Priming for enhanced defense

Priming of inducible defenses protects Norway spruce against tree-killing bark beetles

Plants can form an immunological memory known as defense priming, whereby exposure to a priming stimulus enables quicker or stronger response to subsequent attack by pests and pathogens. Such priming of inducible defenses provides increased protection and reduces allocation costs of defense. Defense priming has been widely studied for short-lived model plants such as Arabidopsis, but little is known about this phenomenon in long-lived plants like spruce. We compared the effects of pretreatment with sublethal fungal inoculations or application of the phytohormone methyl jasmonate (MeJA) on the resistance of 48-year-old Norway spruce (Picea abies) trees to mass attack by a tree-killing bark beetle beginning 35 days later. Bark beetles heavily infested and killed untreated trees but largely avoided fungus-inoculated trees and MeJA-treated trees. Quantification of defensive terpenes at the time of bark beetle attack showed fungal inoculation induced 91-fold higher terpene concentrations compared with untreated trees, whereas application of MeJA did not significantly increase terpenes. These results indicate that resistance in fungus-inoculated trees is a result of direct induction of defenses, whereas resistance in MeJA-treated trees is due to defense priming. This work extends our knowledge of defense priming from model plants to an ecologically important tree species.

Empirical patterns of environmental variation favor adaptive transgenerational plasticity

Effects of parental environment on offspring traits have been well known for decades. Interest in this transgenerational form of phenotypic plasticity has recently surged due to advances in our understanding of its mechanistic basis. Theoretical research has simultaneously advanced by predicting the environmental conditions that should favor the adaptive evolution of transgenerational plasticity. Yet whether such conditions actually exist in nature remains largely unexplored. Here, using long-term climate data, we modeled optimal levels of transgenerational plasticity for an organism with a one-year life cycle at a spatial resolution of 4 km2 across the continental United States. Both annual temperature and precipitation levels were often autocorrelated, but the strength and direction of these autocorrelations varied considerably even among nearby sites. When present, such environmental autocorrelations render offspring environments statistically predictable based on the parental environment, a key condition for the adaptive evolution of transgenerational plasticity. Results of our optimality models were consistent with this prediction: High levels of transgenerational plasticity were favored at sites with strong environmental autocorrelations, and little-to-no transgenerational plasticity was favored at sites with weak or nonexistent autocorrelations. These results are among the first to show that natural patterns of environmental variation favor the evolution of adaptive transgenerational plasticity. Furthermore, these findings suggest that transgenerational plasticity is likely variable in nature, depending on site-specific patterns of environmental variation.

Rhizobacteria-induced systemic tolerance against drought stress in Sorghum bicolor (L.) Moench

Induction of systemic tolerance in sorghum [Sorghum bicolor (L.) Moench] against drought stress was studied by screening a large collection of rhizobacterial isolates for their potential to exhibit this essential plant growth-promoting trait. This was done by means of a greenhouse assay that measured the relative change in both plant height and -biomass (roots and shoots) between rhizobacteria-primed versus non-primed (naïve) plants under drought stress conditions. In order to elucidate the metabolomic changes in S. bicolor that conferred the drought stress tolerance after treatment (priming) with selected isolates, untargeted ultra-high performance liquid chromatography-high definition mass spectrometry (UHPLC-HDMS)-based metabolomics was carried out. Intracellular metabolites were methanol-extracted from rhizobacteria-primed and naïve S. bicolor roots and shoots. Extracts were analysed on a UHPLC-HDMS system and the generated data were chemometrically mined to determine signatory metabolic profiles and bio-markers related to induced systemic tolerance. The metabolomic results showed significant treatment-related differential metabolic reprogramming between rhizobacteria-primed and naïve plants, correlating to the ability of the selected isolates to protect S. bicolor against drought stress. The selected isolates, identified by means of 16S rRNA gene sequencing as members of the genera Bacillus and Pseudomonas, were screened for 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity by means of an in vitro assay and the presence of the acdS gene was subsequently confirmed by PCR for strain N66 (Pseudomonas sp.). The underlying key metabolic changes in the enhanced drought stress tolerance observed in rhizobacteria-primed S. bicolor plants included (1) augmented antioxidant capacity; (2) growth promotion and root architecture modification as a result of the upregulation of the hormones gibberellic acid, indole acetic acid and cytokinin; (3) the early activation of induce systemic tolerance through the signalling hormones brassinolides, salicylic acid and jasmonic acid and signalling molecules sphingosine and psychosine; (4) the production of the osmolytes proline, glutamic acid and choline; (5) the production of the epicuticular wax docosanoic acid and (6) ACC deaminase activity resulting in lowered ethylene levels. These results unravelled key molecular details underlying the PGPR-induced systemic tolerance in sorghum plants, providing insights for the plant priming for abiotic stress.

Elicitor and Receptor Molecules: Orchestrators of Plant Defense and Immunity

Pathogen-associated molecular patterns (PAMPs), microbe-associated molecular patterns (MAMPs), herbivore-associated molecular patterns (HAMPs), and damage-associated molecular patterns (DAMPs) are molecules produced by microorganisms and insects in the event of infection, microbial priming, and insect predation. These molecules are then recognized by receptor molecules on or within the plant, which activates the defense signaling pathways, resulting in plant’s ability to overcome pathogenic invasion, induce systemic resistance, and protect against insect predation and damage. These small molecular motifs are conserved in all organisms. Fungi, bacteria, and insects have their own specific molecular patterns that induce defenses in plants. Most of the molecular patterns are either present as part of the pathogen’s structure or exudates (in bacteria and fungi), or insect saliva and honeydew. Since biotic stresses such as pathogens and insects can impair crop yield and production, understanding the interaction between these organisms and the host via the elicitor–receptor interaction is essential to equip us with the knowledge necessary to design durable resistance in plants. In addition, it is also important to look into the role played by beneficial microbes and synthetic elicitors in activating plants’ defense and protection against disease and predation. This review addresses receptors, elicitors, and the receptor–elicitor interactions where these components in fungi, bacteria, and insects will be elaborated, giving special emphasis to the molecules, responses, and mechanisms at play, variations between organisms where applicable, and applications and prospects.

Dodder-transmitted mobile signals prime host plants for enhanced salt tolerance

The dodders (Cuscuta spp.) are a genus of shoot parasites. In nature, a dodder often simultaneously parasitizes two or more neighboring hosts. Salt stress is a common abiotic stress for plants. It is unclear whether dodder transmits physiologically relevant salt stress-induced systemic signals among its hosts and whether these systemic signals affect the hosts' tolerance to salt stress. Here, we simultaneously parasitized two or more cucumber plants with dodder. We found that salt treatment of one host highly primed the connected host, which showed strong decreases in the extent of leaf withering and cell death in response to subsequent salt stress. Transcriptomic analysis indicated that 24 h after salt treatment of one cucumber, the transcriptome of the other dodder-connected cucumber largely resembled that of the salt-treated one, indicating that inter-plant systemic signals primed these dodder-connected cucumbers at least partly through transcriptomic reconfiguration. Furthermore, salt treatment of one of the cucumbers induced physiological changes, including altered proline contents, stomatal conductance, and photosynthetic rates, in both of the dodder-connected cucumbers. This study reveals a role of dodder in mediating salt-induced inter-plant signaling among dodder-connected hosts and highlights the physiological function of these mobile signals in plant-plant interactions under salt stress.

Acclimation, priming and memory in the response of Arabidopsis thaliana seedlings to cold stress

Because stress experiences are often recurrent plants have developed strategies to remember a first so-called priming stress to eventually respond more effectively to a second triggering stress. Here, we have studied the impact of discontinuous or sustained cold stress (4 °C) on in vitro grown Arabidopsis thaliana seedlings of different age and their ability to get primed and respond differently to a later triggering stress. Cold treatment of 7-d-old seedlings induced the expression of cold response genes but did not cause a significantly enhanced freezing resistance. The competence to increase the freezing resistance in response to cold was associated with the formation of true leaves. Discontinuous exposure to cold only during the night led to a stepwise modest increase in freezing tolerance provided that the intermittent phase at ambient temperature was less than 32 h. Seedlings exposed to sustained cold treatment developed a higher freezing tolerance which was further increased in response to a triggering stress during three days after the priming treatment had ended indicating cold memory. Interestingly, in all scenarios the primed state was lost as soon as the freezing tolerance had reached the level of naïve plants indicating that an effective memory was associated with an altered physiological state. Known mutants of the cold stress response (cbfs, erf105) and heat stress memory (fgt1) did not show an altered behaviour indicating that their roles do not extend to memory of cold stress in Arabidopsis seedlings.

Arabidopsis CAMTA Transcription Factors Regulate Pipecolic Acid Biosynthesis and Priming of Immunity Genes

The Arabidopsis thaliana Calmodulin-binding Transcription Activator (CAMTA) transcription factors CAMTA1, CAMTA2, and CAMTA3 (CAMTA123) serve as master regulators of salicylic acid (SA)-mediated immunity, repressing the biosynthesis of SA in healthy plants. Here, we show that CAMTA123 also repress the biosynthesis of pipecolic acid (Pip) in healthy plants. Loss of CAMTA123 function resulted in the induction of AGD2-like defense response protein 1 (ALD1), which encodes an enzyme involved in Pip biosynthesis. Induction of ALD1 resulted in the accumulation of high levels of Pip, which brought about increased levels of the SA receptor protein NPR1 without induction of NPR1 expression or requirement for an increase in SA levels. Pip-mediated induction of ALD1 and genes regulating the biosynthesis of SA-CBP60g, SARD1, PAD4, and EDS1-was largely dependent on NPR1. Furthermore, Pip-mediated increase in NPR1 protein levels was associated with priming of Pip and SA biosynthesis genes to induction by low levels of SA. Taken together, our findings expand the role for CAMTA123 in regulating key immunity genes and suggest a working model whereby loss of CAMTA123 repression leads to the induction of plant defense genes and initiation of SAR.

The Use of PTI-Marker Genes to Identify Novel Compounds that Establish Induced Resistance in Rice

Compounds that establish induced resistance (IR) in plants are promising alternatives for the pesticides that are progressively being banned worldwide. Screening platforms to identify IR-establishing compounds have been developed, but none were specifically designed for monocot plants. Here, we propose the use of an RT-qPCR screening platform, based on conserved immunity marker genes of rice as proxy for IR induction. Central regulators of biotic stress responses of rice were identified with a weighted gene co-expression network analysis (WGCNA), using more than 350 microarray datasets of rice under various sorts of biotic stress. Candidate genes were narrowed down to six immunity marker genes, based on consistent association with pattern-triggered immunity (PTI), both in rice plants as in rice cell suspension cultures (RCSCs). By monitoring the expression of these genes in RCSCs upon treatment with candidate IR-inducing compounds, we showed that our marker genes can predict IR induction in rice. Diproline, a novel IR-establishing compound for monocots that was detected with these marker genes, was shown to induce rice resistance against root-knot nematodes, without fitness costs. Gene expression profiling of the here-described PTI-marker genes can be executed on fully-grown plants or in RCSCs, providing a novel and versatile tool to predict IR induction.

Plant Epigenetic Stress Memory Induced by Drought: A Physiological and Molecular Perspective

Drought stress is one of the most common stresses encountered by crops and other plants and leads to significant productivity losses. It commonly happens that drought stress occurs more than once during the plant's life cycle. Plants suffering from drought stress can adapt their life strategies to acclimate and survive in many different ways. Interestingly, some plants have evolved a stress response strategy referred to as stress memory which leads to an enhanced response the next time the stress is encountered. The acquisition of stress memory leads to a reprogrammed transcriptional response during subsequent stress and subsequent changes both at the physiological and molecular level. Recent advances in understanding chromatin dynamics have demonstrated the involvement of chromatin modifications, especially histone marks, associated with drought stress-responsive memory genes and subsequent enhanced transcriptional responses to repeated drought stress. In this chapter, we describe recent progress in this area and summarize techniques for the study of plant epigenetic responses to stress, including the roles of ABA and transcription factors in superinduced transcriptional activation during recurrent drought stress. We also review the possible use of seed priming to induce stress memory later in the plant life cycle. Finally, we discuss the potential implications of understanding the epigenetic mechanisms involved in plant stress memory for future applications in crop improvement and drought resistance.

Catalase, glutathione, and protein phosphatase 2A-dependent organellar redox signalling regulate aphid fecundity under moderate and high irradiance

Redox processes regulate plant/insect responses, but the precise roles of environmental triggers and specific molecular components remain poorly defined. Aphid fecundity and plant responses were therefore measured in Arabidopsis thaliana mutants deficient in either catalase 2 (cat2), different protein phosphatase 2A (PP2A) subunits or glutathione (cad2, pad2, and clt1) under either moderate (250 μmol m^-2 s^-1 ) or high (800 μmol m^-2 s^-1 ) light. Aphid fecundity was decreased in pp2a-b'γ, cat2 and the cat2 pp2a-b'γ double mutants relative to the wild type under moderate irradiance. High light decreased aphid numbers in all genotypes except for cat2. Aphid fecundity was similar in the cat2 and glutathione-, phytoalexin-, and glucosinolate-deficient cat2cad2 double mutants under both irradiances. Aphid-induced increases in transcripts encoding the abscisic acid-related ARABIDOPSIS ZINC-FINGER PROTEIN 1 transcription factor were observed only under moderate light. Conversely, aphid induced increases in transcripts encoding the jasmonate-synthesis enzyme ALLENE OXIDE CYCLASE 3 was observed in all genotypes only under high light. Aphid-induced increases in REDOX RESPONSIVE TRANSCRIPTION FACTOR 1 mRNAs were observed in all genotypes except pp2a-b'ζ1-1 under both irradiances. Aphid fecundity is therefore regulated by cellular redox signalling that is mediated, at least in part, through PP2A-dependent mitochondria to nucleus signalling pathways.

Combined Proteome and Transcriptome Analysis of Heat-Primed Azalea Reveals New Insights Into Plant Heat Acclimation Memory

Plants can obtain superinduction of defense against unpredictable challenges based on prior acclimation, but the mechanisms involved in the acclimation memory are little known. The objective of this study was to characterize mechanisms of heat acclimation memory in Rhododendron hainanense, a thermotolerant wild species of azalea. Pretreatment of a 2-d recovery (25/18°C, day/night) after heat acclimation (37°C, 1 h) (AR-pt) did not weaken but enhanced acquired thermotolerance in R. hainanense with less damaged phenotype, net photosynthetic rate, and membrane stability than non-acclimation pretreated (NA-pt) plants. Combined transcriptome and proteome analysis revealed that a lot of heat-responsive genes still maintained high protein abundance rather than transcript level after the 2-d recovery. Photosynthesis-related genes were highly enriched and most decreased under heat stress (HS: 42°C, 1 h) with a less degree in AR-pt plants compared to NA-pt. Sustainably accumulated chloroplast-localized heat shock proteins (HSPs), Rubisco activase 1 (RCA1), beta-subunit of chaperonin-60 (CPN60β), and plastid transcriptionally active chromosome 5 (pTAC5) in the recovery period probably provided equipped protection of AR-pt plants against the subsequent HS, with less damaged photochemical efficiency and chloroplast structure. In addition, significant higher levels of RCA1 transcripts in AR-pt compared to NA-pt plants in early stage of HS showed a more important role of RCA1 than other chaperonins in heat acclimation memory. The novel heat-induced RCA1, rather than constitutively expressed RCA2 and RCA3, showed excellent thermostability after long-term HS (LHS: 42/35°C, 7 d) and maintained balanced Rubisco activation state in photosynthetic acclimation. This study provides new insights into plant heat acclimation memory and indicates candidate genes for genetic modification and molecular breeding in thermotolerance improvement.

Calcium-dependent protein kinase 5 links calcium signaling with N-hydroxy-l-pipecolic acid- and SARD1-dependent immune memory in systemic acquired resistance

Systemic acquired resistance (SAR) prepares infected plants for faster and stronger defense activation upon subsequent attacks. SAR requires an information relay from primary infection to distal tissue and the initiation and maintenance of a self-maintaining phytohormone salicylic acid (SA)-defense loop. In spatial and temporal resolution, we show that calcium-dependent protein kinase CPK5 contributes to immunity and SAR. In local basal resistance, CPK5 functions upstream of SA synthesis, perception, and signaling. In systemic tissue, CPK5 signaling leads to accumulation of SAR-inducing metabolite N-hydroxy-L-pipecolic acid (NHP) and SAR marker genes, including Systemic Acquired Resistance Deficient 1 (SARD1) Plants of increased CPK5, but not CPK6, signaling display an 'enhanced SAR' phenotype towards a secondary bacterial infection. In the sard1-1 background, CPK5-mediated basal resistance is still mounted, but NHP concentration is reduced and enhanced SAR is lost. The biochemical analysis estimated CPK5 half maximal kinase activity for calcium, K50 [Ca²⁺ ], to be c. 100 nM, close to the cytoplasmic resting level. This low threshold uniquely qualifies CPK5 to decode subtle changes in calcium, a prerequisite to signal relay and onset and maintenance of priming at later time points in distal tissue. Our data explain why CPK5 functions as a hub in basal and systemic plant immunity.

Primary Metabolism Is Distinctly Modulated by Plant Resistance Inducers in Coffea arabica Leaves Infected by Hemileia vastatrix

Epidemics of coffee leaf rust (CLR) leads to great yield losses and huge depreciation of coffee marketing values, if no control measures are applied. Societal expectations of a more sustainable coffee production are increasingly imposing the replacement of fungicide treatments by alternative solutions. A protection strategy is to take advantage of the plant immune system by eliciting constitutive defenses. Based on such concept, plant resistance inducers (PRIs) have been developed. The Greenforce CuCa formulation, similarly to acibenzolar-S-methyl (ASM), shows promising results in the control of CLR (Hemileia vastatrix) in Coffea arabica cv. Mundo Novo. The molecular mechanisms of PRIs action are poorly understood. In order to contribute to its elucidation a proteomic, physiological (leaf gas-exchange) and biochemical (enzymatic) analyses were performed. Coffee leaves treated with Greenforce CuCa and ASM and inoculation with H. vastatrix were considered. Proteomics revealed that both PRIs lead to metabolic adjustments but, inducing distinct proteins. These proteins were related with photosynthesis, protein metabolism and stress responses. Greenforce CuCa increased photosynthesis and stomatal conductance, while ASM caused a decrease in these parameters. It was further observed that Greenforce CuCa reinforces the redox homeostasis of the leaf, while ASM seems to affect preferentially the secondary metabolism and the stress-related proteins. So, the PRIs prepare the plant to resist CLR but, inducing different defense mechanisms upon pathogen infection. The existence of a link between the primary metabolism and defense responses was evidenced. The identification of components of the plant primary metabolism, essential for plant growth and development that, simultaneously, participate in the plant defense responses can open new perspectives for plant breeding programs.

Ascorbate Oxidase Induces Systemic Resistance in Sugar Beet Against Cyst Nematode Heterodera schachtii

Ascorbate oxidase (AO) is an enzyme involved in catalyzing the oxidation of apoplastic ascorbic acid (AA) to dehydroascorbic acid (DHA). In this research, the potential of AO spraying to induce systemic resistance was demonstrated in the interaction between sugar beet root and cyst nematode Heterodera schachtii and the mechanism was elucidated. Plant bioassays showed that roots of AO-sprayed plants were infested by a significantly lower number of females and cysts when compared with mock-sprayed control plants. Hormone measurements showed an elevated level of jasmonic acid (JA) salicylic acid (SA) and ethylene (ET) in the roots of AO-sprayed plants, with a dynamic temporal pattern of activation. Experiments with chemical inhibitors showed that AO-induced systemic resistance is partially dependent on the JA, ET and SA pathways. Biochemical analyses revealed a primed accumulation of hydrogen peroxide (H₂O₂), and phenylalanine ammonia lyase (PAL) activity in the roots of AO-sprayed plants upon infection by cyst nematodes. In conclusion, our data shows that AO works as an effective systemic defense priming agent in sugar beet against cyst nematode infection, through activation of multiple basal plant defense pathways.

Selected isolates of Trichoderma gamsii induce different pathways of systemic resistance in maize upon Fusarium verticillioides challenge

The pink ear rot is one of the most damaging maize diseases, caused by the mycotoxigenic fungal pathogen, Fusarium verticillioides. The application of biological control agents, like antagonistic and/or resistance inducer microorganisms, is an option to reduce fungal infection and kernel contamination in a sustainable and environmentally friendly way. It is well known that Trichoderma species are non-pathogenic fungi able to antagonize plant pathogens and to induce systemic resistance in plants. The present work aimed to verify if Trichoderma spp., applied to maize kernels, affect the plant growth and induce systemic responses to F. verticillioides. Besides, the capability to reduce fumonisin concentration in liquid cultures was investigated. Two T. gamsii (IMO5 and B21), and one T. afroharzianum (B75) isolates, selected both for antagonism and for the ability to reduce root infections, significantly reduced the endophytic development of the stem-inoculated pathogen, compared to the control. The mechanisms of action appeared to be strain-specific, with IMO5 enhancing transcript levels of marker genes of Induced Systemic Resistance (ZmLOX10, ZmAOS, and ZmHPL) while B21 enhancing marker genes of Systemic Acquired Resistance (ZmPR1 and ZmPR5), as evinced by measuring their expression profiles in the leaves. Moreover, IMO5 promoted plant growth, while B21 was able to significantly reduce the fumonisin content in a liquid medium. The results of this work give new evidence that the seed application of T. gamsii is a promising tool for controlling F. verticillioides to be integrated with breeding and the adoption of good agricultural practices.

Osa-miR7695 enhances transcriptional priming in defense responses against the rice blast fungus

BACKGROUND

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the post-transcriptional level in eukaryotes. In rice, MIR7695 expression is regulated by infection with the rice blast fungus Magnaporthe oryzae with subsequent down-regulation of an alternatively spliced transcript of natural resistance-associated macrophage protein 6 (OsNramp6). NRAMP6 functions as an iron transporter in rice.

RESULTS

Rice plants grown under high iron supply showed blast resistance, which supports that iron is a factor in controlling blast resistance. During pathogen infection, iron accumulated in the vicinity of M. oryzae appressoria, the sites of pathogen entry, and in cells surrounding infected regions of the rice leaf. Activation-tagged MIR7695 rice plants (MIR7695-Ac) exhibited enhanced iron accumulation and resistance to M. oryzae infection. RNA-seq analysis revealed that blast resistance in MIR7695-Ac plants was associated with strong induction of defense-related genes, including pathogenesis-related and diterpenoid biosynthetic genes. Levels of phytoalexins during pathogen infection were higher in MIR7695-Ac than wild-type plants. Early phytoalexin biosynthetic genes, OsCPS2 and OsCPS4, were also highly upregulated in wild-type rice plants grown under high iron supply.

CONCLUSIONS

Our data support a positive role of miR7695 in regulating rice immunity that further underpin links between defense and iron signaling in rice. These findings provides a basis to better understand regulatory mechanisms involved in rice immunity in which miR7695 participates which has a great potential for the development of strategies to improve blast resistance in rice.

Bio-control agents activate plant immune response and prime susceptible tomato against root-knot nematodes

Beneficial microorganisms are generally known to activate plant defense against biotic challenges. However, the molecular mechanisms by which activated plants react more rapidly and actively to pests remain still largely unclear. Tomato plants pre-treated with a mixture of beneficial bio-control agents (BCAs), as soil-drenches, were less sensitive to infection of the root-knot nematode (RKN) Meloidogyne incognita. To unravel the molecular mechanisms of this induced resistance against RKNs, we used qRT-PCR to monitor the expression, in tomato roots and leaves, of 6 key defense genes. Gene transcripts were detected until the 12th day after BCA treatment(3, 7, 8, 12 dpt) and3 and 7 days after nematode inoculation of pre-treated plants. Early after BCA treatment, the salicylic acid (SA)-dependent pathogenesis related gene (PR-gene), PR-1b, marker of the systemic acquired resistance (SAR), was systemically over-expressed. Another PR-gene, PR-5, was over-expressed at later stages of BCA-plant interaction, and only in roots. Activation of defense against RKNs was attested by the early up-regulation of 4 genes (PR-1, PR-3, PR-5, ACO) in pre-treated plants after inoculation. Conversely, the expression of the JA/ET-dependent gene JERF3 did not increase after nematode inoculation in primed plants. A catalase gene (CAT)was highly over-expressed by nematode infection, however, this over-expression was annulled at the earliest stages or limited at the later stages of infection toBCA-treated roots. Enzyme activities, such as glucanase and endochitinase, were enhanced in roots of pre-treated inoculated plants with respect to plants left not inoculated as a control. These findings indicate that BCA interaction with roots primes plants against RKNs. BCA-mediated immunity seems to rely on SA-mediated SAR and to be associated with both the activation of chitinase and glucanase enzyme activities and the inhibition of the plant antioxidant enzyme system. Immunity is triggered at the penetration and movements inside the roots of the invading nematode juveniles but probably acts at the feeding site building stage of nematode infection.

Organellar protein multi-functionality and phenotypic plasticity in plants

With the increasing impact of climate instability on agricultural and ecological systems has come a heightened sense of urgency to understand plant adaptation mechanisms in more detail. Plant species have a remarkable ability to disperse their progeny to a wide range of environments, demonstrating extraordinary resiliency mechanisms that incorporate epigenetics and transgenerational stability. Surprisingly, some of the underlying versatility of plants to adapt to abiotic and biotic stress emerges from the neofunctionalization of organelles and organellar proteins. We describe evidence of possible plastid specialization and multi-functional organellar protein features that serve to enhance plant phenotypic plasticity. These features appear to rely on, for example, spatio-temporal regulation of plastid composition, and unusual interorganellar protein targeting and retrograde signalling features that facilitate multi-functionalization. Although we report in detail on three such specializations, involving MSH1, WHIRLY1 and CUE1 proteins in Arabidopsis, there is ample reason to believe that these represent only a fraction of what is yet to be discovered as we begin to elaborate cross-species diversity. Recent observations suggest that plant proteins previously defined in one context may soon be rediscovered in new roles and that much more detailed investigation of proteins that show subcellular multi-targeting may be warranted.

This article is part of the theme issue ‘Linking the mitochondrial genotype to phenotype: a complex endeavour’.

Bacillus velezensis 5113 Induced Metabolic and Molecular Reprogramming during Abiotic Stress Tolerance in Wheat

Abiotic stresses are main limiting factors for agricultural production around the world. Plant growth promoting rhizobacteria (PGPR) have been shown to improve abiotic stress tolerance in several plants. However, the molecular and physiological changes connected with PGPR priming of stress management are poorly understood. The present investigation aimed to explore major metabolic and molecular changes connected with the ability of Bacillus velezensis 5113 to mediate abiotic stress tolerance in wheat. Seedlings treated with Bacillus were exposed to heat, cold/freezing or drought stress. Bacillus improved wheat survival in all stress conditions. SPAD readings showed higher chlorophyll content in 5113-treated stressed seedlings. Metabolite profiling using NMR and ESI-MS provided evidences for metabolic reprograming in 5113-treated seedlings and showed that several common stress metabolites were significantly accumulated in stressed wheat. Two-dimensional gel electrophoresis of wheat leaves resolved more than 300 proteins of which several were differentially expressed between different treatments and that cold stress had a stronger impact on the protein pattern compared to heat and drought. Peptides maps or sequences were used for database searches which identified several homologs. The present study suggests that 5113 treatment provides systemic effects that involve metabolic and regulatory functions supporting both growth and stress management.

Histone modifications and their regulatory roles in plant development and environmental memory

Plants grow in dynamic environments where they receive diverse environmental signals. Swift and precise control of gene expression is essential for plants to align their development and metabolism with fluctuating surroundings. Modifications on histones serve as "histone code" to specify chromatin and gene activities. Different modifications execute distinct functions on the chromatin, promoting either active transcription or gene silencing. Histone writers, erasers, and readers mediate the regulation of histone modifications by catalyzing, removing, and recognizing modifications, respectively. Growing evidence indicates the important function of histone modifications in plant development and environmental responses. Histone modifications also serve as environmental memory for plants to adapt to environmental changes. Here we review recent progress on the regulation of histone modifications in plants, the impact of histone modifications on environment-controlled developmental transitions including germination and flowering, and the role of histone modifications in environmental memory.

An engineered pathway for N-hydroxy-pipecolic acid synthesis enhances systemic acquired resistance in tomato

Systemic acquired resistance (SAR) is a powerful immune response that triggers broad-spectrum disease resistance throughout a plant. In the model plant Arabidopsis thaliana, long-distance signaling and SAR activation in uninfected tissues occur without circulating immune cells and instead rely on the metabolite N-hydroxy-pipecolic acid (NHP). Engineering SAR in crop plants would enable external control of a plant's ability to mount a global defense response upon sudden changes in the environment. Such a metabolite-engineering approach would require the molecular machinery for producing and responding to NHP in the crop plant. Here, we used heterologous expression in Nicotiana benthamiana leaves to identify a minimal set of Arabidopsis genes necessary for the biosynthesis of NHP. Local expression of these genes in tomato leaves triggered SAR in distal tissues in the absence of a pathogen, suggesting that the SAR trait can be engineered to enhance a plant's endogenous ability to respond to pathogens. We also showed tomato produces endogenous NHP in response to a bacterial pathogen and that NHP is present across the plant kingdom, raising the possibility that an engineering strategy to enhance NHP-induced defenses could be possible in many crop plants.

A Microbial Fermentation Mixture Primes for Resistance Against Powdery Mildew in Wheat

Since many fungal pathogens develop resistance to fungicides, novel and low-cost alternative methods to improve plant health and fitness need to be developed. An approach to improve productivity in crops is to stimulate the plant's own defence mechanisms via priming. Therefore, we investigated if a fermentation-based elicitor could prime plant defences against powdery mildew in wheat by inducing the expression of endogenous defence-related genes. Wheat seedlings were spray-treated with a fermentation-based elicitor 8 days prior to inoculation with powdery mildew. Disease assays showed a significantly reduced number of powdery mildew pustules were formed on wheat treated with the mixed elicitor. In vitro sensitivity assays tested the ability of powdery mildew conidia to germinate on agar amended with the fermentation-based product and concluded that fungal germination and differentiation were also inhibited. Tissue samples were taken at time points pertaining to different developmental stages of powdery mildew infection. Significantly higher expression of PR genes (PR1, PR4, PR5, and PR9) was observed in the microbial fermentation mixture-treated plants compared with untreated plants. These genes are often associated with the elicitation of plant defence responses to specific biotrophic pathogens, such as powdery mildew, suggesting an elicitor-mediated response in the wheat plants tested. The product components were assessed, and the components were found to act synergistically in the microbial fermentation mixture. Therefore, this fermentation-based elicitor provides an effective method for powdery mildew control.

Pipecolic Acid Is Induced in Barley upon Infection and Triggers Immune Responses Associated with Elevated Nitric Oxide Accumulation

Pipecolic acid (Pip) is an essential component of systemic acquired resistance, priming resistance in Arabidopsis thaliana against (hemi)biotrophic pathogens. Here, we studied the potential role of Pip in bacteria-induced systemic immunity in barley. Exudates of barley leaves infected with the systemic immunity-inducing pathogen Pseudomonas syringae pv. japonica induced immune responses in A. thaliana. The same leaf exudates contained elevated Pip levels compared with those of mock-treated barley leaves. Exogenous application of Pip induced resistance in barley against the hemibiotrophic bacterial pathogen Xanthomonas translucens pv. cerealis. Furthermore, both a systemic immunity-inducing infection and exogenous application of Pip enhanced the resistance of barley against the biotrophic powdery mildew pathogen Blumeria graminis f. sp. hordei. In contrast to a systemic immunity-inducing infection, Pip application did not influence lesion formation by a systemically applied inoculum of the necrotrophic fungus Pyrenophora teres. Nitric oxide (NO) levels in barley leaves increased after Pip application. Furthermore, X. translucens pv. cerealis induced the accumulation of superoxide anion radicals and this response was stronger in Pip-pretreated compared with mock-pretreated plants. Thus, the data suggest that Pip induces barley innate immune responses by triggering NO and priming reactive oxygen species accumulation.

Roles of plant volatiles in defence against microbial pathogens and microbial exploitation of volatiles

Plants emit a large variety of volatile organic compounds during infection by pathogenic microbes, including terpenes, aromatics, nitrogen-containing compounds, and fatty acid derivatives, as well as the volatile plant hormones, methyl jasmonate, and methyl salicylate. Given the general antimicrobial activity of plant volatiles and the timing of emission following infection, these compounds have often been assumed to function in defence against pathogens without much solid evidence. In this review, we critically evaluate current knowledge on the toxicity of volatiles to fungi, bacteria, and viruses and their role in plant resistance as well as how they act to induce systemic resistance in uninfected parts of the plant and in neighbouring plants. We also discuss how microbes can detoxify plant volatiles and exploit them as nutrients, attractants for insect vectors, and inducers of volatile emissions, which stimulate immune responses that make plants more susceptible to infection. Although much more is known about plant volatile-herbivore interactions, knowledge of volatile-microbe interactions is growing and it may eventually be possible to harness plant volatiles to reduce disease in agriculture and forestry. Future research in this field can be facilitated by making use of the analytical and molecular tools generated by the prolific research on plant-herbivore interactions.

Distribution of the Specialist Aphid Uroleucon nigrotuberculatum (Homoptera: Aphididae) in Response to Host Plant Semiochemical Induction by the Gall Fly Eurosta solidaginis (Diptera: Tephritidae)

Many plants use terpenoids and other volatile compounds as semiochemicals. Reception of plant volatiles by conspecifics may trigger a defensive phytochemical response. These same compounds can also function as host recognition signals for phytophagous insects. In this experiment, we find that when the specialist gall-forming fly Eurosta solidaginis (Fitch; Diptera: Tephritidae) attacks its tall goldenrod (Solidago altissima (L.; Asterales: Asteraceae)) host plant, the fly indirectly induces a phytochemical response in nearby tall goldenrod plants. This phytochemical response may, in turn, act as a positive signal attracting the goldenrod specialist aphid Uroleucon nigrotuberculatum (Olive; Hemiptera: Aphididae). Laboratory-based experiments exposing ungalled tall goldenrod plants to the volatiles released by E. solidaginis galls demonstrated a consistent increase in foliar terpenoid concentrations in ungalled plants. Analysis of tall goldenrod stem and gall tissue chemistry revealed induction of terpenoids in gall tissue, with a simultaneous decrease in green leaf volatile concentrations. Field experiments demonstrated a consistent spatial relationship in tall goldenrod foliar terpenoid concentrations with distance from an E. solidaginis gall. Both laboratory and field experiments establish consistent induction of the terpene β-farnesene, and that this compound is a strong positive predictor of U. nigrotuberculatum aphid presence on goldenrod plants along with plant biomass and several other foliar terpenoids. These findings suggest E. solidaginis induced phytochemistry, especially β-farnesene, may be acting as a kairomone, driving aphid distribution in the field.

Unravelling the Metabolic Reconfiguration of the Post-Challenge Primed State in Sorghum bicolor Responding to Colletotrichum sublineolum Infection

Priming is a natural phenomenon that pre-conditions plants for enhanced defence against a wide range of pathogens. It represents a complementary strategy, or sustainable alternative that can provide protection against disease. However, a comprehensive functional and mechanistic understanding of the various layers of priming events is still limited. A non-targeted metabolomics approach was used to investigate metabolic changes in plant growth-promoting rhizobacteria (PGPR)-primed Sorghum bicolor seedlings infected with the anthracnose-causing fungal pathogen, Colletotrichum sublineolum, with a focus on the post-challenge primed state phase. At the 4-leaf growth stage, the plants were treated with a strain of Paenibacillus alvei at 10⁸ cfu mL⁻¹. Following a 24 h PGPR application, the plants were inoculated with a C. sublineolum spore suspension (10⁶ spores mL⁻¹), and the infection monitored over time: 1, 3, 5, 7 and 9 days post-inoculation. Non-infected plants served as negative controls. Intracellular metabolites from both inoculated and non-inoculated plants were extracted with 80% methanol-water. The extracts were chromatographically and spectrometrically analysed on an ultra-high performance liquid chromatography (UHPLC) system coupled to high-definition mass spectrometry. The acquired multidimensional data were processed to create data matrices for chemometric modelling. The computed models indicated time-related metabolic perturbations that reflect primed responses to the fungal infection. Evaluation of orthogonal projection to latent structure-discriminant analysis (OPLS-DA) loading shared and unique structures (SUS)-plots uncovered the differential stronger defence responses against the fungal infection observed in primed plants. These involved enhanced levels of amino acids (tyrosine, tryptophan), phytohormones (jasmonic acid and salicylic acid conjugates, and zeatin), and defence-related components of the lipidome. Furthermore, other defence responses in both naïve and primed plants were characterised by a complex mobilisation of phenolic compounds and de novo biosynthesis of the flavones, apigenin and luteolin and the 3-deoxyanthocyanidin phytoalexins, apigeninidin and luteolinidin, as well as some related conjugates.

Folate Metabolism Interferes with Plant Immunity through 1C Methionine Synthase-Directed Genome-wide DNA Methylation Enhancement

Plants rely on primary metabolism for flexible adaptation to environmental changes. Here, through a combination of chemical genetics and forward genetic studies in Arabidopsis plants, we identified that the essential folate metabolic pathway exerts a salicylic acid-independent negative control on plant immunity. Disruption of the folate pathway promotes enhanced resistance to Pseudomonas syringae DC3000 via activation of a primed immune state in plants, whereas its implementation results in enhanced susceptibility. Comparative proteomics analysis using immune-defective mutants identified a methionine synthase (METS1), in charge of the synthesis of Met through the folate-dependent 1C metabolism, acting as a nexus between the folate pathway and plant immunity. Overexpression of METS1 represses plant immunity and is accompanied by genome-wide global increase in DNA methylation, revealing that imposing a methylation pressure at the genomic level compromises plant immunity. Take together, these results indicate that the folate pathway represents a new layer of complexity in the regulation of plant defense responses.

Systemic defense activation by COS-OGA in rice against root-knot nematodes depends on stimulation of the phenylpropanoid pathway

Activation of induced plant resistance to control pests and diseases is regaining attention in the current climate where chemical pesticides are being progressively banned. Formulations of chitosan oligomers (COS) and pectin-derived oligogalacturonides (OGA), COS-OGA, have previously been described to induce resistance against fungal diseases in different crop plants. Here, we investigated their potential and mode-of-action as preventive measures to control root-knot nematode Meloidogyne graminicola infection in rice. The results show a significant reduction in root-galling and nematode development in rice plants that were treated through foliar application with the COS-OGA formulations FytoSol^® and FytoSave^® 24 h before nematode inoculation. Hormone measurements, gene expression analyses, corroborated by treatments on salicylic acid (SA) and jasmonic acid (JA)-mutants indicated that the systemic COS-OGA induced defense mechanism against nematodes is not based on SA or JA activation. However, phenylalanine ammonia lyase (PAL) gene expression in roots as well as enzymatic PAL activity in the shoots were significantly induced 24 h after foliar COS-OGA spraying in comparison with untreated plants. COS-OGA-induced systemic defense was abolished in the rice OsPAL4-mutant, demonstrating that COS-OGA-induced defense is dependent on OsPAL4 activation in rice plants.

Nodulation Induces Systemic Resistance of Medicago truncatula and Pisum sativum Against Erysiphe pisi and Primes for Powdery Mildew-Triggered Salicylic Acid Accumulation

Plants encounter beneficial and detrimental microorganisms both above- and belowground and the health status of the plant depends on the composition of this pan-microbiome. Beneficial microorganisms contribute to plant nutrition or systemically or locally protect plants against pathogens, thus facilitating adaptation to a variety of environments. Induced systemic resistance, caused by root-associated microbes, manifests as aboveground resistance against necrotrophic pathogens and is mediated by jasmonic acid/ethylene-dependent signaling. By contrast, systemic acquired resistance relies on salicylic acid (SA) signaling and confers resistance against secondary infection by (hemi)biotrophic pathogens. To investigate whether symbiotic rhizobia that are ubiquitously found in natural ecosystems are able to modulate resistance against biotrophs, we tested the impact of preestablished nodulation of Medicago truncatula and pea (Pisum sativum) plants against infection by the powdery mildew fungus Erysiphe pisi. We found that root symbiosis interfered with fungal penetration of M. truncatula and reduced asexual spore formation on pea leaves independently of symbiotic nitrogen fixation. Improved resistance of nodulated plants correlated with elevated levels of free SA and SA-dependent marker gene expression upon powdery mildew infection. Our results suggest that nodulation primes the plants systemically for E. pisi-triggered SA accumulation and defense gene expression, resulting in increased resistance.

Cold stress activates disease resistance in Arabidopsis thaliana through a salicylic acid dependent pathway

Exposure to short-term cold stress influences disease resistance by mechanisms that remain poorly characterized. The molecular basis of cold-activated immunity was therefore investigated in Arabidopsis thaliana inoculated with the bacterial pathogen Pst DC3000, using a transcriptomic analysis. Exposure to cold stress for 10 hr was sufficient to activate immunity, as well as H₂ O₂ accumulation and callose deposition. Transcriptome changes induced by the 10-hr cold treatment were similar to those caused by pathogen infection, including increased expression of the salicylic acid (SA) pathway marker genes, PR2 and PR5, and genes playing positive roles in defence against (hemi)-biotrophs. In contrast, transcripts encoding jasmonic acid (JA) pathway markers such as PR4 and MYC2 and transcripts with positive roles in defence against necrotrophs were less abundant following the 10-hr cold treatment. Cold-activated immunity was dependent on SA, being partially dependent on NPR1 and ICS1/SID2. In addition, transcripts encoding SA biosynthesis enzymes such as ICS2, PAL1, PAL2, and PAL4 (but not ICS1/SID2) and MES9 were more abundant, whereas GH3.5/WES1 and SOT12 transcripts that encode components involved in SA modification were less abundant following cold stress treatment. These findings show that cold stress cross-activates innate immune responses via a SA-dependent pathway.

Surviving in a Hostile World: Plant Strategies to Resist Pests and Diseases

As primary producers, plants are under constant pressure to defend themselves against potentially deadly pathogens and herbivores. In this review, we describe short- and long-term strategies that enable plants to cope with these stresses. Apart from internal immunological strategies that involve physiological and (epi)genetic modifications at the cellular level, plants also employ external strategies that rely on recruitment of beneficial organisms. We discuss these strategies along a gradient of increasing timescales, ranging from rapid immune responses that are initiated within seconds to (epi)genetic adaptations that occur over multiple plant generations. We cover the latest insights into the mechanistic and evolutionary underpinnings of these strategies and present explanatory models. Finally, we discuss how knowledge from short-lived model species can be translated to economically and ecologically important perennials to exploit adaptive plant strategies and mitigate future impacts of pests and diseases in an increasingly interconnected and changing world.

Comparative analysis of biocontrol agent Trichoderma asperellum ACCC30536 transcriptome during its interaction with Populus davidiana × P. alba var. pyramidalis

After exposure to with Populus davidiana × P. alba var. pyramidalis, the expression of genes in Trichoderma asperellum were compared in four transcriptomes. The top 20 high expression genes included six heat shock proteins and three hydrophobins, indicating that Trichoderma can rapidly adapt to environment stresses and elicit a plant defense response. The genes, involved in the interaction between Trichoderma and plant, showed an increasing expression level, for example sugar transporters, EPL1s, endoxylanases, pectin lyases, and nitrilases. Interestingly, sugar transporters also showed high expression when T. asperellum was cultured on medium lacking a carbon substrate, which would contribute to T. asperellum's survival and domination in ecological niche competition. And the genes related to mycoparasitism were expressed abundantly following T. asperellum's interaction with PdPap, indicating the PdPap induction could enhance the mycoparasitic ability of T. asperellum. Twelve chitinases and five glucanases showed higher expression in transcriptome Cs, indicating that T. asperellum secretes both types of enzyme before interacting with pathogens, allowing T. asperellum to implement mycoparasitism and obtain more energy. Many novel transcripts were obtained in each transcriptome, which may play important roles in the biocontrol process of T. asperellum. Interestingly, T. asperellum undergo constitutive alternative splicing in the biocontrol process: Seven biocontrol genes were alternative spliced via intron retention. qRT-PCR analysis proved that intron retention is negatively associated with the expression of chitinase, oligopeptide transporters, and beta-lactamase. However, the percentage of MAPK intron retention was quite low, suggesting that intron retention has little effect on the function of MAPK.

The differential response of cold-experienced Arabidopsis thaliana to larval herbivory benefits an insect generalist, but not a specialist

BACKGROUND

In native environments plants frequently experience simultaneous or sequential unfavourable abiotic and biotic stresses. The plant's response to combined stresses is usually not the sum of the individual responses. Here we investigated the impact of cold on plant defense against subsequent herbivory by a generalist and specialist insect.

RESULTS

We determined transcriptional responses of Arabidopsis thaliana to low temperature stress (4 °C) and subsequent larval feeding damage by the lepidopteran herbivores Mamestra brassicae (generalist), Pieris brassicae (specialist) or artificial wounding. Furthermore, we compared the performance of larvae feeding upon cold-experienced or untreated plants. Prior experience of cold strongly affected the plant's transcriptional anti-herbivore and wounding response. Feeding by P. brassicae, M. brassicae and artificial wounding induced transcriptional changes of 1975, 1695, and 2239 genes, respectively. Of these, 125, 360, and 681 genes were differentially regulated when cold preceded the tissue damage. Overall, prior experience of cold mostly reduced the transcriptional response of genes to damage. The percentage of damage-responsive genes, which showed attenuated transcriptional regulation when cold preceded the tissue damage, was highest in M. brassicae damaged plants (98%), intermediate in artificially damaged plants (89%), and lowest in P. brassicae damaged plants (69%). Consistently, the generalist M. brassicae performed better on cold-treated than on untreated plants, whereas the performance of the specialist P. brassicae did not differ.

CONCLUSIONS

The transcriptional defense response of Arabidopsis leaves to feeding by herbivorous insects and artificial wounding is attenuated by a prior exposure of the plant to cold. This attenuation correlates with improved performance of the generalist herbivore M. brassicae, but not the specialist P. brassicae, a herbivore of the same feeding guild.

The Arabidopsis non-host defence-associated coumarin scopoletin protects soybean from Asian soybean rust

The fungus Phakopsora pachyrhizi (Pp) causes Asian soybean rust (SBR) disease which provokes tremendous losses in global soybean production. Pp is mainly controlled with synthetic fungicides to which the fungus swiftly develops fungicide resistance. To substitute or complement synthetic fungicides in Asian soybean rust control, we aimed to identify antifungal metabolites in Arabidopsis which is not a host for Pp. Comparative transcriptional and metabolic profiling of the Pp-inoculated Arabidopsis non-host and the soybean host revealed induction of phenylpropanoid metabolism-associated genes in both species but activation of scopoletin biosynthesis only in the resistant non-host. Scopoletin is a coumarin and an antioxidant. In vitro experiments disclosed fungistatic activity of scopoletin against Pp, associated with reduced accumulation of reactive oxygen species (ROS) in fungal pre-infection structures. Non-antioxidant and antioxidant molecules including coumarins with a similar structure to scopoletin were inactive or much less effective at inhibiting fungal accumulation of ROS and germination of Pp spores. When sprayed onto Arabidopsis leaves, scopoletin also suppressed the formation of Pp pre-infection structures and penetration of the plant. However, scopoletin neither directly activated defence nor did it prime Arabidopsis for enhanced defence, therefore emphasizing fungistatic activity as the exclusive mode of action of scopoletin against Pp. Because scopletin also protected soybean from Pp infection, the coumarin may serve as a natural fungicide or as a lead for the development of near-to-nature fungicides against Asian soybean rust.

Mode of Action of Microbial Biological Control Agents Against Plant Diseases: Relevance Beyond Efficacy

Microbial biological control agents (MBCAs) are applied to crops for biological control of plant pathogens where they act via a range of modes of action. Some MBCAs interact with plants by inducing resistance or priming plants without any direct interaction with the targeted pathogen. Other MBCAs act via nutrient competition or other mechanisms modulating the growth conditions for the pathogen. Antagonists acting through hyperparasitism and antibiosis are directly interfering with the pathogen. Such interactions are highly regulated cascades of metabolic events, often combining different modes of action. Compounds involved such as signaling compounds, enzymes and other interfering metabolites are produced in situ at low concentrations during interaction. The potential of microorganisms to produce such a compound in vitro does not necessarily correlate with their in situ antagonism. Understanding the mode of action of MBCAs is essential to achieve optimum disease control. Also understanding the mode of action is important to be able to characterize possible risks for humans or the environment and risks for resistance development against the MBCA. Preferences for certain modes of action for an envisaged application of a MBCA also have impact on the screening methods used to select new microbials. Screening of MBCAs in bioassays on plants or plant tissues has the advantage that MBCAs with multiple modes of action and their combinations potentially can be detected whereas simplified assays on nutrient media strongly bias the selection toward in vitro production of antimicrobial metabolites which may not be responsible for in situ antagonism. Risks assessments for MBCAs are relevant if they contain antimicrobial metabolites at effective concentration in the product. However, in most cases antimicrobial metabolites are produced by antagonists directly on the spot where the targeted organism is harmful. Such ubiquitous metabolites involved in natural, complex, highly regulated interactions between microbial cells and/or plants are not relevant for risk assessments. Currently, risks of microbial metabolites involved in antagonistic modes of action are often assessed similar to assessments of single molecule fungicides. The nature of the mode of action of antagonists requires a rethinking of data requirements for the registration of MBCAs.

Trichoderma atroviride P1 Colonization of Tomato Plants Enhances Both Direct and Indirect Defense Barriers Against Insects

Numerous microbial root symbionts are known to induce different levels of enhanced plant protection against a variety of pathogens. However, more recent studies have demonstrated that beneficial microbes are able to induce plant systemic resistance that confers some degree of protection against insects. Here, we report how treatments with the fungal biocontrol agent Trichoderma atroviride strain P1 in tomato plants induce responses that affect pest insects with different feeding habits: the noctuid moth Spodoptera littoralis (Boisduval) and the aphid Macrosiphum euphorbiae (Thomas). We observed that the tomato plant-Trichoderma P1 interaction had a negative impact on the development of moth larvae and on aphid longevity. These effects were attributed to a plant response induced by Trichoderma that was associated with transcriptional changes of a wide array of defense-related genes. While the impact on aphids could be related to the up-regulation of genes involved in the oxidative burst reaction, which occur early in the defense reaction, the negative performance of moth larvae was associated with the enhanced expression of genes encoding for protective enzymes (i.e., Proteinase inhibitor I (PI), Threonine deaminase, Leucine aminopeptidase A1, Arginase 2, and Polyphenol oxidase) that are activated downstream in the defense cascade. In addition, Trichoderma P1 produced alterations in plant metabolic pathways leading to the production and release of volatile organic compounds (VOCs) that are involved in the attraction of the aphid parasitoid Aphidius ervi, thus reinforcing the indirect plant defense barriers. Our findings, along with the evidence available in the literature, indicate that the outcome of the tripartite interaction among plant, Trichoderma, and pests is highly specific and only a comprehensive approach, integrating both insect phenotypic changes and plant transcriptomic alterations, can allow a reliable prediction of its potential for plant protection.

Polyphenols from Rheum Roots Inhibit Growth of Fungal and Oomycete Phytopathogens and Induce Plant Disease Resistance

A growing world population requires an increase in the quality and quantity of food production. However, field losses due to biotic stresses are currently estimated to be between 10 and 20% worldwide. The risk of resistance and strict pesticide legislation necessitate innovative agronomical practices to adequately protect crops in the future, such as the identification of new substances with novel modes of action. In the present study, liquid chromatography mass spectrometry was used to characterize Rheum rhabarbarum root extracts that were primarily composed of the stilbenes rhaponticin, desoxyrhaponticin, and resveratrol. Minor components were the flavonoids catechin, epicatechin gallate, and procyanidin B1. Specific polyphenolic mixtures inhibited mycelial growth of several phytopathogenic fungi and oomycetes. Foliar spray applications with fractions containing stilbenes and flavonoids inhibited spore germination of powdery mildew in Hordeum vulgare with indications of synergistic interactions. Formulated extracts led to a significant reduction in the incidence of brown rust in Triticum aestivum under field conditions. Arabidopsis thaliana mutant and quantitative reverse-transcription polymerase chain reaction studies suggested that the stilbenes induce salicylic acid-mediated resistance. Thus, the identified substances of Rheum roots represent an excellent source of antifungal agents that can be used in horticulture and agriculture.

Review: Phytostimulation and root architectural responses to quorum-sensing signals and related molecules from rhizobacteria

Bacteria rely on chemical communication to sense the environment and to retrieve information on their population densities. Accordingly, a vast repertoire of molecules is released, which synchronizes expression of genes, coordinates behavior through a process termed quorum-sensing (QS), and determines the relationships with eukaryotic species. Already identified QS molecules from Gram negative bacteria can be grouped into two main classes, N-acyl-L-homoserine lactones (AHLs) and cyclodipeptides (CDPs), with roles in biofilm formation, bacterial virulence or symbiotic interactions. Noteworthy, plants detect each of these molecules, change their own gene expression programs, re-configurate root architecture, and activate defense responses, improving in this manner their adaptation to natural and agricultural ecosystems. AHLs may act as alarm signals, pathogen and/or microbe-associated molecular patterns, whereas CDPs function as hormonal mimics for plants via their putative interactions with the auxin receptor Transport Inhibitor Response1 (TIR1). A major challenge is to identify the molecular pathways of QS-mediated crosstalk and the plant receptors and interacting proteins for AHLs, CDPs and related signals.

Transcriptome and Metabolome Reprogramming in Tomato Plants by Trichoderma harzianum strain T22 Primes and Enhances Defense Responses Against Aphids

Beneficial fungi in the genus Trichoderma are among the most widespread biocontrol agents of plant pathogens. Their role in triggering plant defenses against pathogens has been intensely investigated, while, in contrast, very limited information is available on induced barriers active against insects. The growing experimental evidence on this latter topic looks promising, and paves the way toward the development of Trichoderma strains and/or consortia active against multiple targets. However, the predictability and reproducibility of the effects that these beneficial fungi is still somewhat limited by the lack of an in-depth understanding of the molecular mechanisms underlying the specificity of their interaction with different crop varieties, and on how the environmental factors modulate this interaction. To fill this research gap, here we studied the transcriptome changes in tomato plants (cultivar "Dwarf San Marzano") induced by Trichoderma harzianum (strain T22) colonization and subsequent infestation by the aphid Macrosiphum euphorbiae. A wide transcriptome reprogramming, related to metabolic processes, regulation of gene expression and defense responses, was induced both by separate experimental treatments, which showed a synergistic interaction when concurrently applied. The most evident expression changes of defense genes were associated with the multitrophic interaction Trichoderma-tomato-aphid. Early and late genes involved in direct defense against insects were induced (i.e., peroxidase, GST, kinases and polyphenol oxidase, miraculin, chitinase), along with indirect defense genes, such as sesquiterpene synthase and geranylgeranyl phosphate synthase. Targeted and untargeted semi-polar metabolome analysis revealed a wide metabolome alteration showing an increased accumulation of isoprenoids in Trichoderma treated plants. The wide array of transcriptomic and metabolomics changes nicely fit with the higher mortality of aphids when feeding on Trichoderma treated plants, herein reported, and with the previously observed attractiveness of these latter toward the aphid parasitoid Aphidius ervi. Moreover, Trichoderma treated plants showed the over-expression of transcripts coding for several families of defense-related transcription factors (bZIP, MYB, NAC, AP2-ERF, WRKY), suggesting that the fungus contributes to the priming of plant responses against pest insects. Collectively, our data indicate that Trichoderma treatment of tomato plants induces transcriptomic and metabolomic changes, which underpin both direct and indirect defense responses.

Identification of Stress Associated microRNAs in Solanum lycopersicum by High-Throughput Sequencing

Tomato (Solanum lycopersicum) is one of the most important crops around the world and also a model plant to study response to stress. High-throughput sequencing was used to analyse the microRNA (miRNA) profile of tomato plants undergoing five biotic and abiotic stress conditions (drought, heat, P. syringae infection, B. cinerea infection, and herbivore insect attack with Leptinotarsa decemlineata larvae) and one chemical treatment with a plant defence inducer, hexanoic acid. We identified 104 conserved miRNAs belonging to 37 families and we predicted 61 novel tomato miRNAs. Among those 165 miRNAs, 41 were stress-responsive. Reverse transcription quantitative PCR (RT-qPCR) was used to validate high-throughput expression analysis data, confirming the expression profiles of 10 out of 11 randomly selected miRNAs. Most of the differentially expressed miRNAs were stress-specific, except for sly-miR167c-3p upregulated in B. cinerea and P. syringae infection, sly-newmiR26-3p upregulated in drought and Hx treatment samples, and sly-newmiR33-3p, sly-newmiR6-3p and sly-newmiR8-3p differentially expressed both in biotic and abiotic stresses. From mature miRNAs sequences of the 41 stress-responsive miRNAs 279 targets were predicted. An inverse correlation between the expression profiles of 4 selected miRNAs (sly-miR171a, sly-miR172c, sly-newmiR22-3p and sly-miR167c-3p) and their target genes (Kinesin, PPR, GRAS40, ABC transporter, GDP and RLP1) was confirmed by RT-qPCR. Altogether, our analysis of miRNAs in different biotic and abiotic stress conditions highlight the interest to understand the functional role of miRNAs in tomato stress response as well as their putative targets which could help to elucidate plants molecular and physiological adaptation to stress.

Proteomic Analysis of MeJa-Induced Defense Responses in Rice against Wounding

The role of jasmonates in defense priming has been widely recognized. Priming is a physiological process by which a plant exposed to low doses of biotic or abiotic elicitors activates faster and/or stronger defense responses when subsequently challenged by a stress. In this work, we investigated the impact of MeJA-induced defense responses to mechanical wounding in rice (Oryza sativa). The proteome reprogramming of plants treated with MeJA, wounding or MeJA+wounding has been in-depth analyzed by using a combination of high throughput profiling techniques and bioinformatics tools. Gene Ontology analysis identified protein classes as defense/immunity proteins, hydrolases and oxidoreductases differentially enriched by the three treatments, although with different amplitude. Remarkably, proteins involved in photosynthesis or oxidative stress were significantly affected upon wounding in MeJA-primed plants. Although these identified proteins had been previously shown to play a role in defense responses, our study revealed that they are specifically associated with MeJA-priming. Additionally, we also showed that at the phenotypic level MeJA protects plants from oxidative stress and photosynthetic damage induced by wounding. Taken together, our results add novel insight into the molecular actors and physiological mechanisms orchestrated by MeJA in enhancing rice plants defenses after wounding.

Targeting the pattern-triggered immunity pathway to enhance resistance to Fusarium graminearum

Fusarium head blight (FHB) is a disease of the floral tissues of wheat and barley for which highly resistant varieties are not available. Thus, there is a need to identify genes/mechanisms that can be targeted for the control of this devastating disease. Fusarium graminearum is the primary causal agent of FHB in North America. In addition, it also causes Fusarium seedling blight. Fusarium graminearum can also cause disease in the model plant Arabidopsis thaliana. The Arabidopsis-F. graminearum pathosystem has facilitated the identification of targets for the control of disease caused by this fungus. Here, we show that resistance against F. graminearum can be enhanced by flg22, a bacterial microbe-associated molecular pattern (MAMP). flg22-induced resistance in Arabidopsis requires its cognate pattern recognition receptor (PRR) FLS2, and is accompanied by the up-regulation of WRKY29. The expression of WRKY29, which is associated with pattern-triggered immunity (PTI), is also induced in response to F. graminearum infection. Furthermore, WRKY29 is required for basal resistance as well as flg22-induced resistance to F. graminearum. Moreover, constitutive expression of WRKY29 in Arabidopsis enhances disease resistance. The PTI pathway is also activated in response to F. graminearum infection of wheat. Furthermore, flg22 application and ectopic expression of WRKY29 enhance FHB resistance in wheat. Thus, we conclude that the PTI pathway provides a target for the control of FHB in wheat. We further show that the ectopic expression of WRKY29 in wheat results in shorter stature and early heading time, traits that are important to wheat breeding.

Intra and Extracellular Journey of the Phytohormone Salicylic Acid

Salicylic acid (SA) is a plant hormone that has been described to play an essential role in the activation and regulation of multiple responses to biotic and to abiotic stresses. In particular, during plant-microbe interactions, as part of the defense mechanisms, SA is initially accumulated at the local infected tissue and then spread all over the plant to induce systemic acquired resistance at non-infected distal parts of the plant. SA can be produced by either the phenylalanine or isochorismate biosynthetic pathways. The first, takes place in the cytosol, while the second occurs in the chloroplasts. Once synthesized, free SA levels are regulated by a number of chemical modifications that produce inactive forms, including glycosylation, methylation and hydroxylation to dihydroxybenzoic acids. Glycosylated SA is stored in the vacuole, until required to activate SA-triggered responses. All this information suggests that SA levels are under a strict control, including its intra and extracellular movement that should be coordinated by the action of transporters. However, our knowledge on this matter is still very limited. In this review, we describe the most significant efforts made to date to identify the molecular mechanisms involved in SA transport throughout the plant. Additionally, we propose new alternatives that might help to understand the journey of this important phytohormone in the future.

Intra and Extracellular Journey of the Phytohormone Salicylic Acid

Salicylic acid (SA) is a plant hormone that has been described to play an essential role in the activation and regulation of multiple responses to biotic and to abiotic stresses. In particular, during plant-microbe interactions, as part of the defense mechanisms, SA is initially accumulated at the local infected tissue and then spread all over the plant to induce systemic acquired resistance at non-infected distal parts of the plant. SA can be produced by either the phenylalanine or isochorismate biosynthetic pathways. The first, takes place in the cytosol, while the second occurs in the chloroplasts. Once synthesized, free SA levels are regulated by a number of chemical modifications that produce inactive forms, including glycosylation, methylation and hydroxylation to dihydroxybenzoic acids. Glycosylated SA is stored in the vacuole, until required to activate SA-triggered responses. All this information suggests that SA levels are under a strict control, including its intra and extracellular movement that should be coordinated by the action of transporters. However, our knowledge on this matter is still very limited. In this review, we describe the most significant efforts made to date to identify the molecular mechanisms involved in SA transport throughout the plant. Additionally, we propose new alternatives that might help to understand the journey of this important phytohormone in the future.

Symbiotic Root-Endophytic Soil Microbes Improve Crop Productivity and Provide Environmental Benefits

Plants should not be regarded as entities unto themselves, but as the visible part of plant-microbe complexes which are best understood as "holobiomes." Some microorganisms when given the opportunity to inhabit plant roots become root symbionts. Such root colonization by symbiotic microbes can raise crop yields by promoting the growth of both shoots and roots, by enhancing uptake, fixation, and/or more efficient use of nutrients, by improving plants' resistance to pests, diseases, and abiotic stresses that include drought, salt, and other environmental conditions, and by enhancing plants' capacity for photosynthesis. We refer plant-microbe associations with these capabilities that have been purposefully established as enhanced plant holobiomes (EPHs). Here, we consider four groups of phylogenetically distinct and distant symbiotic endophytes: (1) Rhizobiaceae bacteria; (2) plant-obligate arbuscular mycorrhizal fungi (AMF); (3) selected endophytic strains of fungi in the genus Trichoderma; and (4) fungi in the Sebicales order, specifically Piriformospora indica. Although these exhibit quite different "lifestyles" when inhabiting plants, all induce beneficial systemic changes in plants' gene expression that are surprisingly similar. For example, all induce gene expression that produces proteins which detoxify reactive oxygen species (ROS). ROS are increased by environmental stresses on plants or by overexcitation of photosynthetic pigments. Gene overexpression results in a cellular environment where ROS levels are controlled and made more compatible with plants' metabolic processes. EPHs also frequently exhibit increased rates of photosynthesis that contribute to greater plant growth and other capabilities. Soil organic matter (SOM) is augmented when plant root growth is increased and roots remain in the soil. The combination of enhanced photosynthesis, increasing sequestration of CO2 from the air, and elevation of SOM removes C from the atmosphere and stores it in the soil. Reductions in global greenhouse gas levels can be accelerated by incentives for carbon farming and carbon cap-and-trade programs that reward such climate-friendly agriculture. The development and spread of EPHs as part of such initiatives has potential both to enhance farm productivity and incomes and to decelerate global warming.

Anthonomus grandis aggregation pheromone induces cotton indirect defence and attracts the parasitic wasp Bracon vulgaris

Insect-derived volatiles seem to provide reliable chemical cues that plants could employ to defend themselves. Here we investigated the effect of pheromone emission from a closely associated (Anthonomus grandis; boll weevil) and an unassociated (Tibraca limbativentris) herbivore on cotton volatile emission. Exposure to A. grandis aggregation pheromone induced cotton defence response by enhancing the emission of volatiles attractive to the natural enemy of A. grandis, the parasitic wasp Bracon vulgaris, but only when the pheromonal blend was complete (all four components). Individual components of A. grandis aggregation pheromone were not able to induce cotton plants to increase the release of volatiles. On the other hand, T. limbativentris sex pheromone did not induce any change in the cotton constitutive volatile profile. Our results support the hypothesis that plants are able to detect pheromones of tightly co-evolved herbivores. Moreover, A. grandis pheromone exposure induced similar volatile compounds to herbivore-induced cotton, such as linalool, (E)-ocimene, (E)-4,8-dimethylnona-1,3,7-triene (DMNT), and (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT). We also showed that the larval ectoparasitoid B. vulgaris relies on boll weevil's aggregation pheromone and pheromone-induced plant volatiles as kairomones to locate suitable hosts.

Rational protein design of Bacillus sp. MN chitosanase for altered substrate binding and production of specific chitosan oligomers

BACKGROUND

Partially acetylated chito-oligosaccharides (paCOS) have a variety of potential applications in different fields, but to harness their benefits, pure paCOS or well-defined paCOS mixtures are essential. For example, if one could produce fully acetylated (A4) and fully deacetylated (D4) tetramers in abundance, all possible variants of tetrameric paCOS could be generated reliably from them. A promising approach for generating defined paCOS is by enzymatic depolymerization of chitosan polymers using chitosanases, since these enzymes' subsite specificities directly influence the composition of the paCOS produced; however, enzymatic production of e.g. D4 is challenging because the substrate is generally hydrolyzed further by most chitosanases. To overcome this, chitosanases could potentially be engineered so that upon hydrolyzing chitosan, they are unable to hydrolyze certain substrates, leaving well-defined oligomers intact in the hydrolysate.

RESULTS

For this purpose, we performed rational protein engineering on the extensively studied GH 8 chitosanase CSN from Bacillus sp. MN. By specifically targeting residues with a predicted function in substrate binding, we created new muteins incapable of efficiently hydrolyzing the fully deacetylated tetramer D4, and we were able to demonstrate efficient large-scale production of D4 with an altered version of CSN. Furthermore, we were able to uncover differences in the substrate positioning and subsite specificities of the muteins, which result in altered paCOS mixtures produced from partially acetylated chitosan polymers, with possibly altered bioactivities.

CONCLUSION

The value of protein engineering as a tool for the more efficient production of pure oligomers and potentially bioactive paCOS mixtures was demonstrated by the results and the suitability of specific muteins for the large-scale production of strictly defined, pure paCOS in a batch process was shown using the example of D4.