Prosystemin Overexpression in Tomato Enhances Resistance to Different Biotic Stresses by Activating Genes of Multiple Signaling Pathways

Pull the fuzes: Processing protein precursors to generate apoplastic danger signals for triggering plant immunity

The apoplast is one of the first cellular compartments outside the plasma membrane encountered by phytopathogenic microbes in the early stages of plant tissue invasion. Plants have developed sophisticated surveillance mechanisms to sense danger events at the cell surface and promptly activate immunity. However, a fine tuning of the activation of immune pathways is necessary to mount a robust and effective defense response. Several endogenous proteins and enzymes are synthesized as inactive precursors, and their post-translational processing has emerged as a critical mechanism for triggering alarms in the apoplast. In this review, we focus on the precursors of phytocytokines, cell wall remodeling enzymes, and proteases. The physiological events that convert inactive precursors into immunomodulatory active peptides or enzymes are described. This review also explores the functional synergies among phytocytokines, cell wall damage-associated molecular patterns, and remodeling, highlighting their roles in boosting extracellular immunity and reinforcing defenses against pests.

Deciphering molecular events behind Systemin-induced resistance to Botrytis cinerea in tomato plants

Plant defence peptides are paramount endogenous danger signals secreted after a challenge, intensifying the plant immune response. The peptidic hormone Systemin (Sys) was shown to participate in resistance in several plant pathosystems, although the mechanisms behind Sys-induced resistance when exogenously applied remain elusive. We performed proteomic, metabolomic, and enzymatic studies to decipher the Sys-induced changes in tomato plants in either the absence or the presence of Botrytis cinerea infection. Sys treatments triggered direct proteomic rearrangement mostly involved in carbon metabolism and photosynthesis. However, the final induction of defence proteins required concurrent challenge, triggering priming of pathogen-targeted proteins. Conversely, at the metabolomic level, Sys-treated plants showed an alternative behaviour following a general priming profile. Of the primed metabolites, the flavonoids rutin and isorhamnetin and two alkaloids correlated with the proteins 4-coumarate-CoA-ligase and chalcone-flavanone-isomerase triggered by Sys treatment. In addition, proteomic and enzymatic analyses revealed that Sys conditioned the primary metabolism towards the production of available sugars that could be fuelling the priming of callose deposition in Sys-treated plants; furthermore, PR1 appeared as a key element in Sys-induced resistance. Collectively, the direct induction of proteins and priming of specific secondary metabolites in Sys-treated plants indicated that post-translational protein regulation is an additional component of priming against necrotrophic fungi.

Plant Protease Inhibitors as Emerging Antimicrobial Peptide Agents: A Comprehensive Review

Antimicrobial peptides (AMPs) are important mediator molecules of the innate defense mechanisms in a wide range of living organisms, including bacteria, mammals, and plants. Among them, peptide protease inhibitors (PPIs) from plants play a central role in their defense mechanisms by directly attacking pathogens or by modulating the plant's defense response. The growing prevalence of microbial resistance to currently available antibiotics has intensified the interest concerning these molecules as novel antimicrobial agents. In this scenario, PPIs isolated from a variety of plants have shown potential in inhibiting the growth of pathogenic bacteria, protozoans, and fungal strains, either by interfering with essential biochemical or physiological processes or by altering the permeability of biological membranes of invading organisms. Moreover, these molecules are active inhibitors of a range of proteases, including aspartic, serine, and cysteine types, with some showing particular efficacy as trypsin and chymotrypsin inhibitors. In this review, we provide a comprehensive analysis of the potential of plant-derived PPIs as novel antimicrobial molecules, highlighting their broad-spectrum antimicrobial efficacy, specificity, and minimal toxicity. These natural compounds exhibit diverse mechanisms of action and often multifunctionality, positioning them as promising molecular scaffolds for developing new therapeutic antibacterial agents.

Intrinsically disordered Prosystemin discloses biologically active repeat motifs

The in-depth studies over the years on the defence barriers by tomato plants have shown that the Systemin peptide controls the response to a wealth of environmental stress agents. This multifaceted stress reaction seems to be related to the intrinsic disorder of its precursor protein, Prosystemin (ProSys). Since latest findings show that ProSys has biological functions besides Systemin sequence, here we wanted to assess if this precursor includes peptide motifs able to trigger stress-related pathways. Candidate peptides were identified in silico and synthesized to test their capacity to trigger defence responses in tomato plants against different biotic stressors. Our results demonstrated that ProSys harbours several repeat motifs which triggered plant immune reactions against pathogens and pest insects. Three of these peptides were detected by mass spectrometry in plants expressing ProSys, demonstrating their effective presence in vivo. These experimental data shed light on unrecognized functions of ProSys, mediated by multiple biologically active sequences which may partly account for the capacity of ProSys to induce defense responses to different stress agents.

Damage-Associated Molecular Patterns and Systemic Signaling

Cellular damage inflicted by wounding, pathogen infection, and herbivory releases a variety of host-derived metabolites, degraded structural components, and peptides into the extracellular space that act as alarm signals when perceived by adjacent cells. These so-called damage-associated molecular patterns (DAMPs) function through plasma membrane localized pattern recognition receptors to regulate wound and immune responses. In plants, DAMPs act as elicitors themselves, often inducing immune outputs such as calcium influx, reactive oxygen species generation, defense gene expression, and phytohormone signaling. Consequently, DAMP perception results in a priming effect that enhances resistance against subsequent pathogen infections. Alongside their established function in local tissues, recent evidence supports a critical role of DAMP signaling in generation and/or amplification of mobile signals that induce systemic immune priming. Here, we summarize the identity, signaling, and synergy of proposed and established plant DAMPs, with a focus on those with published roles in systemic signaling.

Small Signals Lead to Big Changes: The Potential of Peptide-Induced Resistance in Plants

The plant immunity system is being revisited more and more and new elements and roles are attributed to participating in the response to biotic stress. The new terminology is also applied in an attempt to identify different players in the whole scenario of immunity: Phytocytokines are one of those elements that are gaining more attention due to the characteristics of processing and perception, showing they are part of a big family of compounds that can amplify the immune response. This review aims to highlight the latest findings on the role of phytocytokines in the whole immune response to biotic stress, including basal and adaptive immunity, and expose the complexity of their action in plant perception and signaling events.

In silico and in vitro approaches allow the identification of the Prosystemin molecular network

Tomato Prosystemin (ProSys), the precursor of Systemin, a small peptidic hormone, is produced at very low concentration in unchallenged plants, while its expression greatly increases in response to several different stressors triggering an array of defence responses. The molecular mechanisms that underpin such a wide array of defence barriers are not fully understood and are likely correlated with the intrinsically disordered (ID) structure of the protein. ID proteins interact with different protein partners forming complexes involved in the modulation of different biological mechanisms. Here we describe the ProSys-protein network that shed light on the molecular mechanisms underpinning ProSys associated defence responses. Three different approaches were used. In silico prediction resulted in 98 direct interactors, most clustering in phytohormone biosynthesis, transcription factors and signal transduction gene classes. The network shows the central role of ProSys during defence responses, that reflects its role as central hub. In vitro ProSys interactors, identified by Affinity Purification-Mass Spectrometry (AP-MS), revealed over three hundred protein partners, while Bimolecular Fluorescent Complementation (BiFC) experiments validated in vivo some interactors predicted in silico and in vitro. Our results demonstrate that ProSys interacts with several proteins and reveal new key molecular events in the ProSys-dependent defence response of tomato plant.

The Use of DArTseq Technology to Identify Markers Linked to Genes Responsible for Seed Germination and Seed Vigor in Maize

Seed vigor and seed germination are very important traits, determined by several factors including genetic and physical purity, mechanical damage, and physiological condition, characterized by maintaining a high seed vigor and stable content after storage. The search for molecular markers related to improvement in seed vigor under adverse condition is an important issue in maize breeding currently. Higher sowing quality of seeds is necessary for the development of the agriculture production and better ability to resist all kinds of adversity in the seeds’ storage. Condition is a very important factor affecting the yield of plants, thanks to the construction of their vitality. Identification of molecular markers associated with seed germination and seed vigor may prove to be very important in the selection of high-yielding maize varieties. The aim of this study was to identify and select new markers for maize (SNP and SilicoDArT) linked to genes influencing the seed germination and seed vigor in inbred lines of maize (Zea mays L.). The plant material used for the research was 152 inbred maize lines. The seed germination and seed vigor were analyzed. For identification of SNP and SilicoDArT markers related to the seed germination and seed vigor, the SilicoDarT technique developed by Diversity Arrays Technology was used. The analysis of variance indicated a statistically significant differentiation between genotypes for both observed traits. Positive (r = 0.41) correlation (p < 0.001) between seed germination and seed vigor was observed. As a result of next-generation sequencing, the molecular markers SilicoDArT (53,031) and SNP (28,571) were obtained. Out of 81,602 identified SilicoDArT and SNP markers, 15,409 (1559 SilicoDArT and 13,850 SNP) were selected as a result of association mapping, which showed them to be significantly related to the analyzed traits. The 890 molecular markers were associated with seed vigor, and 1323 with seed germination. Fifty-six markers (47 SilicoDArT and nine SNP) were significant for both traits. Of these 56 markers, the 20 most significant were selected (five of these markers were significant at the level of 0.001 for seed vigor and at the level of 0.05 for seed germination, another five markers were significant at the level of 0.001 for seed germination and at the level of 0.05 for seed vigor, five markers significant at the level of 0.001 only for seed vigor and five significant at the level of 0.001 only for seed germination also selected). These markers were used for physical mapping to determine their location on the genetic map. Finally, it was found that six of these markers (five silicoDArT—2,435,784, 4,772,587, 4,776,334, 2,507,310, 25,981,291, and one SNP—2,386,217) are located inside genes, the action of which may affect both seed germination and seed vigor. These markers can be used to select genotypes with high vigor and good seed germination.

Transgenic Improvement for Biotic Resistance of Crops

Biotic constraints, including pathogenic fungi, viruses and bacteria, herbivory insects, as well as parasitic nematodes, cause significant yield loss and quality deterioration of crops. The effect of conventional management of these biotic constraints is limited. The advances in transgenic technologies provide a direct and directional approach to improve crops for biotic resistance. More than a hundred transgenic events and hundreds of cultivars resistant to herbivory insects, pathogenic viruses, and fungi have been developed by the heterologous expression of exogenous genes and RNAi, authorized for cultivation and market, and resulted in a significant reduction in yield loss and quality deterioration. However, the exploration of transgenic improvement for resistance to bacteria and nematodes by overexpression of endogenous genes and RNAi remains at the testing stage. Recent advances in RNAi and CRISPR/Cas technologies open up possibilities to improve the resistance of crops to pathogenic bacteria and plant parasitic nematodes, as well as other biotic constraints.

Tomato Systemin induces resistance against Plectosphaerella cucumerina in Arabidopsis through the induction of phenolic compounds and priming of tryptophan derivatives

Phytocytokines are endogenous danger peptides that are actively released after a pest or pathogen attack, triggering an amplification of plant immune responses. Here, we found that Systemin, a peptide from tomato, has a substantial impact at the molecular level in Arabidopsis plants that leads to induced resistance against Plectosphaerella cucumerina. Using transcriptional and metabolomics approaches, and loss-of-function mutants to analyse the molecular mechanisms underlying induced resistance against the necrotroph, we decipher the enhanced molecular responses in Systemin-treated plants following infection. Some protein complexes involved in the response to other damage signals, including the BAK1-BIK1 protein complex and heterotrimeric G proteins, as well as MPK activation, were among the early signalling events triggered by Systemin in Arabidopsis upon infection. Non-targeted analysis of the late responses underlying Systemin-Induced Resistance¹ (Sys-IR) showed that phenolic and indolic compounds were the most representative groups in the Systemin metabolic fingerprint. Lack of flavonoids resulted in the impairment of Sys-IR. On the other hand, some indolic compounds showed a priming profile and were also essential for functional Sys-IR. Evidence presented here shows that plants can sense heterologous peptides from other species as danger signals driving the participation of common protein cascades activated in the PTI and promoting enhanced resistance against necrotrophic fungus.

Not Only Systemin: Prosystemin Harbors Other Active Regions Able to Protect Tomato Plants

Prosystemin is a 200-amino acid precursor expressed in Solanaceae plants which releases at the C-terminal part a peptidic hormone called Systemin in response to wounding and herbivore attack. We recently showed that Prosystemin is not only a mere scaffold of Systemin but, even when deprived of Systemin, is biologically active. These results, combined with recent discoveries that Prosystemin is an intrinsically disordered protein containing disordered regions within its sequence, prompted us to investigate the N-terminal portions of the precursor, which contribute to the greatest disorder within the sequence. To this aim, PS1-70 and PS1-120 were designed, produced, and structurally and functionally characterized. Both the fragments, which maintained their intrinsic disorder, were able to induce defense-related genes and to protect tomato plants against Botrytis cinerea and Spodoptera littoralis larvae. Intriguingly, the biological activity of each of the two N-terminal fragments and of Systemin is similar but not quite the same and does not show any toxicity on experimental non-targets considered. These regions account for different anti-stress activities conferred to tomato plants by their overexpression. The two N-terminal fragments identified in this study may represent new promising tools for sustainable crop protection.

Tomato Prosystemin Is Much More than a Simple Systemin Precursor

Simple Summary

Prosystemin is a 200 amino acid precursor that releases, upon wounding and biotic attacks, an 18 amino acid peptide called Systemin. This peptide was traditionally considered as the principal actor of the resistance of tomato plants induced by triggering multiple defense pathways in response to a wide range of biotic/abiotic stress agents. Recent findings from our group discovered the disordered structure of Prosystemin that promotes the binding of different molecular partners and the possible activation of multiple stress-related pathways. All of our recent findings suggest that Prosystemin could be more than a simple precursor of Systemin peptide. Indeed, we hypothesized that it contains other sequences able to activate multiple stress-related responses. To verify this hypothesis, we produced a truncated Prosystemin protein deprived of the Systemin peptide and the relative deleted gene. Experiments with transgenic tomato plants overexpressing the truncated Prosystemin and with plants exogenously treated with the recombinant truncated protein demonstrated that both transgenic and treated plants modulated the expression of defense-related genes and were protected against a noctuid moth and a fungal pathogen. Taken together, our results demonstrated that Prosystemin is not a mere scaffold of Systemin, but itself contains other biologically active regions.

Abstract

Systemin (Sys) is an octadecapeptide, which upon wounding, is released from the carboxy terminus of its precursor, Prosystemin (ProSys), to promote plant defenses. Recent findings on the disordered structure of ProSys prompted us to investigate a putative biological role of the whole precursor deprived of the Sys peptide. We produced transgenic tomato plants expressing a truncated ProSys gene in which the exon coding for Sys was removed and compared their defense response with that induced by the exogenous application of the recombinant truncated ProSys (ProSys_(1-178), the Prosystemin sequence devoid of Sys region). By combining protein structure analyses, transcriptomic analysis, gene expression profiling and bioassays with different pests, we demonstrate that truncated ProSys promotes defense barriers in tomato plants through a hormone-independent defense pathway, likely associated with the production of oligogalacturonides (OGs). Both transgenic and plants treated with the recombinant protein showed the modulation of the expression of genes linked with defense responses and resulted in protection against the lepidopteran pest Spodoptera littoralis and the fungus Botrytis cinerea. Our results suggest that the overall function of the wild-type ProSys is more complex than previously shown, as it might activate at least two tomato defense pathways: the well-known Sys-dependent pathway connected with the induction of jasmonic acid biosynthesis and the successive activation of a set of defense-related genes, and the ProSys_(1-178)-dependent pathway associated with OGs production leading to the OGs mediate plant immunity.

Selection of Endophytic Beauveria bassiana as a Dual Biocontrol Agent of Tomato Pathogens and Pests

Endophytic fungi (EF) can enhance both plant growth and defense barriers against pests and pathogens, contributing to the reduction of chemical pesticides and fertilizers use in agriculture. Beauveria bassiana is an entomopathogenic fungus showing endophytism in several crops, often associated with a good capacity to limit the development of pests and disease agents. However, the diversity of the protective efficacy and plant response to different strains can be remarkable and needs to be carefully assessed for the successful and predictable use of these beneficial microorganisms. This study aims to select B. bassiana strains able to colonize tomato plants as endophytes as well as to control two important disease agents, Botrytis cinerea and Alternaria alternata, and the pest aphid, Macrosiphum euphorbiae. Nine wild-type isolates and one commercial strain were screened for endophytism, then further characterized for plant-growth promotion plus inhibition of disease development and pest infestation. Four isolates proved to have a good control activity against the biotic stressors tested, but only Bb716 was also able to promote plant growth. This work provides a simple workflow for the selection of beneficial EF, paving the way towards more effective use of B. bassiana in Integrate Pest Management (IPM) of tomato.

Trichoderma harzianum Strain T22 Modulates Direct Defense of Tomato Plants in Response to Nezara viridula Feeding Activity

Plant growth-promoting fungi belonging to genus Trichoderma are known to help plants when dealing with biotic stressors by enhancing plant defenses. While beneficial effects of Trichoderma spp. against plant pathogens have long been documented, fewer studies have investigated their effect on insect pests. Here, we studied the impact of Trichoderma root colonization on the plant defense responses against stink bug feeding attack. For this purpose, a model system consisting of tomato plant, Solanum lycopersicum cv Dwarf San Marzano, Trichoderma harzianum strain T22 and the southern green stink bug, Nezara viridula, was used. We firstly determined stink bug performance in terms of relative growth rate and survival on tomato plants inoculated by T. harzianum T22. Then, we evaluated relative expression of plant defense-related genes on inoculated plants induced by stink bug feeding. We found evidence that T. harzianum T22 affects tomato defense responses against N. viridula nymphs leading to reduction of growth rate. Our results also showed that T. harzianum T22 enhances plant direct defenses by an early increase of transcript levels of jasmonic acid marker genes. Yet this effect was time-dependent and only detected 8 h after herbivore induction. Taken together, our findings provide better understanding on the mechanisms underlying tomato induced resistance against herbivorous stink bugs.

Colonization of Solanum melongena and Vitis vinifera Plants by Botrytis cinerea Is Strongly Reduced by the Exogenous Application of Tomato Systemin

Plant defense peptides are able to control immune barriers and represent a potential novel resource for crop protection. One of the best-characterized plant peptides is tomato Systemin (Sys) an octadecapeptide synthesized as part of a larger precursor protein. Upon pest attack, Sys interacts with a leucine-rich repeat receptor kinase, systemin receptor SYR, activating a complex intracellular signaling pathway that leads to the wound response. Here, we demonstrated, for the first time, that the direct delivery of the peptide to Solanum melongena and Vitis vinifera plants protects from the agent of Grey mould (Botrytis cinerea). The observed disease tolerance is associated with the increase of total soluble phenolic content, the activation of antioxidant enzymes, and the up-regulation of defense-related genes in plants treated with the peptide. Our results suggest that in treated plants, the biotic defense system is triggered by the Sys signaling pathway as a consequence of Sys interaction with a SYR-like receptor recently found in several plant species, including those under investigation. We propose that this biotechnological use of Sys, promoting defense responses against invaders, represents a useful tool to integrate into pest management programs for the development of novel strategies of crop protection.

Genome-wide analyses of direct target genes of an ERF11 transcription factor involved in plant defense against bacterial pathogens

Ethylene responsive factor ERF11 containing the ERF-associated amphiphilic repression (EAR) motif enhances plant resistance to bacterial pathogens. However, the underlying molecular mechanisms regulated by transcription factor ERF11 are poorly understood, in tobacco or other model plants. Here, we revealed the genome-wide binding landscape of BrERF11b in Nicotiana benthamian by conducting chromatin immunoprecipitation experiments followed by high-throughput sequencing (ChIP-seq) and bioinformatic analyses. Our results also revealed a GCCbox-like consensus BrERF11b-binding DNA motif: VCGCCGCC. By further integrative analysis of ChIP-seq and RNA-seq data, and the confirmation of electrophoretic mobility shift assay (EMSA), we screened three direct target genes NbNIMIN2, NbTAF15b and NbERF4. These results suggest that ERF11 may be involved in NPR1-mediated systemic acquired resistance (SAR), nucleotide-binding leucine-rich repeat immune receptors (NLR) -mediated autoimmunity, and H2O2 generation, by direct transcriptional repression of NIM1-INTERACTING2 (NIMIN2), and transcriptional activation of TATA-binding protein-associated factor 15b (TAF15b) and ERF4. Our findings provide insightful information and valuable gene resource in unraveling the regulatory networks of plant defense responses to bacterial pathogens.

Arabidopsis Plants Sense Non-self Peptides to Promote Resistance Against Plectosphaerella cucumerina

Peptides are important regulators that participate in the modulation of almost every physiological event in plants, including defense. Recently, many of these peptides have been described as defense elicitors, termed phytocytokines, that are released upon pest or pathogen attack, triggering an amplification of plant defenses. However, little is known about peptides sensing and inducing resistance activities in heterologous plants. In the present study, exogenous peptides from solanaceous species, Systemins and HypSys, are sensed and induce resistance to the necrotrophic fungus Plectosphaerella cucumerina in the taxonomically distant species Arabidopsis thaliana. Surprisingly, other peptides from closer taxonomic clades have very little or no effect on plant protection. In vitro bioassays showed that the studied peptides do not have direct antifungal activities, suggesting that they protect the plant through the promotion of the plant immune system. Interestingly, tomato Systemin was able to induce resistance at very low concentrations (0.1 and 1 nM) and displays a maximum threshold being ineffective above at higher concentrations. Here, we show evidence of the possible involvement of the JA-signaling pathway in the Systemin-Induced Resistance (Sys-IR) in Arabidopsis. Additionally, Systemin treated plants display enhanced BAK1 and BIK1 gene expression following infection as well as increased production of ROS after PAMP treatment suggesting that Systemin sensitizes Arabidopsis perception to pathogens and PAMPs.

Plant immunity in signal integration between biotic and abiotic stress responses

Plants constantly monitor and cope with the fluctuating environment while hosting a diversity of plant-inhabiting microbes. The mode and outcome of plant-microbe interactions, including plant disease epidemics, are dynamically and profoundly influenced by abiotic factors, such as light, temperature, water and nutrients. Plants also utilize associations with beneficial microbes during adaptation to adverse conditions. Elucidation of the molecular bases for the plant-microbe-environment interactions is therefore of fundamental importance in the plant sciences. Following advances into individual stress signaling pathways, recent studies are beginning to reveal molecular intersections between biotic and abiotic stress responses and regulatory principles in combined stress responses. We outline mechanisms underlying environmental modulation of plant immunity and emerging roles for immune regulators in abiotic stress tolerance. Furthermore, we discuss how plants coordinate conflicting demands when exposed to combinations of different stresses, with attention to a possible determinant that links initial stress response to broad-spectrum stress tolerance or prioritization of specific stress tolerance.

Tomato Plants Treated with Systemin Peptide Show Enhanced Levels of Direct and Indirect Defense Associated with Increased Expression of Defense-Related Genes

Plant defense peptides represent an important class of compounds active against pathogens and insects. These molecules controlling immune barriers can potentially be used as novel tools for plant protection, which mimic natural defense mechanisms against invaders. The constitutive expression in tomato plants of the precursor of the defense peptide systemin was previously demonstrated to increase tolerance against moth larvae and aphids and to hamper the colonization by phytopathogenic fungi, through the expression of a wealth of defense-related genes. In this work we studied the impact of the exogenous supply of systemin to tomato plants on pests to evaluate the use of the peptide as a tool for crop protection in non-transgenic approaches. By combining gene expression studies and bioassays with different pests we demonstrate that the exogenous supply of systemin to tomato plants enhances both direct and indirect defense barriers. Experimental plants, exposed to this peptide by foliar spotting or root uptake through hydroponic culture, impaired larval growth and development of the noctuid moth Spodoptera littoralis, even across generations, reduced the leaf colonization by the fungal pathogen Botrytis cinerea and were more attractive towards natural herbivore antagonists. The induction of these defense responses was found to be associated with molecular and biochemical changes under control of the systemin signalling cascade. Our results indicate that the direct delivery of systemin, likely characterized by a null effect on non-target organisms, represents an interesting tool for the sustainable protection of tomato plants.

Mycorrhizal Fungi Enhance Resistance to Herbivores in Tomato Plants with Reduced Jasmonic Acid Production

Arbuscular mycorrhizal (AM) fungi favor plant growth by improving nutrient acquisition, but also by increasing their resistance against abiotic and biotic stressors, including herbivory. Mechanisms of AM fungal mediated increased resistance include a direct effect of AM fungi on plant vigor, but also a manipulation of the hormonal cascades, such as the systemic activation of jasmonic acid (JA) dependent defenses. However, how AM fungal inoculation and variation in the endogenous JA production interact to produce increased resistance against insect herbivores remains to be further elucidated. To address this question, three genotypes of Solanum lycopersicum L., a JA-biosynthesis deficient mutant, a JA over-accumulating mutant, and their wild-type were either inoculated with AM fungi or left un-inoculated. Plant growth-related traits and resistance against Spodoptera littoralis (Boisduval) caterpillars, a major crop pest, were measured. Overall, we found that deficiency in JA production reduced plant development and were the least resistant against S. littoralis. Moreover, AM fungi increased plant resistance against S. littoralis, but such beneficial effect was more pronounced in JA-deficient plant than on JA over-accumulating plants. These results highlight that AM fungi-driven increased plant resistance is negatively affected by the ability of plants to produce JA and that AM fungi complement JA-mediated endogenous plant defenses in this system.

Modulation of steroidal glycoalkaloid biosynthesis in tomato (Solanum lycopersicum) by jasmonic acid

Jasmonic acid (JA) is a phytohormone involved in plant development and defense. A major role of JA is the enhancement of secondary metabolite production, such as response to herbivory. Systemin is a bioactive plant peptide of 18 amino acids that contributes to the induction of local and systemic defense responses in tomato (Solanum lycopersicum) through JA biosynthesis. The overexpression of systemin (PS-OE) results in constitutive JA accumulation and enhances pest resistance in plants. Conversely, mutant plants affected in linolenic acid synthesis (spr2) are negatively compromised in the production of JA which favors damage and oviposition by insect herbivores. With undirected mass fingerprinting analyses, we found global metabolic differences between genotypes with modified jasmonic acid production. The spr2 mutants were enriched in di-unsaturated fatty acids and generally showed more changes. The PS-OE genotype produced an unidentified compound with a mass-to-charge ratio of 695 (MZ695). Most strikingly, the steroidal glycoalkaloid biosynthesis was negatively affected in the spr2 genotype. Complementation with jasmonic acid could restore the tomatine pathway, which strongly suggests the control of steroidal glycoalkaloid biosynthesis by jasmonic acid. spr2 plants were more susceptible to fungal infection with Fusarium oxysporum f.sp. ciceris, but not to bacterial infection with Clavibacter michiganensis subsp. michiganensis which supports the involvement of steroidal glycoalkaloids in the plant response against fungi.

TPS Genes Silencing Alters Constitutive Indirect and Direct Defense in Tomato

Following herbivore attacks, plants modify a blend of volatiles organic compounds (VOCs) released, resulting in the attraction of their antagonists. However, volatiles released constitutively may affect herbivores and natural enemies' fitness too. In tomato there is still a lack of information on the genetic bases responsible for the constitutive release of VOC involved in direct and indirect defenses. Here we studied the constitutive emissions related to the two most abundant sesquiterpene synthase genes expressed in tomato and their functional role in plant defense. Using an RNA interference approach, we silenced the expression of TPS9 and TPS12 genes and assessed the effect of this transformation on herbivores and parasitoids. We found that silenced plants displayed a different constitutive volatiles emission from controls, resulting in reduced attractiveness for the aphid parasitoid Aphidius ervi and in an impaired development of Spodoptera exigua larvae. We discussed these data considering the transcriptional regulation of key-genes involved in the pathway of VOC metabolism. We provide several lines of evidence on the metabolic flux from terpenoids to phenylpropanoids. Our results shed more light on constitutive defenses mediated by plant volatiles and on the molecular mechanisms involved in their metabolic regulation.

Tomato PEPR1 ORTHOLOG RECEPTOR-LIKE KINASE1 Regulates Responses to Systemin, Necrotrophic Fungi, and Insect Herbivory

Endogenous peptides regulate plant immunity and growth. Systemin, a peptide specific to the Solanaceae, is known for its functions in plant responses to insect herbivory and pathogen infections. Here, we describe the identification of the tomato (Solanum lycopersicum) PEPR1/2 ORTHOLOG RECEPTOR-LIKE KINASE1 (PORK1) as the TOMATO PROTEIN KINASE1b (TPK1b) interacting protein and demonstrate its biological functions in systemin signaling and tomato immune responses. Tomato PORK1 RNA interference (RNAi) plants with significantly reduced PORK1 expression showed increased susceptibility to tobacco hornworm (Manduca sexta), reduced seedling growth sensitivity to the systemin peptide, and compromised systemin-mediated resistance to Botrytis cinerea. Systemin-induced expression of Proteinase Inhibitor II (PI-II), a classical marker for systemin signaling, was abrogated in PORK1 RNAi plants. Similarly, in response to systemin and wounding, the expression of jasmonate pathway genes was attenuated in PORK1 RNAi plants. TPK1b, a key regulator of tomato defense against B. cinerea and M. sexta, was phosphorylated by PORK1. Interestingly, wounding- and systemin-induced phosphorylation of TPK1b was attenuated when PORK1 expression was suppressed. Our data suggest that resistance to B. cinerea and M. sexta is dependent on PORK1-mediated responses to systemin and subsequent phosphorylation of TPK1b. Altogether, PORK1 regulates tomato systemin, wounding, and immune responses.

Plant Perception and Short-Term Responses to Phytophagous Insects and Mites

Plant⁻pest relationships involve complex processes encompassing a network of molecules, signals, and regulators for overcoming defenses they develop against each other. Phytophagous arthropods identify plants mainly as a source of food. In turn, plants develop a variety of strategies to avoid damage and survive. The success of plant defenses depends on rapid and specific recognition of the phytophagous threat. Subsequently, plants trigger a cascade of short-term responses that eventually result in the production of a wide range of compounds with defense properties. This review deals with the main features involved in the interaction between plants and phytophagous insects and acari, focusing on early responses from the plant side. A general landscape of the diverse strategies employed by plants within the first hours after pest perception to block the capability of phytophagous insects to develop mechanisms of resistance is presented, with the potential of providing alternatives for pest control.

Chemical signaling involved in plant-microbe interactions

Microorganisms are found everywhere, and they are closely associated with plants. Because the establishment of any plant-microbe association involves chemical communication, understanding crosstalk processes is fundamental to defining the type of relationship. Although several metabolites from plants and microbes have been fully characterized, their roles in the chemical interplay between these partners are not well understood in most cases, and they require further investigation. In this review, we describe different plant-microbe associations from colonization to microbial establishment processes in plants along with future prospects, including agricultural benefits.

Expression of tomato prosystemin gene in Arabidopsis reveals systemic translocation of its mRNA and confers necrotrophic fungal resistance

Systemin (SYS), an octadecapeptide hormone processed from a 200-amino-acid precursor (prosystemin, PS), plays a central role in the systemic activation of defense genes in tomato in response to herbivore and pathogen attacks. However, whether PS mRNA is transferable and its role in systemic defense responses remain unknown. We created the transgenic tomato PS gene tagged with the green fluorescent protein (PS-GFP) using a shoot- or root-specific promoter, and the constitutive 35S promoter in Arabidopsis. Subcellular localization of PS-/SYS-GFP was observed using confocal laser scanning microscopy and gene transcripts were determined using quantitative real-time PCR. In Arabidopsis, PS protein can be processed and SYS is secreted. Shoot-/root-specific expression of PS-GFP in Arabidopsis, and grafting experiments, revealed that the PS mRNA moves in a bi-directional manner. We also found that ectopic expression of PS improves Arabidopsis resistance to the necrotrophic fungus Botrytis cinerea, consistent with substantial upregulation of the transcript levels of specific pathogen-responsive genes. Our results provide novel insights into the multifaceted mechanism of SYS signaling transport and its potential application in genetic engineering for increasing pathogen resistance across diverse plant families.

Accurate and easy method for systemin quantification and examining metabolic changes under different endogenous levels

BACKGROUND

Systemin has been extensively studied since it was discovered and is described as a peptidic hormone in tomato plants and other Solanaceae. Jasmonic acid and systemin are proposed to act through a positive feed-back loop with jasmonic acid, playing synergistic roles in response to both wounding and insect attack. Despite its biological relevance, most studies regarding the function of systemin in defence have been studied via PROSYSTEMIN (PROSYS) gene expression, which encodes the propeptide prosystemin that is later cleaved to systemin (SYS). Interestingly, hardly any studies have been based on quantification of the peptide.

RESULTS

In this study, a simple and accurate method for systemin quantification was developed to understand its impact on plant metabolism. The basal levels of systemin were found to be extremely low. To study the role of endogenous systemin on plant metabolism, systemin was quantified in a transgenic line overexpressing the PROSYS gene (PS+) and in a silenced antisense line (PS-). We evaluated the relevance of systemin in plant metabolism by analysing the metabolomic profiles of both lines compared to wildtype plants through untargeted metabolomic profiling. Compounds within the lignan biosynthesis and tyrosine metabolism pathways strongly accumulated in PS+ compared to wild-type plants and to plants from the PS- line. The exogenous treatments with SYS enhanced accumulation of lignans, which confirms the role of SYS in cell wall reinforcement. Unexpectedly, PS+ plants displayed wild-type levels of jasmonic acid (JA) but elevated accumulation of 12-oxo-phytodienoic acid (OPDA), suggesting that PS+ should not be used as an over-accumulator of JA in experimental setups.

CONCLUSIONS

A simple method, requiring notably little sample manipulation to quantify the peptide SYS, is described. Previous studies were based on genetic changes. In our study, SYS accumulated at extremely low levels in wild-type tomato leaves, showed slightly higher levels in the PROSYSTEMIN-overexpressing plants and was absent in the silenced lines. These small changes have a significant impact on plant metabolism. SA and OPDA, but not JA, were higher in the PROSYS-overexpressing plants.

Prosystemin, a prohormone that modulates plant defense barriers, is an intrinsically disordered protein

Prosystemin, originally isolated from Lycopersicon esculentum, is a tomato pro-hormone of 200 aminoacid residues which releases a bioactive peptide of 18 aminoacids called Systemin. This signaling peptide is involved in the activation of defense genes in solanaceous plants in response to herbivore feeding damage. Using biochemical, biophysical and bioinformatics approaches we characterized Prosystemin, showing that it is an intrinsically disordered protein possessing a few secondary structure elements within the sequence. Plant treatment with recombinant Prosystemin promotes early and late plant defense genes, which limit the development and survival of Spodoptera littoralis larvae fed with treated plants.

Plant-to-plant communication triggered by systemin primes anti-herbivore resistance in tomato

Plants actively respond to herbivory by inducing various defense mechanisms in both damaged (locally) and non-damaged tissues (systemically). In addition, it is currently widely accepted that plant-to-plant communication allows specific neighbors to be warned of likely incoming stress (defense priming). Systemin is a plant peptide hormone promoting the systemic response to herbivory in tomato. This 18-aa peptide is also able to induce the release of bioactive Volatile Organic Compounds, thus also promoting the interaction between the tomato and the third trophic level (e.g. predators and parasitoids of insect pests). In this work, using a combination of gene expression (RNA-Seq and qRT-PCR), behavioral and chemical approaches, we demonstrate that systemin triggers metabolic changes of the plant that are capable of inducing a primed state in neighboring unchallenged plants. At the molecular level, the primed state is mainly associated with an elevated transcription of pattern -recognition receptors, signaling enzymes and transcription factors. Compared to naïve plants, systemin-primed plants were significantly more resistant to herbivorous pests, more attractive to parasitoids and showed an increased response to wounding. Small peptides are nowadays considered fundamental signaling molecules in many plant processes and this work extends the range of downstream effects of this class of molecules to intraspecific plant-to-plant communication.

A comparison between constitutive and inducible transgenic expression of the PhRIP I gene for broad-spectrum resistance against phytopathogens in potato

OBJECTIVES

To engineer broad spectrum resistance in potato using different expression strategies.

RESULTS

The previously identified Ribosome-Inactivating Protein from Phytolacca heterotepala was expressed in potato under a constitutive or a wound-inducible promoter. Leaves and tubers of the plants constitutively expressing the transgene were resistant to Botrytis cinerea and Rhizoctonia solani, respectively. The wound-inducible promoter was useful in driving the expression upon wounding and fungal damage, and conferred resistance to B. cinerea. The observed differences between the expression strategies are discussed considering the benefits and features offered by the two systems.

CONCLUSIONS

Evidence is provided of the possible impact of promoter sequences to engineer BSR in plants, highlighting that the selection of a suitable expression strategy has to balance specific needs and target species.

Prosystemin overexpression induces transcriptional modifications of defense-related and receptor-like kinase genes and reduces the susceptibility to Cucumber mosaic virus and its satellite RNAs in transgenic tomato plants

Systemin is a plant signal peptide hormone involved in the responses to wounding and insect damage in the Solanaceae family. It works in the same signaling pathway of jasmonic acid (JA) and enhances the expression of proteinase inhibitors. With the aim of studying a role for systemin in plant antiviral responses, a tomato (Solanum lycopersicum) transgenic line overexpressing the prosystemin cDNA, i.e. the systemin precursor, was inoculated with Cucumber mosaic virus (CMV) strain Fny supporting either a necrogenic or a non-necrogenic satellite RNA (satRNA) variant. Transgenic plants showed reduced susceptibility to both CMV/satRNA combinations. While symptoms of the non-necrogenic inoculum were completely suppressed, a delayed onset of lethal disease occurred in about half of plants challenged with the necrogenic inoculum. RT-qPCR analysis showed a correlation between the systemin-mediated reduced susceptibility and the JA biosynthetic and signaling pathways (e.g. transcriptional alteration of lipoxygenase D and proteinase inhibitor II). Moreover, transgenically overexpressed systemin modulated the expression of a selected set of receptor-like protein kinase (RLK) genes, including some playing a known role in plant innate immunity. A significant correlation was found between the expression profiles of some RLKs and the systemin-mediated reduced susceptibility to CMV/satRNA. These results show that systemin can increase plant defenses against CMV/satRNA through transcriptional reprogramming of diverse signaling pathways.

Emerging Trends in Molecular Interactions between Plants and the Broad Host Range Fungal Pathogens Botrytis cinerea and Sclerotinia sclerotiorum

Fungal plant pathogens are major threats to food security worldwide. Sclerotinia sclerotiorum and Botrytis cinerea are closely related Ascomycete plant pathogens causing mold diseases on hundreds of plant species. There is no genetic source of complete plant resistance to these broad host range pathogens known to date. Instead, natural plant populations show a continuum of resistance levels controlled by multiple genes, a phenotype designated as quantitative disease resistance. Little is known about the molecular mechanisms controlling the interaction between plants and S. sclerotiorum and B. cinerea but significant advances were made on this topic in the last years. This minireview highlights a selection of nine themes that emerged in recent research reports on the molecular bases of plant-S. sclerotiorum and plant-B. cinerea interactions. On the fungal side, this includes progress on understanding the role of oxalic acid, on the study of fungal small secreted proteins. Next, we discuss the exchanges of small RNA between organisms and the control of cell death in plant and fungi during pathogenic interactions. Finally on the plant side, we highlight defense priming by mechanical signals, the characterization of plant Receptor-like proteins and the hormone abscisic acid in the response to B. cinerea and S. sclerotiorum, the role of plant general transcription machinery and plant small bioactive peptides. These represent nine trends we selected as remarkable in our understanding of fungal molecules causing disease and plant mechanisms associated with disease resistance to two devastating broad host range fungi.