ZmPep1, an ortholog of Arabidopsis elicitor peptide 1, regulates maize innate immunity and enhances disease resistance

In Silico Phylogenetic and Structural Analyses of Plant Endogenous Danger Signaling Molecules upon Stress

The plant innate immune system has two major branches, the pathogen-triggered immunity and the effector-triggered immunity (ETI). The effectors are molecules released by plant attackers to evade host immunity. In addition to the foreign intruders, plants possess endogenous instigators produced in response to general cellular injury termed as damage-associated molecular patterns (DAMPs). In plants, DAMPs or alarmins are released by damaged, stressed, or dying cells following abiotic stress such as radiation, oxidative and drought stresses. In turn, a cascade of downstream signaling events is initiated leading to the upregulation of defense or response-related genes. In the present study, we have investigated more thoroughly the conservation status of the molecular mechanisms implicated in the danger signaling primarily in plants. Towards this direction, we have performed in silico phylogenetic and structural analyses of the associated biomolecules in taxonomically diverse plant species. On the basis of our results, the defense mechanisms appear to be largely conserved within the plant kingdom. Of note, the sequence and/or function of several components of these mechanisms was found to be conserved in animals, as well. At the same time, the molecules involved in plant defense were found to form a dense protein-protein interaction (PPi) network, suggesting a crosstalk between the various defense mechanisms to a variety of stresses, like oxidative stress.

Damage-Associated Molecular Pattern-Triggered Immunity in Plants

As a universal process in multicellular organisms, including animals and plants, cells usually emit danger signals when suffering from attacks of microbes and herbivores, or physical damage. These signals, termed as damage-associated molecular patterns (DAMPs), mainly include cell wall or extracellular protein fragments, peptides, nucleotides, and amino acids. Once exposed on cell surfaces, DAMPs are detected by plasma membrane-localized receptors of surrounding cells to regulate immune responses against the invading organisms and promote damage repair. DAMPs may also act as long-distance mobile signals to mediate systemic wounding responses. Generation, release, and perception of DAMPs, and signaling events downstream of DAMP perception are all rigorously modulated by plants. These processes integrate together to determine intricate mechanisms of DAMP-triggered immunity in plants. In this review, we present an extensive overview on our current understanding of DAMPs in plant immune system.

Differential and tissue-specific activation pattern of the AtPROPEP and AtPEPR genes in response to biotic and abiotic stress in Arabidopsis thaliana

In Arabidopsis thaliana AtPEPR1 and AtPEPR2 act as the receptors for the endogenous AtPROPEP-derived Pep peptides and subsequently initiate defense-signaling cascades. In the previous work,⁹ the expression pattern of the genes encoding the PEPR receptors and the AtPROPEP peptide precursor proteins was studied using promoter-GUS reporter constructs. Here, using the same constructs to study their expression pattern under biotic and abiotic stress, including AtPep1, flg22, methyl jasmonate (MeJA), and NaCl treatments, we observed that in response to AtPep1 and flg22, the activation of AtPEPR1 promoter was different from AtPEPR2. We also found that these promoters were differentially activated in response to NaCl. Remarkably, we showed that it is possible to classify the genes of the AtPROPEP family, based on the response of their promoters to the various stimuli employed: thus, we classify AtPROPEP1 in one group; AtPROPEP2 and AtPROPEP3 in a second group; AtPROPEP4, AtPROPEP7 and AtPROPEP8 in a third group and AtPROPEP5 in a fourth group. Our finding, confirm non-redundant roles among the members of the AtPROPEP family and their corresponding receptors.

Pyramiding of nine transgenes in maize generates high-level resistance against necrotrophic maize pathogens

Key message Nine transgenes from different categories, viz. plant defense response genes and anti-apoptosis genes, played combined roles in maize to inhibit the necrotrophic pathogens Rhizoctonia solani and Bipolaris maydis. Maize sheath blight and southern corn leaf blight are major global threats to maize production. The management of these necrotrophic pathogens has encountered limited success due to the characteristics of their lifestyle. Here, we presented a transgenic pyramiding breeding strategy to achieve nine different resistance genes integrated in one transgenic maize line to combat different aspects of necrotrophic pathogens. These nine genes, selected from two different categories, plant defense response genes (Chi, Glu, Ace-AMP1, Tlp, Rs-AFP2, ZmPROPEP1 and Pti4), and anti-apoptosis genes (Iap and p35), were successfully transferred into maize and further implicated in resistance against the necrotrophic pathogens Rhizoctonia solani and Bipolaris maydis. Furthermore, the transgenic maize line 910, with high expression levels of the nine integrated genes, was selected from 49 lines. Under greenhouse and field trial conditions, line 910 showed significant resistance against maize sheath blight and southern corn leaf blight diseases. Higher-level resistance was obtained after the pyramiding of more resistance transgenes from different categories that function via different mechanisms. The present study provides a successful strategy for the management of necrotrophic pathogens.

Pyramiding of nine transgenes in maize generates high-level resistance against necrotrophic maize pathogens

Key message Nine transgenes from different categories, viz. plant defense response genes and anti-apoptosis genes, played combined roles in maize to inhibit the necrotrophic pathogens Rhizoctonia solani and Bipolaris maydis. Maize sheath blight and southern corn leaf blight are major global threats to maize production. The management of these necrotrophic pathogens has encountered limited success due to the characteristics of their lifestyle. Here, we presented a transgenic pyramiding breeding strategy to achieve nine different resistance genes integrated in one transgenic maize line to combat different aspects of necrotrophic pathogens. These nine genes, selected from two different categories, plant defense response genes (Chi, Glu, Ace-AMP1, Tlp, Rs-AFP2, ZmPROPEP1 and Pti4), and anti-apoptosis genes (Iap and p35), were successfully transferred into maize and further implicated in resistance against the necrotrophic pathogens Rhizoctonia solani and Bipolaris maydis. Furthermore, the transgenic maize line 910, with high expression levels of the nine integrated genes, was selected from 49 lines. Under greenhouse and field trial conditions, line 910 showed significant resistance against maize sheath blight and southern corn leaf blight diseases. Higher-level resistance was obtained after the pyramiding of more resistance transgenes from different categories that function via different mechanisms. The present study provides a successful strategy for the management of necrotrophic pathogens.

Wound-induced polypeptides improve resistance against Pseudomonas syringae pv. tomato DC3000 in Arabidopsis

Wound-induced polypeptides (WIPs) are a novel class of polypeptides with the length less than 100 amino acids. Our previous research has identified a number of WIP genes in soybean (Glycine max) root nodules. However, functions of WIPs in planta remains largely unknown. Here, we identified five WIP-encoding genes, AtWIP1-5, in Arabidopsis. Among them, AtWIP1 and -2 are ubiquitously expressed in a partially overlapping pattern as revealed by both qRT-PCR and promoter:GUS assays. Subcellular localization analyses reveal that both AtWIP1 and -2 are localized at the plasma membrane while AtWIP1 shows a punctate distribution pattern. AtWIP1, -2 are transcriptionally induced by flg22 treatment, but repressed by effector(s) of Pseudomonas syringae pv. tomato DC3000 (Pst DC3000). Heterologous overexpression of GmWIP genes enhances resistance of Arabidopsis to Pst DC3000 at the cost of growth inhibition. Moreover, overexpression of GmWIP genes promotes pattern-triggered immunity (PTI) evidenced by increased expressions of flg22-inducible genes and enhanced seedling growth inhibition under flg22 treatment. Taken together, our results indicate that WIPs positively regulate plant resistance against Pst DC3000 by enhancing PTI responses.

Plant Protection by Benzoxazinoids—Recent Insights into Biosynthesis and Function

Benzoxazinoids (BXs) are secondary metabolites present in many Poaceae including the major crops maize, wheat, and rye. In contrast to other potentially toxic secondary metabolites, BXs have not been targets of counter selection during breeding and the effect of BXs on insects, microbes, and neighbouring plants has been recognised. A broad knowledge about the mode of action and metabolisation in target organisms including herbivorous insects, aphids, and plants has been gathered in the last decades. BX biosynthesis has been elucidated on a molecular level in crop cereals. Recent advances, mainly made by investigations in maize, uncovered a significant diversity in the composition of BXs within one species. The pattern can be specific for single plant lines and dynamic changes triggered by biotic and abiotic stresses were observed. Single BXs might be toxic, repelling, attractive, and even growth-promoting for insects, depending on the particular species. BXs delivered into the soil influence plant and microbial communities. Furthermore, BXs can possibly be used as signalling molecules within the plant. In this review we intend to give an overview of the current data on the biosynthesis, structure, and function of BXs, beyond their characterisation as mere phytotoxins.

Beyond Defense: Multiple Functions of Benzoxazinoids in Maize Metabolism

Benzoxazinoids are a class of indole-derived plant metabolites that function in defense against numerous pests and pathogens. Due to their abundance in maize (Zea mays) and other important cereal crops, benzoxazinoids have been the subject of extensive research for >50 years. Whereas benzoxazinoids can account for 1% or more of the dry weight in young seedlings constitutively, their accumulation in older plants is induced locally by pest and pathogen attack. Although the biosynthetic pathways for most maize benzoxazinoids have been identified, unanswered questions remain about the developmental and defense-induced regulation of benzoxazinoid metabolism. Recent research shows that, in addition to their central role in the maize chemical defense repertoire, benzoxazinoids may have important functions in regulating other defense responses, flowering time, auxin metabolism, iron uptake and perhaps aluminum tolerance. Investigation of natural variation in maize benzoxazinoid accumulation, which is greatly facilitated by recent genomics advances, will have a major impact in this research area by leading to the discovery of previously unknown genes and functions of benzoxazinoid metabolism.

Danger-Associated Peptides Close Stomata by OST1-Independent Activation of Anion Channels in Guard Cells

The plant elicitor peptides (Peps), a family of damage/danger-associated molecular patterns (DAMPs), are perceived by two receptors, PEPR1 and PEPR2, and contribute to plant defense against pathogen attack and abiotic stress. Here, we show that the Peps-PEPR signaling pathway functions in stomatal immunity by activating guard cell anion channels in Arabidopsis thaliana The mutant plants lacking both PEPR1 and PEPR2 (pepr1 pepr2) displayed enhanced bacterial growth after being sprayed with Pseudomonas syringae pv tomato (Pst) DC3000, but not after pathogen infiltration into leaves, implicating PEPR function in stomatal immunity. Indeed, synthetic Arabidopsis Peps (AtPeps) effectively induced stomatal closure in wild-type but not pepr1 pepr2 mutant leaves, suggesting that the AtPeps-PEPR signaling pathway triggers stomatal closure. Consistent with this finding, patch-clamp recording revealed AtPep1-induced activation of anion channels in the guard cells of wild-type but not pepr1 pepr2 mutant plants. We further identified two guard cell-expressed anion channels, SLOW ANION CHANNEL1 (SLAC1) and its homolog SLAH3, as functionally overlapping components responsible for AtPep1-induced stomatal closure. The slac1 slah3 double mutant, but not slac1 or slah3 single mutants, failed to respond to AtPep1 in stomatal closure assays. Interestingly, disruption of OPEN STOMATA1 (OST1), an essential gene for abscisic acid-triggered stomatal closure, did not affect the AtPep1-induced anion channel activity and stomatal response. Together, these results illustrate a DAMP-triggered signaling pathway that, unlike the flagellin22-FLAGELLIN-SENSITIVE2 pathway, triggers stomata immunity through an OST1-independent mechanism.

Plant elicitor peptides promote plant defences against nematodes in soybean

Plant elicitor peptides (Peps) are widely distributed among angiosperms, and have been shown to amplify immune responses in multiple plant families. Here, we characterize three Peps from soybean (Glycine max) and describe their effects on plant defences against two damaging agricultural pests, the root-knot nematode (Meloidogyne incognita) and the soybean cyst nematode (Heterodera glycines). Seed treatments with exogenous GmPep1, GmPep2 or GmPep3 significantly reduced the reproduction of both nematodes. Pep treatment also protected plants from the inhibitory effects of root-knot nematodes on above-ground growth, and up-regulated basal expression levels of nematode-responsive defence genes. GmPep1 induced the expression of its propeptide precursor (GmPROPEP1), a nucleotide-binding site leucine-rich repeat protein (NBS-LRR), a pectin methylesterase inhibitor (PMEI), Respiratory Burst Oxidase Protein D (RBOHD) and the accumulation of reactive oxygen species (ROS) in leaves. In addition, GmPep2 and GmPep3 seed treatments up-regulated RBOHD expression and ROS accumulation in roots and leaves. These results suggest that GmPeps activate plant defences through systemic transcriptional reprogramming and ROS signalling, and that Pep seed treatments represent a potential strategy for nematode management.

Pathogen Trojan Horse Delivers Bioactive Host Protein to Alter Maize Anther Cell Behavior in Situ

Small proteins are crucial signals during development, host defense, and physiology. The highly spatiotemporal restricted functions of signaling proteins remain challenging to study in planta. The several month span required to assess transgene expression, particularly in flowers, combined with the uncertainties from transgene position effects and ubiquitous or overexpression, makes monitoring of spatiotemporally restricted signaling proteins lengthy and difficult. This situation could be rectified with a transient assay in which protein deployment is tightly controlled spatially and temporally in planta to assess protein functions, timing, and cellular targets as well as to facilitate rapid mutagenesis to define functional protein domains. In maize (Zea mays), secreted ZmMAC1 (MULTIPLE ARCHESPORIAL CELLS1) was proposed to trigger somatic niche formation during anther development by participating in a ligand-receptor module. Inspired by Homer's Trojan horse myth, we engineered a protein delivery system that exploits the secretory capabilities of the maize smut fungus Ustilago maydis, to allow protein delivery to individual cells in certain cell layers at precise time points. Pathogen-supplied ZmMAC1 cell-autonomously corrected both somatic cell division and differentiation defects in mutant Zmmac1-1 anthers. These results suggest that exploiting host-pathogen interactions may become a generally useful method for targeting host proteins to cell and tissue types to clarify cellular autonomy and to analyze steps in cell responses.

An apoplastic peptide activates salicylic acid signalling in maize

Localized control of cell death is crucial for the resistance of plants to pathogens. Papain-like cysteine proteases (PLCPs) regulate plant defence to drive cell death and protection against biotrophic pathogens. In maize (Zea mays), PLCPs are crucial in the orchestration of salicylic acid (SA)-dependent defence signalling. Despite this central role in immunity, it remains unknown how PLCPs are activated, and which downstream signals they induce to trigger plant immunity. Here, we discover an immune signalling peptide, Z. mays immune signalling peptide 1 (Zip1), which is produced after salicylic acid (SA) treatment. In vitro studies demonstrate that PLCPs are required to release bioactive Zip1 from its propeptide precursor. Conversely, Zip1 treatment strongly elicits SA accumulation in leaves. Moreover, transcriptome analyses revealed that Zip1 and SA induce highly overlapping transcriptional changes. Consequently, Zip1 promotes the infection of the necrotrophic fungus Botrytis cinerea, while it reduces virulence of the biotrophic fungus Ustilago maydis. Thus, Zip1 represents the previously missing signal that is released by PLCPs to activate SA defence signalling.

Novel Rosaceae plant elicitor peptides as sustainable tools to control Xanthomonas arboricola pv. pruni in Prunus spp

Fruit crops are regarded as important health promoters and constitute a major part of global agricultural production, and Rosaceae species are of high economic impact. Their culture is threatened by bacterial diseases, whose control is based on preventative treatments using compounds of limited efficacy and negative environmental impact. One of the most economically relevant examples is the pathogen Xanthomonas arboricola pv. pruni (Xap) affecting Prunus spp. The plant immune response against pathogens can be triggered and amplified by plant elicitor peptides (Peps), perceived by specific receptors (PEPRs). Although they have been described in various angiosperms, scarce information is available on Rosaceae species. Here, we identified the Pep precursor (PROPEP), Pep and PEPR orthologues of 10 Rosaceae species and confirmed the presence of the Pep/PEPR system in this family. We showed the perception and elicitor activity of Rosaceae Peps using the Prunus-Xap pathosystem as proof-of-concept. Treatment with nanomolar doses of Peps induced the corresponding PROPEP and a set of defence-related genes in Prunus leaves, and enhanced resistance against Xap. Peps from the same species had the highest efficiencies. Rosaceae Peps could potentially be used to develop natural, targeted and environmentally friendly strategies to enhance the resistance of Prunus species against biotic attackers.

Pattern recognition receptors and signaling in plant-microbe interactions

Plants solely rely on innate immunity of each individual cell to deal with a diversity of microbes in the environment. Extracellular recognition of microbe- and host damage-associated molecular patterns leads to the first layer of inducible defenses, termed pattern-triggered immunity (PTI). In plants, pattern recognition receptors (PRRs) described to date are all membrane-associated receptor-like kinases or receptor-like proteins, reflecting the prevalence of apoplastic colonization of plant-infecting microbes. An increasing inventory of elicitor-active patterns and PRRs indicates that a large number of them are limited to a certain range of plant groups/species, pointing to dynamic and convergent evolution of pattern recognition specificities. In addition to common molecular principles of PRR signaling, recent studies have revealed substantial diversification between PRRs in their functions and regulatory mechanisms. This serves to confer robustness and plasticity to the whole PTI system in natural infections, wherein different PRRs are simultaneously engaged and faced with microbial assaults. We review the functional significance and molecular basis of PRR-mediated pathogen recognition and disease resistance, and also an emerging role for PRRs in homeostatic association with beneficial or commensal microbes.

Diversity of plant defense elicitor peptides within the Rosaceae

BACKGROUND

Plant elicitor peptides (Peps) are endogenous molecules that induce and amplify the first line of inducible plant defense, known as pattern-triggered immunity, contributing to protect plants against attack by bacteria, fungi and herbivores. Pep topic application and transgenic expression have been found to enhance disease resistance in a small number of model plant-pathogen systems. The action of Peps relies on perception by specific receptors, so displaying a family-specific activity. Recently, the presence and activity of Peps within the Rosaceae has been demonstrated. Here we characterized the population of Pep sequences within the economically important plant family of Rosaceae, with special emphasis on the Amygdaleae and Pyreae tribes, which include the most relevant edible species such as apple, pear and peach, and numerous ornamental and wild species (e.g. photinia, firethorn and hawthorn).

RESULTS

The systematic experimental search for Pep and the corresponding precursor PROPEP sequences within 36 Amygdaleae and Pyreae species, and 100 cultivars had a highly homogeneous pattern, with two tribe-specific Pep types per plant, i.e. Pep1 and Pep2 (Amygdaleae) or Pep3 and Pep4 (Pyreae). Pep2 and Pep3 are highly conserved, reaching identity percentages similar to those of genes used in plant phylogenetic analyses, while Pep1 and Pep4 are somewhat more variable, with similar values to the corresponding PROPEPs. In contrast to Pep3 and Pep4, Pep1 and Pep2 sequences of different species paralleled their phylogenetic relationships, and putative ancestor sequences were identified. The large amount of sequences allowed refining of a C-terminal consensus sequence that would support the protective activity of Pep1-4 in a Prunus spp. and Xanthomonas arboricola pv. pruni system. Moreover, tribe-specific consensus sequences were deduced at the center and C-terminal regions of Peps, which might explain the higher protection efficiencies described upon topic treatments with Peps from the same tribe.

CONCLUSIONS

The present study substantially enhances the knowledge on Peps within the Amygdaleae and Pyreae species. It can be the basis to design and fine-tune new control tools against important plant pathogens affecting Prunus, Pyrus and Malus species.

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.

Plant peptides in plant defense responses

Plant peptides secreted as signal molecular to trigger cell-to-cell signaling are indispensable for plant growth and defense processes. Preciously, it is regraded some plant peptides function in plant growth and development, whereas others regulate defense response in plant-microbe interactions. However, this prejudice is got rid due to more and more evidence showed growth-related plant peptides also exhibit bifunctional roles in plant defense response against different microbial pathogens. Here we provide a mini-review of reported types of plant peptides, including their basic information, reported receptor ligands, and especially direct or indirect roles in plant immune responses.

Fungal-induced protein hyperacetylation in maize identified by acetylome profiling

Lysine acetylation is a key posttranslational modification that regulates diverse proteins involved in a range of biological processes. The role of histone acetylation in plant defense is well established, and it is known that pathogen effector proteins encoding acetyltransferases can directly acetylate host proteins to alter immunity. However, it is unclear whether endogenous plant enzymes can modulate protein acetylation during an immune response. Here, we investigate how the effector molecule HC-toxin (HCT), a histone deacetylase inhibitor produced by the fungal pathogen Cochliobolus carbonum race 1, promotes virulence in maize through altering protein acetylation. Using mass spectrometry, we globally quantified the abundance of 3,636 proteins and the levels of acetylation at 2,791 sites in maize plants treated with HCT as well as HCT-deficient or HCT-producing strains of C. carbonum Analyses of these data demonstrate that acetylation is a widespread posttranslational modification impacting proteins encoded by many intensively studied maize genes. Furthermore, the application of exogenous HCT enabled us to show that the activity of plant-encoded enzymes (histone deacetylases) can be modulated to alter acetylation of nonhistone proteins during an immune response. Collectively, these results provide a resource for further mechanistic studies examining the regulation of protein function by reversible acetylation and offer insight into the complex immune response triggered by virulent C. carbonum.

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.

In roots of Arabidopsis thaliana, the damage-associated molecular pattern AtPep1 is a stronger elicitor of immune signalling than flg22 or the chitin heptamer

Plants interpret their immediate environment through perception of small molecules. Microbe-associated molecular patterns (MAMPs) such as flagellin and chitin are likely to be more abundant in the rhizosphere than plant-derived damage-associated molecular patterns (DAMPs). We investigated how the Arabidopsis thaliana root interprets MAMPs and DAMPs as danger signals. We monitored root development during exposure to increasing concentrations of the MAMPs flg22 and the chitin heptamer as well as of the DAMP AtPep1. The tissue-specific expression of defence-related genes in roots was analysed using a toolkit of promoter::YFPN lines reporting jasmonic acid (JA)-, salicylic acid (SA)-, ethylene (ET)- and reactive oxygen species (ROS)- dependent signalling. Finally, marker responses were analysed during invasion by the root pathogen Fusarium oxysporum. The DAMP AtPep1 triggered a stronger activation of the defence markers compared to flg22 and the chitin heptamer. In contrast to the tested MAMPs, AtPep1 induced SA- and JA-signalling markers in the root and caused a severe inhibition of root growth. Fungal attack resulted in a strong activation of defence genes in tissues close to the invading fungal hyphae. The results collectively suggest that AtPep1 presents a stronger danger signal to the Arabidopsis root than the MAMPs flg22 and chitin heptamer.

Sensing Danger: Key to Activating Plant Immunity

In both plants and animals, defense against pathogens relies on a complex surveillance system for signs of danger. Danger signals may originate from the infectious agent or from the host itself. Immunogenic plant host factors can be roughly divided into two categories: molecules which are passively released upon cell damage ('classical' damage-associated molecular patterns, DAMPs), and peptides which are processed and/or secreted upon infection to modulate the immune response (phytocytokines). We highlight the ongoing challenge to understand how plants sense various danger signals and integrate this information to produce an appropriate immune response to diverse challenges.

Genetic dissection of the maize (Zea mays L.) MAMP response

Loci associated with variation in maize responses to two microbe-associated molecular patterns (MAMPs) were identified. MAMP responses were correlated. No relationship between MAMP responses and quantitative disease resistance was identified. Microbe-associated molecular patterns (MAMPs) are highly conserved molecules commonly found in microbes which can be recognized by plant pattern recognition receptors. Recognition triggers a suite of responses including production of reactive oxygen species (ROS) and nitric oxide (NO) and expression changes of defense-related genes. In this study, we used two well-studied MAMPs (flg22 and chitooctaose) to challenge different maize lines to determine whether there was variation in the level of responses to these MAMPs, to dissect the genetic basis underlying that variation and to understand the relationship between MAMP response and quantitative disease resistance (QDR). Naturally occurring quantitative variation in ROS, NO production, and defense genes expression levels triggered by MAMPs was observed. A major quantitative traits locus (QTL) associated with variation in the ROS production response to both flg22 and chitooctaose was identified on chromosome 2 in a recombinant inbred line (RIL) population derived from the maize inbred lines B73 and CML228. Minor QTL associated with variation in the flg22 ROS response was identified on chromosomes 1 and 4. Comparison of these results with data previously obtained for variation in QDR and the defense response in the same RIL population did not provide any evidence for a common genetic basis controlling variation in these traits.

Identification and VIGS-based characterization of Bx1 ortholog in rye (Secale cereale L.)

The first step of the benzoxazinoid (BX) synthesis pathway is catalyzed by an enzyme with indole-3-glycerol phosphate lyase activity encoded by 3 genes, Bx1, TSA and Igl. A gene highly homologous to maize and wheat Bx1 has been identified in rye. The goal of the study was to analyze the gene and to experimentally verify its role in the rye BX biosynthesis pathway as a rye ortholog of the Bx1 gene. Expression of the gene showed peak values 3 days after imbibition (dai) and at 21 dai it was undetectable. Changes of the BX content in leaves were highly correlated with the expression pattern until 21 dai. In plants older than 21 dai despite the undetectable expression of the analyzed gene there was still low accumulation of BXs. Function of the gene was verified by correlating its native expression and virus-induced silencing with BX accumulation. Barley stripe mosaic virus (BSMV)-based vectors were used to induce transcriptional (TGS) and posttranscriptional (PTGS) silencing of the analyzed gene. Both strategies (PTGS and TGS) significantly reduced the transcript level of the analyzed gene, and this was highly correlated with lowered BX content. Inoculation with virus-based vectors specifically induced expression of the analyzed gene, indicating up-regulation by biotic stressors. This is the first report of using the BSMV-based system for functional analysis of rye gene. The findings prove that the analyzed gene is a rye ortholog of the Bx1 gene. Its expression is developmentally regulated and is strongly induced by biotic stress. Stable accumulation of BXs in plants older than 21 dai associated with undetectable expression of ScBx1 indicates that the function of the ScBx1 in the BX biosynthesis is redundant with another gene. We anticipate that the unknown gene is a putative ortholog of the Igl, which still remains to be identified in rye.

Ligand Receptor-Mediated Regulation of Growth in Plants

Growth and development of multicellular organisms are coordinately regulated by various signaling pathways involving the communication of inter- and intracellular components. To form the appropriate body patterns, cellular growth and development are modulated by either stimulating or inhibiting these pathways. Hormones and second messengers help to mediate the initiation and/or interaction of the various signaling pathways in all complex multicellular eukaryotes. In plants, hormones include small organic molecules, as well as larger peptides and small proteins, which, as in animals, act as ligands and interact with receptor proteins to trigger rapid biochemical changes and induce the intracellular transcriptional and long-term physiological responses. During the past two decades, the availability of genetic and genomic resources in the model plant species, Arabidopsis thaliana, has greatly helped in the discovery of plant hormone receptors and the components of signal transduction pathways and mechanisms used by these immobile but highly complex organisms. Recently, it has been shown that two of the most important plant hormones, auxin and abscisic acid (ABA), act through signaling pathways that have not yet been recognized in animals. For example, auxins stimulate cell elongation by bringing negatively acting transcriptional repressor proteins to the proteasome to be degraded, thus unleashing the gene expression program required for increasing cell size. The "dormancy" inducing hormone, ABA, binds to soluble receptor proteins and inhibits a specific class of protein phosphatases (PP2C), which activates phosphorylation signaling leading to transcriptional changes needed for the desiccation of the seeds prior to entering dormancy. While these two hormone receptors have no known animal counterparts, there are also many similarities between animal and plant signaling pathways. For example, in plants, the largest single gene family in the genome is the protein kinase family (approximately 5% of the protein coding genes), although the specific function for only a few dozen of these kinases is clearly established. Recent comparative genomics studies have revealed that parasitic nematodes and pathogenic microbes produce plant peptide hormone mimics that target specific plant plasma membrane receptor-like protein kinases, thus usurping endogenous signaling pathways for their own pathogenic purposes. With biochemical, genetic, and physiological analyses of the regulation of hormone receptor signal pathways, we are thus just now beginning to understand how plants optimize the development of their body shape and cope with constantly changing environmental conditions.

Synthesis and Functions of Jasmonates in Maize

Of the over 600 oxylipins present in all plants, the phytohormone jasmonic acid (JA) remains the best understood in terms of its biosynthesis, function and signaling. Much like their eicosanoid analogues in mammalian system, evidence is growing for the role of the other oxylipins in diverse physiological processes. JA serves as the model plant oxylipin species and regulates defense and development. For several decades, the biology of JA has been characterized in a few dicot species, yet the function of JA in monocots has only recently begun to be elucidated. In this work, the synthesis and function of JA in maize is presented from the perspective of oxylipin biology. The maize genes responsible for catalyzing the reactions in the JA biosynthesis are clarified and described. Recent studies into the function of JA in maize defense against insect herbivory, pathogens and its role in growth and development are highlighted. Additionally, a list of JA-responsive genes is presented for use as biological markers for improving future investigations into JA signaling in maize.

DAMPs, MAMPs, and NAMPs in plant innate immunity

BACKGROUND

Multicellular organisms have evolved systems/mechanisms to detect various forms of danger, including attack by microbial pathogens and a variety of pests, as well as tissue and cellular damage. Detection via cell-surface receptors activates an ancient and evolutionarily conserved innate immune system.

RESULT

Potentially harmful microorganisms are recognized by the presence of molecules or parts of molecules that have structures or chemical patterns unique to microbes and thus are perceived as non-self/foreign. They are referred to as Microbe-Associated Molecular Patterns (MAMPs). Recently, a class of small molecules that is made only by nematodes, and that functions as pheromones in these organisms, was shown to be recognized by a wide range of plants. In the presence of these molecules, termed Nematode-Associated Molecular Patterns (NAMPs), plants activate innate immune responses and display enhanced resistance to a broad spectrum of microbial and nematode pathogens. In addition to pathogen attack, the relocation of various endogenous molecules or parts of molecules, generally to the extracellular milieu, as a result of tissue or cellular damage is perceived as a danger signal, and it leads to the induction of innate immune responses. These relocated endogenous inducers are called Damage-Associated Molecular Patterns (DAMPs).

CONCLUSIONS

This mini-review is focused on plant DAMPs, including the recently discovered Arabidopsis HMGB3, which is the counterpart of the prototypic animal DAMP HMGB1. The plant DAMPs will be presented in the context of plant MAMPs and NAMPs, as well as animal DAMPs.

DAMPs, MAMPs, and NAMPs in plant innate immunity

BACKGROUND

Multicellular organisms have evolved systems/mechanisms to detect various forms of danger, including attack by microbial pathogens and a variety of pests, as well as tissue and cellular damage. Detection via cell-surface receptors activates an ancient and evolutionarily conserved innate immune system.

RESULT

Potentially harmful microorganisms are recognized by the presence of molecules or parts of molecules that have structures or chemical patterns unique to microbes and thus are perceived as non-self/foreign. They are referred to as Microbe-Associated Molecular Patterns (MAMPs). Recently, a class of small molecules that is made only by nematodes, and that functions as pheromones in these organisms, was shown to be recognized by a wide range of plants. In the presence of these molecules, termed Nematode-Associated Molecular Patterns (NAMPs), plants activate innate immune responses and display enhanced resistance to a broad spectrum of microbial and nematode pathogens. In addition to pathogen attack, the relocation of various endogenous molecules or parts of molecules, generally to the extracellular milieu, as a result of tissue or cellular damage is perceived as a danger signal, and it leads to the induction of innate immune responses. These relocated endogenous inducers are called Damage-Associated Molecular Patterns (DAMPs).

CONCLUSIONS

This mini-review is focused on plant DAMPs, including the recently discovered Arabidopsis HMGB3, which is the counterpart of the prototypic animal DAMP HMGB1. The plant DAMPs will be presented in the context of plant MAMPs and NAMPs, as well as animal DAMPs.

Interactive Effects of Elevated [CO2] and Drought on the Maize Phytochemical Defense Response against Mycotoxigenic Fusarium verticillioides

Changes in climate due to rising atmospheric carbon dioxide concentration ([CO2]) are predicted to intensify episodes of drought, but our understanding of how these combined conditions will influence crop-pathogen interactions is limited. We recently demonstrated that elevated [CO2] alone enhances maize susceptibility to the mycotoxigenic pathogen, Fusarium verticillioides (Fv) but fumonisin levels remain unaffected. In this study we show that maize simultaneously exposed to elevated [CO2] and drought are even more susceptible to Fv proliferation and also prone to higher levels of fumonisin contamination. Despite the increase in fumonisin levels, the amount of fumonisin produced in relation to pathogen biomass remained lower than corresponding plants grown at ambient [CO2]. Therefore, the increase in fumonisin contamination was likely due to even greater pathogen biomass rather than an increase in host-derived stimulants. Drought did not negate the compromising effects of elevated [CO2] on the accumulation of maize phytohormones and metabolites. However, since elevated [CO2] does not influence the drought-induced accumulation of abscisic acid (ABA) or root terpenoid phytoalexins, the effects elevated [CO2] are negated belowground, but the stifled defense response aboveground may be a consequence of resource redirection to the roots.

Signal transduction and regulation of IbpreproHypSys in sweet potato

Hydroxyproline-rich glycopeptides (HypSys) are small signalling peptides containing 18-20 amino acids. The expression of IbpreproHypSys, encoding the precursor of IbHypSys, was induced in sweet potato (Ipomoea batatas cv. Tainung 57) through wounding and IbHypSys treatments by using jasmonate and H2 O2 . Transgenic sweet potatoes overexpressing (OE) and silencing [RNA interference (RNAi)] IbpreproHypSys were created. The expression of the wound-inducible gene for ipomoelin (IPO) in the local and systemic leaves of OE plants was stronger than the expression in wild-type (WT) and RNAi plants after wounding. Furthermore, grafting experiments indicated that IPO expression was considerably higher in WT stocks receiving wounding signals from OE than from RNAi scions. However, wounding WT scions highly induced IPO expression in OE stocks. These results indicated that IbpreproHypSys expression contributed towards sending and receiving the systemic signals that induced IPO expression. Analysing the genes involved in the phenylpropanoid pathway demonstrated that lignin biosynthesis was activated after synthetic IbHypSys treatment. IbpreproHypSys expression in sweet potato suppressed Spodoptera litura growth. In conclusion, wounding induced the expression of IbpreproHypSys, whose protein product was processed into IbHypSys. IbHypSys stimulated IbpreproHypSys and IPO expression and enhanced lignin biosynthesis, thus protecting plants from insects.

Cooperative interaction of antimicrobial peptides with the interrelated immune pathways in plants

Plants express a diverse repertoire of functionally and structurally distinct antimicrobial peptides (AMPs) which provide innate immunity by acting directly against a wide range of pathogens. AMPs are expressed in nearly all plant organs, either constitutively or in response to microbial infections. In addition to their direct activity, they also contribute to plant immunity by modulating defence responses resulting from pathogen-associated molecular pattern/effector-triggered immunity, and also interact with other AMPs and pathways involving mitogen-activated protein kinases, reactive oxygen species, hormonal cross-talk and sugar signalling. Such links among AMPs and defence signalling pathways are poorly understood and there is no clear model for their interactions. This article provides a critical review of the empirical data to shed light on the wider role of AMPs in the robust and resource-effective defence responses of plants.

Activation of Plant Innate Immunity by Extracellular High Mobility Group Box 3 and Its Inhibition by Salicylic Acid

Damage-associated molecular pattern molecules (DAMPs) signal the presence of tissue damage to induce immune responses in plants and animals. Here, we report that High Mobility Group Box 3 (HMGB3) is a novel plant DAMP. Extracellular HMGB3, through receptor-like kinases BAK1 and BKK1, induced hallmark innate immune responses, including i) MAPK activation, ii) defense-related gene expression, iii) callose deposition, and iv) enhanced resistance to Botrytis cinerea. Infection by necrotrophic B. cinerea released HMGB3 into the extracellular space (apoplast). Silencing HMGBs enhanced susceptibility to B. cinerea, while HMGB3 injection into apoplast restored resistance. Like its human counterpart, HMGB3 binds salicylic acid (SA), which results in inhibition of its DAMP activity. An SA-binding site mutant of HMGB3 retained its DAMP activity, which was no longer inhibited by SA, consistent with its reduced SA-binding activity. These results provide cross-kingdom evidence that HMGB proteins function as DAMPs and that SA is their conserved inhibitor.

Elicitation, an Effective Strategy for the Biotechnological Production of Bioactive High-Added Value Compounds in Plant Cell Factories

Plant in vitro cultures represent an attractive and cost-effective alternative to classical approaches to plant secondary metabolite (PSM) production (the “Plant Cell Factory” concept). Among other advantages, they constitute the only sustainable and eco-friendly system to obtain complex chemical structures biosynthesized by rare or endangered plant species that resist domestication. For successful results, the biotechnological production of PSM requires an optimized system, for which elicitation has proved one of the most effective strategies. In plant cell cultures, an elicitor can be defined as a compound introduced in small concentrations to a living system to promote the biosynthesis of the target metabolite. Traditionally, elicitors have been classified in two types, abiotic or biotic, according to their chemical nature and exogenous or endogenous origin, and notably include yeast extract, methyl jasmonate, salicylic acid, vanadyl sulphate and chitosan. In this review, we summarize the enhancing effects of elicitors on the production of high-added value plant compounds such as taxanes, ginsenosides, aryltetralin lignans and other types of polyphenols, focusing particularly on the use of a new generation of elicitors such as coronatine and cyclodextrins.

Tissue-specific gene expression in maize seeds during colonization by Aspergillus flavus and Fusarium verticillioides

Aspergillus flavus and Fusarium verticillioides are fungal pathogens that colonize maize kernels and produce the harmful mycotoxins aflatoxin and fumonisin, respectively. Management practice based on potential host resistance to reduce contamination by these mycotoxins has proven difficult, resulting in the need for a better understanding of the infection process by these fungi and the response of maize seeds to infection. In this study, we followed the colonization of seeds by histological methods and the transcriptional changes of two maize defence-related genes in specific seed tissues by RNA in situ hybridization. Maize kernels were inoculated with either A. flavus or F. verticillioides 21-22 days after pollination, and harvested at 4, 12, 24, 48, 72, 96 and 120 h post-inoculation. The fungi colonized all tissues of maize seed, but differed in their interactions with aleurone and germ tissues. RNA in situ hybridization showed the induction of the maize pathogenesis-related protein, maize seed (PRms) gene in the aleurone and scutellum on infection by either fungus. Transcripts of the maize sucrose synthase-encoding gene, shrunken-1 (Sh1), were observed in the embryo of non-infected kernels, but were induced on infection by each fungus in the aleurone and scutellum. By comparing histological and RNA in situ hybridization results from adjacent serial sections, we found that the transcripts of these two genes accumulated in tissue prior to the arrival of the advancing pathogens in the seeds. A knowledge of the patterns of colonization and tissue-specific gene expression in response to these fungi will be helpful in the development of resistance.

Evolutionary divergence of the plant elicitor peptides (Peps) and their receptors: interfamily incompatibility of perception but compatibility of downstream signalling

Plant elicitor peptides (Peps) are potent inducers of pattern-triggered immunity and amplify the immune response against diverse pathogens. Peps have been discovered and studied extensively in Arabidopsis and only recently orthologues in maize were also identified and characterized in more detail.Here, the presence of PROPEPs, the Pep precursors, and PEPRs, the Pep receptors, was investigated within the plant kingdom. PROPEPs and PEPRs were identified in most sequenced species of the angiosperms. The conservation and compatibility of the Pep-PEPR-system was analysed by using plants of two distantly related dicot families, Brassicaceae and Solanaceae, and a representative family of monocot plants, the Poaceae. All three plant families contain important crop plants, including maize, rice, tomato, potato, and canola. Peps were not recognized by species outside of their plant family of origin, apparently because of a divergence of the Pep sequences. Three family-specific Pep motifs were defined and the integration of such a motif into the Pep sequence of an unrelated Pep enabled its perception. Transient transformation of Nicotiana benthamiana with the coding sequences of the AtPEPR1 and ZmPEPR1a led to the recognition of Pep peptides of Brassicaceae or Poaceae origin, respectively, and to the proper activation of downstream signalling. It was concluded that signalling machinery downstream of the PEPRs is highly conserved whereas the leucine-rich repeat domains of the PEPRs co-evolved with the Peps, leading to distinct motifs and, with it, interfamily incompatibility.

Quo vadis, Pep? Plant elicitor peptides at the crossroads of immunity, stress, and development

The first line of inducible plant defence, pattern-triggered immunity (PTI), is activated by the recognition of exogenous as well as endogenous elicitors. Exogenous elicitors, also called microbe-associated molecular patterns, signal the presence of microbes. In contrast, endogenous elicitors seem to be generated and recognized under more diverse circumstances, making the evaluation of their biological relevance much more complex. Plant elicitor peptides (Peps) are one class of such endogenous elicitors, which contribute to immunity against attack by bacteria, fungi, as well as herbivores. Recent studies indicate that the Pep-triggered signalling pathways also operate during the response to a more diverse set of stresses including starvation stress. In addition, in silico data point to an involvement in the regulation of plant development, and a study on Pep-mediated inhibition of root growth supports this indication. Importantly, Peps are neither limited to the model plant Arabidopsis nor to a specific plant family like the previously intensively studied systemin peptides. On the contrary, they are present and active in angiosperms all across the phylogenetic tree, including many important crop plants. Here we summarize the progress made in research on Peps from their discovery in 2006 until now. We discuss the two main models which describe their likely function in plant immunity, highlight the studies supporting additional roles of Pep-triggered signalling and identify urgent research tasks to further uncover their biological relevance.

The Plant Peptidome: An Expanding Repertoire of Structural Features and Biological Functions

Peptides fulfill a plethora of functions in plant growth, development, and stress responses. They act as key components of cell-to-cell communication, interfere with signaling and response pathways, or display antimicrobial activity. Strikingly, both the diversity and amount of plant peptides have been largely underestimated. Most characterized plant peptides to date acting as small signaling peptides or antimicrobial peptides are derived from nonfunctional precursor proteins. However, evidence is emerging on peptides derived from a functional protein, directly translated from small open reading frames (without the involvement of a precursor) or even encoded by primary transcripts of microRNAs. These novel types of peptides further add to the complexity of the plant peptidome, even though their number is still limited and functional characterization as well as translational evidence are often controversial. Here, we provide a comprehensive overview of the reported types of plant peptides, including their described functional and structural properties. We propose a novel, unifying peptide classification system to emphasize the enormous diversity in peptide synthesis and consequent complexity of the still expanding knowledge on the plant peptidome.

The Arabidopsis Pep-PEPR system is induced by herbivore feeding and contributes to JA-mediated plant defence against herbivory

A number of plant endogenous elicitors have been identified that induce pattern-triggered immunity upon perception. In Arabidopsis thaliana eight small precursor proteins, called PROPEPs, are thought to be cleaved upon danger to release eight peptides known as the plant elicitor peptides Peps. As the expression of some PROPEPs is induced upon biotic stress and perception of any of the eight Peps triggers a defence response, they are regarded as amplifiers of immunity. Besides the induction of defences directed against microbial colonization Peps have also been connected with herbivore deterrence as they share certain similarities to systemins, known mediators of defence signalling against herbivores in solanaceous plants, and they positively interact with the phytohormone jasmonic acid. A recent study using maize indicated that the application of ZmPep3, a maize AtPep-orthologue, elicits anti-herbivore responses. However, as this study only assessed the responses triggered by the exogenous application of Peps, the biological significance of these findings remained open. By using Arabidopsis GUS-reporter lines, it is now shown that the promoters of both Pep-receptors, PEPR1 and PEPR2, as well as PROPEP3 are strongly activated upon herbivore attack. Moreover, pepr1 pepr2 double mutant plants, which are insensitive to Peps, display a reduced resistance to feeding Spodoptera littoralis larvae and a reduced accumulation of jasmonic acid upon exposure to herbivore oral secretions. Taken together, these lines of evidence extend the role of the AtPep-PEPR system as a danger detection mechanism from microbial pathogens to herbivores and further underline its strong interaction with jasmonic acid signalling.

Plant elicitor peptides in induced defense against insects

Over 20 years ago the peptide systemin was discovered to be an integral regulator of anti-herbivore defense responses in Solanaceous plants. However, other peptides of similar function have remained elusive. Plant Elicitor Peptides (Peps) were initially discovered in Arabidopsis as mediators of basal immune responses protective against invading pathogens. Recently a Pep from maize, ZmPep3, was demonstrated to be a potent regulator of anti-herbivore defenses. ZmPep3 was as active as the Lepidopteran elicitor N-linolenoyl-l-glutamine (Gln-18:3) in stimulating volatile emission and accumulation of defense transcripts and metabolites, resulting in both attraction of the parasitoid Cotesia marginiventris and suppressed growth of Spodoptera exigua larvae. Orthologues of Peps in Solanaceous and Fabaceous plants also trigger emission of herbivore-associated volatiles, indicating that Peps have a conserved role as regulators of plant defense against herbivores in diverse species. This conservation of a peptide signal and cognate receptor for activation of plant defense responses reveals a widespread regulatory motif and provides opportunities for manipulation of plant resistance.

Accumulation of 5-hydroxynorvaline in maize (Zea mays) leaves is induced by insect feeding and abiotic stress

Plants produce a wide variety of defensive metabolites to protect themselves against herbivores and pathogens. Non-protein amino acids, which are present in many plant species, can have a defensive function through their mis-incorporation during protein synthesis and/or inhibition of biosynthetic pathways in primary metabolism. 5-Hydroxynorvaline was identified in a targeted search for previously unknown non-protein amino acids in the leaves of maize (Zea mays) inbred line B73. Accumulation of this compound increases during herbivory by aphids (Rhopalosiphum maidis, corn leaf aphid) and caterpillars (Spodoptera exigua, beet armyworm), as well as in response to treatment with the plant signalling molecules methyl jasmonate, salicylic acid and abscisic acid. In contrast, ethylene signalling reduced 5-hydroxynorvaline abundance. Drought stress induced 5-hydroxynorvaline accumulation to a higher level than insect feeding or treatment with defence signalling molecules. In field-grown plants, the 5-hydroxynorvaline concentration was highest in above-ground vegetative tissue, but it was also detectable in roots and dry seeds. When 5-hydroxynorvaline was added to aphid artificial diet at concentrations similar to those found in maize leaves and stems, R. maidis reproduction was reduced, indicating that this maize metabolite may have a defensive function. Among 27 tested maize inbred lines there was a greater than 10-fold range in the accumulation of foliar 5-hydroxynorvaline. Genetic mapping populations derived from a subset of these inbred lines were used to map quantitative trait loci for 5-hydroxynorvaline accumulation to maize chromosomes 5 and 7.

Surveying the potential of secreted antimicrobial peptides to enhance plant disease resistance

Antimicrobial peptides (AMPs) are natural products found across diverse taxa as part of the innate immune system against pathogen attacks. Some AMPs are synthesized through the canonical gene expression machinery and are called ribosomal AMPs. Other AMPs are assembled by modular enzymes generating nonribosomal AMPs and harbor unusual structural diversity. Plants synthesize an array of AMPs, yet are still subject to many pathogen invasions. Crop breeding programs struggle to release new cultivars in which complete disease resistance is achieved, and usually such resistance becomes quickly overcome by the targeted pathogens which have a shorter generation time. AMPs could offer a solution by exploring not only plant-derived AMPs, related or unrelated to the crop of interest, but also non-plant AMPs produced by bacteria, fungi, oomycetes or animals. This review highlights some promising candidates within the plant kingdom and elsewhere, and offers some perspectives on how to identify and validate their bioactivities. Technological advances, particularly in mass spectrometry (MS) and nuclear magnetic resonance (NMR), have been instrumental in identifying and elucidating the structure of novel AMPs, especially nonribosomal peptides which cannot be identified through genomics approaches. The majority of non-plant AMPs showing potential for plant disease immunity are often tested using in vitro assays. The greatest challenge remains the functional validation of candidate AMPs in plants through transgenic experiments, particularly introducing nonribosomal AMPs into crops.

Molecular effects of resistance elicitors from biological origin and their potential for crop protection

Plants contain a sophisticated innate immune network to prevent pathogenic microbes from gaining access to nutrients and from colonizing internal structures. The first layer of inducible response is governed by the plant following the perception of microbe- or modified plant-derived molecules. As the perception of these molecules results in a plant response that can provide efficient resistance toward non-adapted pathogens they can also be described as "defense elicitors." In compatible plant/microbe interactions, adapted microorganisms have means to avoid or disable this resistance response and promote virulence. However, this requires a detailed spatial and temporal response from the invading pathogens. In agricultural practice, treating plants with isolated defense elicitors in the absence of pathogens can promote plant resistance by uncoupling defense activation from the effects of pathogen virulence determinants. The plant responses to plant, bacterial, oomycete, or fungal-derived elicitors are not, in all cases, universal and need elucidating prior to the application in agriculture. This review provides an overview of currently known elicitors of biological rather than synthetic origin and places their activity into a molecular context.

Identification of small secreted peptides (SSPs) in maize and expression analysis of partial SSP genes in reproductive tissues

Maize 1,491 small secreted peptides were identified, which were classified according to the character of peptide sequences. Partial SSP gene expressions in reproductive tissues were determined by qRT-PCR. Small secreted peptides (SSPs) are important cell-cell communication messengers in plants. Most information on plant SSPs come from Arabidopsis thaliana and Oryza sativa, while little is known about the SSPs of other grass species such as maize (Zea mays). In this study, we identified 1,491 SSP genes from maize genomic sequences. These putative SSP genes were distributed throughout the ten maize chromosomes. Among them, 611 SSPs were classified into 198 superfamilies according to their conserved domains, and 725 SSPs with four or more cysteines at their C-termini shared similar cysteine arrangements with their counterparts in other plant species. Moreover, the SSPs requiring post-translational modification, as well as defensin-like (DEFL) proteins, were identified. Further, the expression levels of 110 SSP genes were analyzed in reproductive tissues, including male flower, pollen, silk, and ovary. Most of the genes encoding basal-layer antifungal peptide-like, small coat proteins-like, thioredoxin-like proteins, γ-thionins-like, and DEFL proteins showed high expression levels in the ovary and male flower compared with their levels in silk and mature pollen. The rapid alkalinization factor-like genes were highly expressed only in the mature ovary and mature pollen, and pollen Ole e 1-like genes showed low expression in silk. The results of this study provide basic information for further analysis of SSP functions in the reproductive process of maize.

Jasmonate-triggered plant immunity

The plant hormone jasmonate (JA) exerts direct control over the production of chemical defense compounds that confer resistance to a remarkable spectrum of plant-associated organisms, ranging from microbial pathogens to vertebrate herbivores. The underlying mechanism of JA-triggered immunity (JATI) can be conceptualized as a multi-stage signal transduction cascade involving: i) pattern recognition receptors (PRRs) that couple the perception of danger signals to rapid synthesis of bioactive JA; ii) an evolutionarily conserved JA signaling module that links fluctuating JA levels to changes in the abundance of transcriptional repressor proteins; and iii) activation (de-repression) of transcription factors that orchestrate the expression of myriad chemical and morphological defense traits. Multiple negative feedback loops act in concert to restrain the duration and amplitude of defense responses, presumably to mitigate potential fitness costs of JATI. The convergence of diverse plant- and non-plant-derived signals on the core JA module indicates that JATI is a general response to perceived danger. However, the modular structure of JATI may accommodate attacker-specific defense responses through evolutionary innovation of PRRs (inputs) and defense traits (outputs). The efficacy of JATI as a defense strategy is highlighted by its capacity to shape natural populations of plant attackers, as well as the propensity of plant-associated organisms to subvert or otherwise manipulate JA signaling. As both a cellular hub for integrating informational cues from the environment and a common target of pathogen effectors, the core JA module provides a focal point for understanding immune system networks and the evolution of chemical diversity in the plant kingdom.

Continuous stimulation of the plant immune system by the peptide elicitor PIP-1 is required for phytoalexin biosynthesis in tobacco cells

The peptide elicitor PIP-1 (YGIHTH-nh2) induced various defense responses in tobacco cells. Types of defense responses induced by PIP-1 were different based on its concentration range: oxidative burst (an early response) was induced at low micromolar levels, but phytoalexin production (a late response) required about 10-50-fold higher concentrations than those required for oxidative burst. We assumed that rapid decreases in the PIP-1 concentration due to enzymatic hydrolysis in the culture media could cause this difference. To examine the potential impact of such degradation particularly on induction of phytoalexin biosynthesis, we designed a degradation-resistant analogue, MePIP-1, in which the amide bond between the fifth and sixth residues was N-methylated. MePIP-1 was considerably more stable than PIP-1 and induced significant phytoalexin production upon treatment at low micromolar levels. Further investigation of the mechanism of action of MePIP-1 showed a requirement of continuous elicitor stimulation for 3-6 h for the phytoalexin production, which is likely to be regulated by long-lasting MAP kinase activation.

Plant peptides in defense and signaling

This review focuses on plant peptides involved in defense against pathogen infection and those involved in the regulation of growth and development. Defense peptides, defensins, cyclotides and anti-microbial peptides are compared and contrasted. Signaling peptides are classified according to their major sites of activity. Finally, a network approach to creating an interactomic peptide map is described.