sodium bisulphite (MSB) and 1,2,3-benzothiadiazole-7-thiocarboxylic acid S-methyl ester (BTH), as inducers of resistance mechanisms of the model plant for defence, Solanum lycopersicum, against the generalist mollusc Theba grasseti (Helicidae). The study was designed to test the feeding behaviour and choice of snails, and also to analyse the expression profile of different genes specifically involved in defence against herbivores and wounds.

RESULTS

Our data suggest that, through the downregulation of the terpene volatile genes and the production of proteinase inhibitors, treated MSB plants may be less apparent to herbivores that use herbivore-induced plant volatiles for host location. By contrast, BTH was not effective in the treatment of the pest, probably owing to an antagonistic effect derived from the induction of both salicylic-acid-dependent and jasmonic-acid-dependent pathways.

CONCLUSIONS

This information is crucial to determine the genetic basis of the choice of terrestrial gastropod herbivores in tomato, providing valuable insight into how the plant defence activators could control herbivore pests in plants. Our work not only reports for the first time the interaction between tomato and a mollusc pest but also presents the action of two plant defence inductors that seems to produce opposed responses by inducing resistance mechanisms through different defence pathways.

Benzothiadiazole effect in the compatible tomato-Meloidogyne incognita interaction: changes in giant cell development and priming of two root anionic peroxidases

BTH application is effective in root-knot nematode-tomato interaction in a way that involves a delay in the formation of nematode feeding site and triggers molecular responses at several levels. The compatible interaction between root-knot nematodes and their hosts requires the nematode to overcome plant defense systems so that a sophisticated permanent feeding site (giant cells) can be produced within the host roots. It has been suggested that activators of plant defenses may provide a novel management strategy for controlling root-knot nematodes but little is known about the molecular basis by which these elicitors operate. The role of pre-treatment with Benzothiadiazole (BTH), a salicylic acid analog, in inducing resistance against Meloidogyne incognita infection was investigated in tomato roots. A decrease in galling in roots and feeding site numbers was observed following BTH treatment. Histological investigations showed a delay in formation of feeding sites in treated plants. BTH-treated galls had higher H2O2 production, lignin accumulation, and increased peroxidase activity than untreated galls. The expression of two tomato genes, Tap1 and Tap2, coding for anionic peroxidases, was examined by qRT-PCR and in situ hybridization in response to BTH. Tap1 was induced at all infection points, reaching the highest level at 15 dpi. Tap2 expression, although slightly delayed in untreated galls, increased during infection in both treated and untreated galls. The expression of Tap1 and Tap2 was observed in giant cells of untreated roots, whereas the transcripts were localized in both giant cells and in parenchyma cells surrounding the developing feeding sites in treated plants. These results show that BTH applied to tomato plants makes them more resistant to infection by nematodes, which become less effective in overcoming root defense pathway.

The role of ROS signaling in cross-tolerance: from model to crop

Reactive oxygen species (ROS) are key signaling molecules produced in response to biotic and abiotic stresses that trigger a variety of plant defense responses. Cross-tolerance, the enhanced ability of a plant to tolerate multiple stresses, has been suggested to result partly from overlap between ROS signaling mechanisms. Cross-tolerance can manifest itself both as a positive genetic correlation between tolerance to different stresses (inherent cross-tolerance), and as the priming of systemic plant tolerance through previous exposure to another type of stress (induced cross-tolerance). Research in model organisms suggests that cross-tolerance could be used to benefit the agronomy and breeding of crop plants. However, research under field conditions has been scarce and critical issues including the timing, duration, and intensity of a stressor, as well as its interactions with other biotic and abiotic factors, remain to be addressed. Potential applications include the use of chemical stressors to screen for stress-resistant genotypes in breeding programs and the agronomic use of chemical inducers of plant defense for plant protection. Success of these applications will rely on improving our understanding of how ROS signals travel systemically and persist over time, and of how genetic correlations between resistance to ROS, biotic, and abiotic stresses are shaped by cooperative and antagonistic interactions within the underlying signaling pathways.

Endophytic colonization of potato (Solanum tuberosum L.) by a novel competent bacterial endophyte, Pseudomonas putida strain P9, and its effect on associated bacterial communities

Pseudomonas putida strain P9 is a novel competent endophyte from potato. P9 causes cultivar-dependent suppression of Phytophthora infestans. Colonization of the rhizoplane and endosphere of potato plants by P9 and its rifampin-resistant derivative P9R was studied. The purposes of this work were to follow the fate of P9 inside growing potato plants and to establish its effect on associated microbial communities. The effects of P9 and P9R inoculation were studied in two separate experiments. The roots of transplants of three different cultivars of potato were dipped in suspensions of P9 or P9R cells, and the plants were planted in soil. The fate of both strains was followed by examining colony growth and by performing PCR-denaturing gradient gel electrophoresis (PCR-DGGE). Colonies of both strains were recovered from rhizoplane and endosphere samples of all three cultivars at two growth stages. A conspicuous band, representing P9 and P9R, was found in all Pseudomonas PCR-DGGE fingerprints for treated plants. The numbers of P9R CFU and the P9R-specific band intensities for the different replicate samples were positively correlated, as determined by linear regression analysis. The effects of plant growth stage, genotype, and the presence of P9R on associated microbial communities were examined by multivariate and unweighted-pair group method with arithmetic mean cluster analyses of PCR-DGGE fingerprints. The presence of strain P9R had an effect on bacterial groups identified as Pseudomonas azotoformans, Pseudomonas veronii, and Pseudomonas syringae. In conclusion, strain P9 is an avid colonizer of potato plants, competing with microbial populations indigenous to the potato phytosphere. Bacterization with a biocontrol agent has an important and previously unexplored effect on plant-associated communities.