Molecular aspects of defence priming

DORMANCY/AUXIN ASSOCIATED FAMILY PROTEIN 2 of Arabidopsis thaliana is a negative regulator of local and systemic acquired resistance

To fine tune defense response output, plants recruit both positive and negative regulators. Here we report Arabidopsis DORMANCY/AUXIN ASSOCIATED FAMILY PROTEIN 2(DAP2) gene as a negative regulator of basal defense against virulent bacterial pathogens. Expression of DAP2 enhances upon pathogen inoculation. Our experiments show that DAP2 suppressed resistance against virulent strains of bacterial pathogens, pathogen-induced callose deposition, and ROS accumulation; however, it did not influence effector-triggered immunity. In addition, DAP2 negatively regulated systemic acquired resistance (SAR). DAP2 expression was enhanced in the pathogen-free systemic tissues of SAR-induced plants. Previously, Arabidopsis Flowering locus D (FLD) gene has been shown to be essential for SAR but not for local resistance. We show here that FLD function is necessary for SAR-induced expression of DAP2, suggesting DAP2 as a target of FLD for activation of SAR in Arabidopsis.

Nitric Oxide Plays an Important Role in β-Aminobutyric Acid-Induced Resistance to Botrytis cinerea in Tomato Plants

β-Aminobutyric acid (BABA) has consistently been reported to enhance plant immunity. However, the specific mechanisms and downstream components that mediate this resistance are not yet agreed upon. Nitric oxide (NO) is an important signal molecule involved in a diverse range of physiological processes, and whether NO is involved in BABA-induced resistance is interesting. In this study, treatment with BABA significantly increased NO accumulation and reduced the sensitivity to Botrytis cinerea in tomato plants. BABA treatment reduced physical signs of infection and increased both the transcription of key defense marker genes and the activity of defensive enzymes. Interestingly, compared to treatment with BABA alone, treatment with BABA plus cPTIO (NO specific scavenger) not only significantly reduced NO accumulation, but also increased disease incidence and lesion area. These results suggest that NO accumulation plays an important role in BABA-induced resistance against B. cinerea in tomato plants.

Tissue-specific synergistic bio-priming of pepper by two Streptomyces species against Phytophthora capsici

Among several studied strains, Streptomyces rochei IT20 and S. vinaceusdrappus SS14 showed a high level of inhibitory effect against Phytophthora capsici, the causal agent of pepper blight. The effect of two mentioned superior antagonists, as single or combination treatments, on suppression of stem and fruit blight diseases and reproductive growth promotion was investigated in pepper. To explore the induced plant defense reactions, ROS generation and transcriptional changes of selected genes in leaf and fruit tissues of the plant were evaluated. The plants exposed to the combination of two species responded differently in terms of H₂O₂ accumulation and expression ratio of GST gene compared to single treatments upon pathogen inoculation. Besides, the increment of shoot length, flowering, and fruit weight were observed in healthy plants compared to control. Likely, these changes depended on the coordinated relationships between PR1, ACCO genes and transcription factors WRKY40 enhanced after pathogen challenge. Our findings indicate that appropriate tissue of the host plant is required for inducing Streptomyces-based priming and relied on the up-regulation of SUS and differential regulation of ethylene-dependent genes.

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.

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.

N-3-oxo-octanoyl-homoserine lactone-mediated priming of resistance to Pseudomonas syringae requires the salicylic acid signaling pathway in Arabidopsis thaliana

BACKGROUD

Many Gram-negative bacteria use N-acyl-homoserine lactones (AHLs) to communicate each other and to coordinate their collective behaviors. Recently, accumulating evidence shows that host plants are able to sense and respond to bacterial AHLs. Once primed, plants are in an altered state that enables plant cells to more quickly and/or strongly respond to subsequent pathogen infection or abiotic stress.

RESULTS

In this study, we report that pretreatment with N-3-oxo-octanoyl-homoserine lactone (3OC8-HSL) confers resistance against the pathogenic bacterium Pseudomonas syringae pv. tomato DC3000 (PstDC3000) in Arabidopsis. Pretreatment with 3OC8-HSL and subsequent pathogen invasion triggered an augmented burst of hydrogen peroxide, salicylic acid accumulation, and fortified expression of the pathogenesis-related genes PR1 and PR5. Upon PstDC3000 challenge, plants treated with 3OC8-HSL showed increased activities of defense-related enzymes including peroxidase, catalase, phenylalanine ammonialyase, and superoxide dismutase. In addition, the 3OC8-HSL-primed resistance to PstDC3000 in wild-type plants was impaired in plants expressing the bacterial NahG gene and in the npr1 mutant. Moreover, the expression levels of isochorismate synthases (ICS1), a critical salicylic acid biosynthesis enzyme, and two regulators of its expression, SARD1 and CBP60g, were potentiated by 3OC8-HSL pretreatment followed by pathogen inoculation.

CONCLUSIONS

Our data indicate that 3OC8-HSL primes the Arabidopsis defense response upon hemibiotrophic bacterial infection and that 3OC8-HSL-primed resistance is dependent on the SA signaling pathway. These findings may help establish a novel strategy for the control of plant disease.

Unravelling the Roles of Nitrogen Nutrition in Plant Disease Defences

Nitrogen (N) is one of the most important elements that has a central impact on plant growth and yield. N is also widely involved in plant stress responses, but its roles in host-pathogen interactions are complex as each affects the other. In this review, we summarize the relationship between N nutrition and plant disease and stress its importance for both host and pathogen. From the perspective of the pathogen, we describe how N can affect the pathogen’s infection strategy, whether necrotrophic or biotrophic. N can influence the deployment of virulence factors such as type III secretion systems in bacterial pathogen or contribute nutrients such as gamma-aminobutyric acid to the invader. Considering the host, the association between N nutrition and plant defence is considered in terms of physical, biochemical and genetic mechanisms. Generally, N has negative effects on physical defences and the production of anti-microbial phytoalexins but positive effects on defence-related enzymes and proteins to affect local defence as well as systemic resistance. N nutrition can also influence defence via amino acid metabolism and hormone production to affect downstream defence-related gene expression via transcriptional regulation and nitric oxide (NO) production, which represents a direct link with N. Although the critical role of N nutrition in plant defences is stressed in this review, further work is urgently needed to provide a comprehensive understanding of how opposing virulence and defence mechanisms are influenced by interacting networks.

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.

Metabolomics Reveal Induction of ROS Production and Glycosylation Events in Wheat Upon Exposure to the Green Leaf Volatile Z-3-Hexenyl Acetate

The activation and priming of plant defense upon perception of green leaf volatiles (GLVs) have often been reported. However, information as to which metabolic pathways in plants are affected by GLVs remains elusive. We report the production of reactive oxygen species in the tip of young wheat leaves followed by activation of antioxidant-related enzyme activity. In this study, we aimed to uncover metabolic signatures upon exposure to the GLV Z-3-hexenyl acetate (Z-3-HAC). By using an untargeted metabolomics approach, we observed changes in the phenylpropanoid pathways which yield metabolites that are involved in many anti-oxidative processes. Furthermore, exposure to GLV, followed by infection with Fusarium graminearum (Fg), induced significantly greater changes in the phenylpropanoid pathway compared to a sole Z-3-HAC treatment. Fragmentation of a selection of metabolites, which are significantly more upregulated in the Z-3-HAC + Fg treatment, showed D-glucose to be present as a substructure. This suggests that Z-3-HAC induces early glycosylation processes in plants. Additionally, we identified the presence of hexenyl diglycosides, which indicates that aerial Z-3-HAC is metabolized in the leaves by glycosyltransferases. Together these data indicate that GLV Z-3-HAC is taken up by leaves and incites oxidative stress. This subsequently results in the modulation of the phenylpropanoid pathway and an induction of glycosylation processes.

Hormopriming to Mitigate Abiotic Stress Effects: A Case Study of N 9-Substituted Cytokinin Derivatives With a Fluorinated Carbohydrate Moiety

Drought and salinity reduce seed germination, seedling emergence, and early seedling establishment, affect plant metabolism, and hence, reduce crop yield. Development of technologies that can increase plant tolerance of these challenging growth conditions is a major current interest among plant scientists and breeders. Seed priming has become established as one of the practical approaches that can alleviate the negative impact of many environmental stresses and improve the germination and overall performance of crops. Hormopriming using different plant growth regulators has been widely demonstrated as effective, but information about using cytokinins (CKs) as priming agents is limited to only a few studies using kinetin or 6-benzylaminopurine (BAP). Moreover, the mode of action of these compounds in improving seed and plant fitness through priming has not yet been studied. For many years, BAP has been one of the CKs most commonly applied exogenously to plants to delay senescence and reduce the impact of stress. However, rapid endogenous N ⁹-glucosylation of BAP can result in negative effects. This can be suppressed by hydroxylation of the benzyl ring or by appropriate N ⁹ purine substitution. Replacement of the 2' or 3' hydroxyl groups of a nucleoside with a fluorine atom has shown promising results in drug research and biochemistry as a means of enhancing biological activity and increasing chemical or metabolic stability. Here, we show that the application of this chemical modification in four new N ⁹-substituted CK derivatives with a fluorinated carbohydrate moiety improved the antisenescence properties of CKs. Besides, detailed phenotypical analysis of the growth and development of Arabidopsis plants primed with the new CK analogs over a broad concentration range and under various environmental conditions revealed that they improve growth regulation and antistress activity. Seed priming with, for example, 6-(3-hydroxybenzylamino)-2'-deoxy-2'-fluoro-9-(β)-D-arabinofuranosylpurine promoted plant growth under control conditions and alleviated the negative effects of the salt and osmotic stress. The mode of action of this hormopriming and its effect on plant metabolism were further analyzed through quantification of the endogenous levels of phytohormones such as CKs, auxins and abscisic acid, and the results are discussed.

BABA application improves soybean resistance to aphid through activation of phenylpropanoid metabolism and callose deposition

BACKGROUND

Beta-aminobutyric acid (BABA) confer plant resistance to a broad spectrum of biotic and abiotic stresses. The soybean aphid (SBA), is native to eastern Asia and is a predominant insect pest of soybean. Both isoflavone and lignin pathway are important branches of the general phenylpropanoid pathway, which would be likely associated with resistance against soybean aphid. However, little is known about the role of the phenylpropanoid pathway in defense response to SBA as induced by BABA application.

RESULTS

The application of BABA effectively enhanced soybean resistance against Aphis glycines, the soybean aphid. Consistent with significantly increased content of isoflavones, especially genistein, the related biosynthetic genes were upregulated by use of BABA. Lignin, another important defense component against arthropods, accumulated at a high level and four lignin biosynthesis related genes were also activated. Additionally, BABA application augmented the expression of callose synthase genes and increased callose deposition in SBA-infested seedlings. In non-caged and caged tests, SBA numbers were significantly reduced in BABA-treated seedlings.

CONCLUSION

These results demonstrate that application of BABA has an obvious positive effect on soybean resistance to aphids, and this defense response partly depends on the potentiation of isoflavone biosynthesis and callose deposition. © 2019 Society of Chemical Industry.

Recurrent Drought Conditions Enhance the Induction of Drought Stress Memory Genes in Glycine max L

Plants remember what they have experienced and are thereby able to confront repeated stresses more promptly and strongly. A subset of the drought responsive genes, called stress memory genes, displayed greatly elevated levels under recurrent drought conditions. To screen for a set of drought stress memory genes in soybean (Glycine max L.), we designed a 180K DNA chip comprising 60-bp probes synthesized in situ to examine 55,589 loci. Through microarray analysis using the DNA chip, we identified 2,162 and 2,385 genes with more than fourfold increases or decreases in transcript levels, respectively, under initial (first) drought stress conditions, when compared with the non-treated control. The transcript levels of the drought-responsive genes returned to basal levels during recovery (watered) states, and 392 and 613 genes displayed more than fourfold elevated or reduced levels, respectively, under subsequent (second) drought conditions, when compared to those observed under the first drought stress conditions. Gene Ontology and MapMan analyses classified the drought-induced memory genes exhibiting elevated levels of transcripts into several functional categories, including those involved in tolerance responses to abiotic stresses, which encode transcription factors, protein phosphatase 2Cs, and late embryogenesis abundant proteins. The drought-repressed memory genes exhibiting reduced levels of transcripts were classified into categories including photosynthesis and primary metabolism. Co-expression network analysis revealed that the soybean drought-induced and -repressed memory genes were equivalent to 172 and 311 Arabidopsis genes, respectively. The soybean drought stress memory genes include genes involved in the dehydration memory responses of Arabidopsis.

Mild water stress-induced priming enhance tolerance to Rosellinia necatrix in susceptible avocado rootstocks

BACKGROUND

White root rot (WRR) disease caused by Rosellinia necatrix is one of the most important threats affecting avocado orchards in temperate regions. The eradication of WRR is a difficult task and environmentally friendly control methods are needed to lessen its impact. Priming plants with a stressor (biotic or abiotic) can be a strategy to enhance plant defense/tolerance against future stress episodes but, despite the known underlying common mechanisms, few studies use abiotic-priming for improving tolerance to forthcoming biotic-stress and vice versa ('cross-factor priming'). To assess whether cross-factor priming can be a potential method for enhancing avocado tolerance to WRR disease, 'Dusa' avocado rootstocks, susceptible to R. necatrix, were subjected to two levels of water stress (mild-WS and severe-WS) and, after drought-recovery, inoculated with R. necatrix. Physiological response and expression of plant defense related genes after drought-priming as well as the disease progression were evaluated.

RESULTS

Water-stressed avocado plants showed lower water potential and stomatal limitations of photosynthesis compared to control plants. In addition, NPQ and qN values increased, indicating the activation of energy dissipating mechanisms closely related to the relief of oxidative stress. This response was proportional to the severity of the water stress and was accompanied by the deregulation of pathogen defense-related genes in the roots. After re-watering, leaf photosynthesis and plant water status recovered rapidly in both treatments, but roots of mild-WS primed plants showed a higher number of overexpressed genes related with plant defense than severe-WS primed plants. Disease progression after inoculating primed plants with R. necatrix was significantly delayed in mild-WS primed plants.

CONCLUSIONS

These findings demonstrate that mild-WS can induce a primed state in the WRR susceptible avocado rootstock 'Dusa' and reveal that 'cross-factor priming' with water stress (abiotic stressor) is effective for increasing avocado tolerance against R. necatrix (biotic stressor), underpinning that plant responses against biotic and abiotic stress rely on common mechanisms. Potential applications of these results may involve an enhancement of WRR tolerance of current avocado groves and optimization of water use via low frequency deficit irrigation strategies.

Enhancement of Pathogen Resistance in Common Bean Plants by Inoculation With Rhizobium etli

Symbiotic Rhizobium-legume associations are mediated by exchange of chemical signals that eventually result in the development of a nitrogen-fixing nodule. Such signal interactions are thought to be at the center of the plants' capacity either to activate a defense response or to suppress the defense response to allow colonization by symbiotic bacteria. In addition, the colonization of plant roots by rhizobacteria activates an induced condition of improved defensive capacity in plants known as induced systemic resistance, based on "defense priming," which protects unexposed plant tissues from biotic stress.Here, we demonstrate that inoculation of common bean plants with Rhizobium etli resulted in a robust resistance against Pseudomonas syringae pv. phaseolicola. Indeed, inoculation with R. etli was associated with a reduction in the lesion size caused by the pathogen and lower colony forming units compared to mock-inoculated plants. Activation of the induced resistance was associated with an accumulation of the reactive oxygen species superoxide anion (O₂ ^-) and a faster and stronger callose deposition. Transcription of defense related genes in plants treated with R. etli exhibit a pattern that is typical of the priming response. In addition, R. etli-primed plants developed a transgenerational defense memory and could produce offspring that were more resistant to halo blight disease. R. etli is a rhizobacteria that could reduce the proliferation of the virulent strain P. syringae pv. phaseolicola in common bean plants and should be considered as a potentially beneficial and eco-friendly tool in plant disease management.

Extracts from cultures of Pseudomonas fluorescens induce defensive patterns of gene expression and enzyme activity while depressing visible injury and reactive oxygen species in Arabidopsis thaliana challenged with pathogenic Pseudomonas syringae

We evaluated the ability of metabolic elicitors extracted from Pseudomonas fluorescens N21.4 to induce systemic resistance (ISR) in Arabidopsis thaliana against the pathogen Pseudomonas syringae DC3000. Metabolic elicitors were obtained from bacteria-free culture medium with n-hexane, ethyl acetate and n-butanol in three consecutive extractions. Each extract showed plant protection activity. The n-hexane fraction was the most effective and was used to study the signal transduction pathways involved by evaluating expression of marker genes of the salicylic acid (SA) signalling pathway (NPR1, PR1, ICS and PR2) and the jasmonic acid/ethylene (JA/ET) signalling pathway (PDF1, MYC2, LOX2 and PR3). In addition, the level of oxidative stress was tested by determining the activity of enzymes related to the ascorbate-glutathione cycle. N-hexane extracts stimulated both pathways based on overexpression of ICS, PR1, PR2, PDF1 and LOX2 genes. In addition, activity of the pathogenesis-related proteins glucanase (PR2) and chitinase (PR3), lipoxygenase and polyphenol oxidase was enhanced together with an increased capacity to remove reactive oxygen species (ROS). This was associated with less oxidative stress as indicated by a decrease in malondialdehyde (MDA), suggesting a causative link between defensive metabolism against P. syringae and ROS scavenging.

Basic-leucine zipper 17 and Hmg-CoA reductase degradation 3A are involved in salt acclimation memory in Arabidopsis

Salt acclimation, which is induced by previous salt exposure, increases the resistance of plants to future exposure to salt stress. However, little is known about the underlying mechanism, particularly how plants store the "memory" of salt exposure. In this study, we established a system to study salt acclimation in Arabidopsis thaliana. Following treatment with a low concentration of salt, seedlings were allowed to recover to allow transitory salt responses to subside while maintaining the sustainable effects of salt acclimation. We performed transcriptome profiling analysis of these seedlings to identify genes related to salt acclimation memory. Notably, the expression of Basic-leucine zipper 17 (bZIP17) and Hmg-CoA reductase degradation 3A (HRD3A), which are important in the unfolded protein response (UPR) and endoplasmic reticulum-associated degradation (ERAD), respectively, increased following treatment with a low concentration of salt and remained at stably high levels after the stimulus was removed, a treatment which improved plant tolerance to future high-salinity challenge. Our findings suggest that the upregulated expression of important genes involved in the UPR and ERAD represents a "memory" of the history of salt exposure and enables more potent responses to future exposure to salt stress, providing new insights into the mechanisms underlying salt acclimation in plants.

10,000-Times Diluted Doses of ACCase-Inhibiting Herbicides Can Permanently Change the Metabolomic Fingerprint of Susceptible Avena fatua L. Plants

Intentional use of low dosage of herbicides has been considered the cause of non-target resistance in weeds. However, herbicide drift could be a source of low dosage that could be detected by weeds and change their metabolism. Furthermore, the minimum dose that a plant can detect in the environment is unknown, and it is unclear whether low doses could modify the response of weeds when they are first exposed to herbicides (priming effects). In this study, we determined the metabolomic fingerprinting using GC-MS of susceptible Avena fatua L. plants exposed to a gradient of doses (1, 0.1, 0.001, 0.0001, and 0x) relative to the recommended dose of clodinafop-propargyl. Additionally, we evaluated the primed plants when they received a second herbicide application. The results showed that even a 10,000-fold dilution of the recommended dose could induce a significant change in the plants’ metabolism and that this change is permanent over the biological cycle. There was no evidence that priming increased its resistance level. However, hormesis increased biomass accumulation and survival in A. fatua plants. Better application methods which prevent herbicide drift should be developed in order to avoid contact with weeds that grow around the crop fields.

Defence priming in Arabidopsis - a Meta-Analysis

Defence priming by organismal and non-organismal stimulants can reduce effects of biotic stress in plants. Thus, it could help efforts to enhance the sustainability of agricultural production by reducing use of agrochemicals in protection of crops from pests and diseases. We have explored effects of applying this approach to both Arabidopsis plants and seeds of various crops in meta-analyses. The results show that its effects on Arabidopsis plants depend on both the priming agent and antagonist. Fungi and vitamins can have strong priming effects, and priming is usually more effective against bacterial pathogens than against herbivores. Moreover, application of bio-stimulants (particularly vitamins and plant defence elicitors) to seeds can have promising defence priming effects. However, the published evidence is scattered, does not include Arabidopsis, and additional studies are required before we can draw general conclusions and understand the molecular mechanisms involved in priming of seeds' defences. In conclusion, defence priming of plants has clear potential and application of bio-stimulants to seeds may protect plants from an early age, promises to be both labour- and resource-efficient, poses very little environmental risk, and is thus both economically and ecologically promising.

β-Aminobutyric Acid Priming Acquisition and Defense Response of Mango Fruit to Colletotrichum gloeosporioides Infection Based on Quantitative Proteomics

β-aminobutyric acid (BABA) is a new environmentally friendly agent to induce disease resistance by priming of defense in plants. However, molecular mechanisms underlying BABA-induced priming defense are not fully understood. Here, comprehensive analysis of priming mechanism of BABA-induced resistance was investigated based on mango-Colletotrichum gloeosporioides interaction system using iTRAQ-based proteome approach. Results showed that BABA treatments effectively inhibited the expansion of anthracnose caused by C. gleosporioides in mango fruit. Proteomic results revealed that stronger response to pathogen in BABA-primed mango fruit after C. gleosporioides inoculation might be attributed to differentially accumulated proteins involved in secondary metabolism, defense signaling and response, transcriptional regulation, protein post-translational modification, etc. Additionally, we testified the involvement of non-specific lipid-transfer protein (nsLTP) in the priming acquisition at early priming stage and memory in BABA-primed mango fruit. Meanwhile, spring effect was found in the primed mango fruit, indicated by inhibition of defense-related proteins at priming phase but stronger activation of defense response when exposure to pathogen compared with non-primed fruit. As an energy-saving strategy, BABA-induced priming might also alter sugar metabolism to provide more backbone for secondary metabolites biosynthesis. In sum, this study provided new clues to elucidate the mechanism of BABA-induced priming defense in harvested fruit.

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.

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

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

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.

Understanding the genetic mechanism of resistance in aphid-treated alfalfa (Medicago sativa L.) through proteomic analysis

To minimize dependency on chemical pesticides, plant breeders are trying to emphasize on important agricultural pests for the development of pest resistant cultivars. However, the molecular approach and associated genetic tools conferring resistance have not been widely studied. In the current study, proteomic analysis of two of the alfalfa cultivars viz. a resistant (R) (Zhongmu-1) and a susceptible (S) (WL343), with (+ A) and without (- A) aphids rearing were carried out. Results indicated that 325 differentially expressed proteins (DEPs) up-regulated while 319 down-regulated with a pattern of R + A/R - A plants, whereas 371 up- and 583 down-regulated DEPs were identified in the S + A/S - A plants. Total number of DEPs found in (S + A/S - A) was around 19.7% greater than that of (R + A/R - A), whereas, the down-regulated DEPs of susceptible variety was 11.6% higher than the resistant cultivar. Applying the KEGG analysis, 96 and 142 DEPs were portrayed to 15 and 10 substantively augmented pathways for Zhongmu-1 and WL343, respectively. We also found that two of the shared pathways (carbon metabolism and pyruvate metabolism) are linking to important traits conferring resistance in alfalfa. Most importantly, the specific role of linoleic acid metabolism was found to be associated with jasmonic acid, flavonoid biosynthesis, and terpenoid backbone biosynthesis that might have been associated with the insect-resistant material synthesis in the resistant alfalfa cultivar. Our study suggested that both alfalfa cultivars (R, S) could govern protein expression through carbon and pyruvate metabolism. But only the resistant alfalfa cultivar (Zhongmu-1) can tune protein expression via linoleic acid metabolism and terpenoid backbone biosynthesis to induce the defensive protein expressions (e.g., jasmonic acid and flavonoid biosynthesis along with terpenoid backbone biosynthesis), to enhance plant defense capacity.

Transcriptome analysis of the model grass Lolium temulentum exposed to green leaf volatiles

BACKGROUND

Forage and turf grasses are routinely cut and grazed upon throughout their lifecycle. When grasses are cut or damaged, they rapidly release a volatile chemical cocktail called green leaf volatiles (GLV). Previously we have shown that mechanical wounding or exposure to GLV released from cut grass, activated a Lt 46 kDa mitogen-activated protein kinase (MAPK) within 3 min and a 44 kDa MAPK within 15-20 min in the model grass species Lolium temulentum (Lt). Currently very little is known concerning the perception, signaling or molecular responses associated with wound stress in grasses. Since GLV are released during wounding, we wanted to investigate what genes and signaling pathways would be induced in undamaged plants exposed to GLV.

RESULTS

RNA-Seq generated transcriptome of Lolium plants exposed to GLV identified 4308 up- and 2794 down-regulated distinct differentially-expressed sequences (DES). Gene Ontology analysis revealed a strong emphasis on signaling, response to stimulus and stress related categories. Transcription factors and kinases comprise over 13% of the total DES found in the up-regulated dataset. The analysis showed a strong initial burst within the first hour of GLV exposure with over 60% of the up-regulated DES being induced. Specifically sequences annotated for enzymes involved in the biosynthesis of jasmonic acid and other plant hormones, mitogen-activated protein kinases and WRKY transcription factors were identified. Interestingly, eleven DES for ferric reductase oxidase, an enzyme involved in iron uptake and transport, were exclusively found in the down-regulated dataset. Twelve DES of interest were selected for qRT-PCR analysis; all displayed a rapid induction one hour after GLV exposure and were also strongly induced by mechanical wounding.

CONCLUSION

The information gained from the analysis of this transcriptome and previous studies suggests that GLV released from cut grasses transiently primes an undamaged plant's wound stress pathways for potential oncoming damage, and may have a dual role for inter- as well as intra-plant signaling.

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.

Chromatin-based mechanisms of temperature memory in plants

For successful growth and development, plants constantly have to gauge their environment. Plants are capable to monitor their current environmental conditions, and they are also able to integrate environmental conditions over time and store the information induced by the cues. In a developmental context, such an environmental memory is used to align developmental transitions with favourable environmental conditions. One temperature-related example of this is the transition to flowering after experiencing winter conditions, that is, vernalization. In the context of adaptation to stress, such an environmental memory is used to improve stress adaptation even when the stress cues are intermittent. A somatic stress memory has now been described for various stresses, including extreme temperatures, drought, and pathogen infection. At the molecular level, such a memory of the environment is often mediated by epigenetic and chromatin modifications. Histone modifications in particular play an important role. In this review, we will discuss and compare different types of temperature memory and the histone modifications, as well as the reader, writer, and eraser proteins involved.

Defence-related priming and responses to recurring drought: Two manifestations of plant transcriptional memory mediated by the ABA and JA signalling pathways

Collective evidence from agricultural practices and from scientific research has demonstrated that plants can alter their phenotypic responses to repeated biotic and abiotic stresses or their elicitors. A coordinated reaction at the organismal, cellular, and genome levels has suggested that plants can "remember" an earlier stress and modify their future responses, accordingly. Stress memory may increase a plant's survival chances by improving its tolerance/avoidance abilities and may provide a mechanism for acclimation and adaptation. Understanding the mechanisms that regulate plant stress memory is not only an intellectually challenging topic but has important implications for agricultural practices as well. Here, I focus exclusively on specific aspects of the transcription memory in response to recurring dehydration stresses and the memory-type responses to insect damage in a process known as "priming." The questions discussed are (a) whether/how the two memory phenomena are connected at the level of transcriptional regulation; (b) how differential transcription is achieved mechanistically under a repeated stress; and (c) whether similar molecular and/or epigenetic mechanisms are involved. Possible biological relevance of transcriptional stress memory and its preservation in plant evolution are also discussed.

Sweet Immunity: Inulin Boosts Resistance of Lettuce (Lactuca sativa) against Grey Mold (Botrytis cinerea) in an Ethylene-Dependent Manner

The concept of "Sweet Immunity" postulates that sugar metabolism and signaling influence plant immune networks. In this study, we tested the potential of commercially available inulin-type fructans to limit disease symptoms caused by Botrytis cinerea in lettuce. Spraying mature lettuce leaves, with inulin-type fructans derived from burdock or chicory was as effective in reducing grey mold disease symptoms caused by Botrytis cinerea as spraying with oligogalacturonides (OGs). OGs are well-known defense elicitors in several plant species. Spraying with inulin and OGs induced accumulation of hydrogen peroxide and levels further increased upon pathogen infection. Inulin and OGs were no longer able to limit Botrytis infection when plants were treated with the ethylene signaling inhibitor 1-methylcyclopropene (1-MCP), indicating that a functional ethylene signaling pathway is needed for the enhanced defense response. Soluble sugars accumulated in leaves primed with OGs, while 1-MCP treatment had an overall negative effect on the sucrose pool. Accumulation of γ-aminobutyric acid (GABA), a stress-associated non-proteinogenic amino acid and possible signaling compound, was observed in inulin-treated samples after infection and negatively affected by the 1-MCP treatment. We have demonstrated for the first time that commercially available inulin-type fructans and OGs can improve the defensive capacity of lettuce, an economically important species. We discuss our results in the context of a possible recognition of fructans as Damage or Microbe Associated Molecular Patterns.

Redox Systemic Signaling and Induced Tolerance Responses During Soybean-Bradyrhizobium japonicum Interaction: Involvement of Nod Factor Receptor and Autoregulation of Nodulation

The symbiotic relationship between legumes and nitrogen-fixing rhizobia induces local and systemic responses, which ultimately lead to nodule formation. The autoregulation of nodulation (AON) is a systemic mechanism related to innate immunity that controls nodule development and involves different components ranging from hormones, peptides, receptors to small RNAs. Here, we characterized a rapid systemic redox changes induced during soybean-Bradyrhizobium japonicum symbiotic interaction. A transient peak of reactive oxygen species (ROS) generation was found in soybean leaves after 30 min of root inoculation with B. japonicum. The ROS response was accompanied by changes in the redox state of glutathione and by activation of antioxidant enzymes. Moreover, the ROS peak and antioxidant enzyme activation were abolished in leaves by the addition, in either root or leaf, of DPI, an NADPH oxidase inhibitor. Likewise, these systemic redox changes primed the plant increasing its tolerance to photooxidative stress. With the use of non-nodulating nfr5-mutant and hyper-nodulating nark-mutant soybean plants, we subsequently studied the systemic redox changes. The nfr5-mutant lacked the systemic redox changes after inoculation, whereas the nark-mutant showed a similar redox systemic signaling than the wild type plants. However, neither nfr5- nor nark-mutant exhibited tolerance to photooxidative stress condition. Altogether, these results demonstrated that (i) the early redox systemic signaling during symbiotic interaction depends on a Nod factor receptor, and that (ii) the induced tolerance response depends on the AON mechanisms.

Physiological and Transcriptome Analyses Reveal Short-Term Responses and Formation of Memory Under Drought Stress in Rice

In some plants, exposure to stress can induce a memory response, which appears to play an important role in adaptation to recurrent stress environments. However, whether rice exhibits drought stress memory and the molecular mechanisms that might underlie this process have remained unclear. Here, we ensured that rice drought memory was established after cycles of mild drought and re-watering treatment, and studied gene expression by whole-transcriptome strand-specific RNA sequencing (ssRNA-seq). We detected 6,885 transcripts and 238 lncRNAs involved in the drought memory response, grouped into 16 distinct patterns. Notably, the identified genes of dosage memory generally did not respond to the initial drought treatment. Our results demonstrate that stress memory can be developed in rice under appropriate water deficient stress, and lncRNA, DNA methylation and endogenous phytohormones (especially abscisic acid) participate in rice short-term drought memory, possibly acting as memory factors to activate drought-related memory transcripts in pathways such as photosynthesis and proline biosynthesis, to respond to the subsequent stresses.

DOTAP, a lipidic transfection reagent, triggers Arabidopsis plant defense responses

DOTAP triggers Arabidopsis thaliana immunity and by priming the defense response is able to reduce bacterial pathogen attack. DOTAP is a cationic lipid widely used as a liposomal transfection reagent and it has recently been identified as a strong activator of the innate immune system in animal cells. Plants are sessile organisms and unlike mammals, that have innate and acquired immunity, plants possess only innate immunity. A key feature of plant immunity is the ability to sense potentially dangerous signals, as it is the case for microbe-associated, pathogen-associated or damage-associated molecular patterns and by doing so, trigger an active defense response to cope with the perturbing stimulus. Here, we evaluated the effect of DOTAP in plant basal innate immunity. An initial plant defense response was induced by the cationic lipid DOTAP in the model plant Arabidopsis thaliana, assessed by callose deposition, reactive oxygen species production, and plant cell death. In addition, a proteomic analysis revealed that these responses are mirrored by changes in the plant proteome, such as up-regulation of proteins related to defense responses, including proteins involved in photorespiration, cysteine and oxylipin synthesis, and oxidative stress response; and down-regulation of enzymes related to photosynthesis. Furthermore, DOTAP was able to prime the defense response for later pathogenic challenges as in the case of the virulent bacterial pathogen Pseudomonas syringae pv. tomato. Disease outcome was diminished in DOTAP-pre-treated leaves and bacterial growth was reduced 100 times compared to mock leaves. Therefore, DOTAP may be considered a good candidate as an elicitor for the study of plant immunity.

Thermopriming reprograms metabolic homeostasis to confer heat tolerance

Heat stress threatens agriculture worldwide. Plants acquire heat stress tolerance through priming, which establishes stress memory during mild or severe transient heat stress. Such induced thermotolerance restructures metabolic networks and helps maintain metabolic homeostasis under heat stress. Here, we used an electrospray ionization mass spectrometry-based platform to explore the composition and dynamics of the metabolome of Arabidopsis thaliana under heat stress and identify metabolites involved in thermopriming. Primed plants performed better than non-primed plants under severe heat stress due to altered energy pathways and increased production of branched-chain amino acids, raffinose family oligosaccharides (RFOs), lipolysis products, and tocopherols. These metabolites serve as osmolytes, antioxidants and growth precursors to help plants recover from heat stress, while lipid metabolites help protect membranes against heat stress. The carbohydrate (e.g., sucrose and RFOs) and lipid superpathway metabolites showed the most significant increases. Under heat stress, there appears to be crosstalk between carbohydrate metabolism (i.e., the thermomemory metabolites stachyose, galactinol, and raffinose) and tyrosine metabolism towards the production of the thermomemory metabolite salidroside, a phenylethanoid glycoside. Crosstalk occurs between two glycerophospholipid pathways (the biosynthetic pathways of the thermomemory metabolite S-adenosyl-L-homocysteine and the terpenoid backbone) and the δ-tocopherol (chloroplast lipid) pathway, which favors the production of glycine betaine and other essential tocopherols, respectively, compounds which are essential for abiotic stress tolerance in plants. Therefore, metabolomic analysis can provide comprehensive insights into the metabolites involved in stress responses, which could facilitate plant breeding to maximize crop yields under adverse conditions.

Biotic Stress-Induced Priming and De-Priming of Transcriptional Memory in Arabidopsis and Apple

Under natural growth conditions, plants experience various and repetitive biotic and abiotic stresses. Salicylic acid (SA) is a key phytohormone involved in the response to biotic challenges. Application of synthetic SA analogues can efficiently prime defense responses, and leads to improved pathogen resistance. Because SA analogues can result in long-term priming and memory, we identified genes for which expression was affected by the SA analogue and explored the role of DNA methylation in this memorization process. We show that treatments with an SA analogue can lead to long-term transcriptional memory of particular genes in Arabidopsis. We found that subsequent challenging of such plants with a bacterial elicitor reverted this transcriptional memory, bringing their expression back to the original pre-treatment level. We also made very similar observations in apple (Malus domestica), suggesting that this expression pattern is highly conserved in plants. Finally, we found a potential role for DNA methylation in the observed transcriptional memory behavior. We show that plants defective in DNA methylation pathways displayed a different memory behavior. Our work improves our understanding of the role of transcriptional memory in priming, and has important implication concerning the application of SA analogues in agricultural settings.

Defensin-like peptides in wheat analyzed by whole-transcriptome sequencing: a focus on structural diversity and role in induced resistance

Antimicrobial peptides (AMPs) are the main components of the plant innate immune system. Defensins represent the most important AMP family involved in defense and non-defense functions. In this work, global RNA sequencing and de novo transcriptome assembly were performed to explore the diversity of defensin-like (DEFL) genes in the wheat Triticum kiharae and to study their role in induced resistance (IR) mediated by the elicitor metabolites of a non-pathogenic strain FS-94 of Fusarium sambucinum. Using a combination of two pipelines for DEFL mining in transcriptome data sets, as many as 143 DEFL genes were identified in T. kiharae, the vast majority of them represent novel genes. According to the number of cysteine residues and the cysteine motif, wheat DEFLs were classified into ten groups. Classical defensins with a characteristic 8-Cys motif assigned to group 1 DEFLs represent the most abundant group comprising 52 family members. DEFLs with a characteristic 4-Cys motif CX{3,5}CX{8,17}CX{4,6}C named group 4 DEFLs previously found only in legumes were discovered in wheat. Within DEFL groups, subgroups of similar sequences originated by duplication events were isolated. Variation among DEFLs within subgroups is due to amino acid substitutions and insertions/deletions of amino acid sequences. To identify IR-related DEFL genes, transcriptional changes in DEFL gene expression during elicitor-mediated IR were monitored. Transcriptional diversity of DEFL genes in wheat seedlings in response to the fungus Fusarium oxysporum, FS-94 elicitors, and the combination of both (elicitors + fungus) was demonstrated, with specific sets of up- and down-regulated DEFL genes. DEFL expression profiling allowed us to gain insight into the mode of action of the elicitors from F. sambucinum. We discovered that the elicitors up-regulated a set of 24 DEFL genes. After challenge inoculation with F. oxysporum, another set of 22 DEFLs showed enhanced expression in IR-displaying seedlings. These DEFLs, in concert with other defense molecules, are suggested to determine enhanced resistance of elicitor-pretreated wheat seedlings. In addition to providing a better understanding of the mode of action of the elicitors from FS-94 in controlling diseases, up-regulated IR-specific DEFL genes represent novel candidates for genetic transformation of plants and development of pathogen-resistant crops.

Long-Lasting Primed State in Maize Plants: Salicylic Acid and Steroid Signaling Pathways as Key Players in the Early Activation of Immune Responses in Silks

In the present study, we investigated the induced systemic resistance (ISR) activated by the beneficial fungus Trichoderma atroviride in maize plants, and the early immunological responses triggered after challenge with the ear rot pathogen Fusarium verticillioides. By transcriptional analysis, we were able to identify the gene core set specifically modulated in silks of maize plants expressing ISR. Our results showed that the main transcriptional reprogramming falls into genes involved in five main functional categories: cell structure or cell wall, amino acid and protein metabolism, stress responses, signaling, and transport. Among these ISR-related genes, it is important to highlight novel findings regarding hormone metabolism and signaling. The expression of hormone-dependent genes was in good agreement with the abscisic acid, jasmonic acid, and salicylic acid (SA) levels detected in the plants under study. The experimental design allowed the identification of novel regulatory elements related to a heightened state of defense in silks and suggests that steroids and SA are central components of a master regulatory network controlling the immunity of silks during ISR. The results presented also provide evidence about the molecular mechanisms used by maize silks against F. verticillioides to counteract pathogenic development and host invasion, including pathogenesis-related genes, plant cell-wall reinforcement, fungal cell-wall-degrading enzymes and secondary metabolism.

The Physiological Functions of Universal Stress Proteins and Their Molecular Mechanism to Protect Plants From Environmental Stresses

Since the original discovery of a Universal Stress Protein (USP) in Escherichia coli, a number of USPs have been identified from diverse sources including archaea, bacteria, plants, and metazoans. As their name implies, these proteins participate in a broad range of cellular responses to biotic and abiotic stresses. Their physiological functions are associated with ion scavenging, hypoxia responses, cellular mobility, and regulation of cell growth and development. Consistent with their roles in resistance to multiple stresses, USPs show a wide range of structural diversity that results from the diverse range of other functional motifs fused with the USP domain. As well as providing structural diversity, these catalytic motifs are responsible for the diverse biochemical properties of USPs and enable them to act in a number of cellular signaling transducers and metabolic regulators. Despite the importance of USP function in many organisms, the molecular mechanisms by which USPs protect cells and provide stress resistance remain largely unknown. This review addresses the diverse roles of USPs in plants and how the proteins enable plants to resist against multiple stresses in ever-changing environment. Bioinformatic tools used for the collection of a set of USPs from various plant species provide more than 2,100 USPs and their functional diversity in plant physiology. Data from previous studies are used to understand how the biochemical activity of plant USPs modulates biotic and abiotic stress signaling. As USPs interact with the redox protein, thioredoxin, in Arabidopsis and reactive oxygen species (ROS) regulates the activity of USPs, the involvement of USPs in redox-mediated defense signaling is also considered. Finally, this review discusses the biotechnological application of USPs in an agricultural context by considering the development of novel stress-resistant crops through manipulating the expression of USP genes.

Plant immune responses - from guard cells and local responses to systemic defense against bacterial pathogens

When plants are infected by pathogens two distinct responses can occur, the early being a local response in the infected area, and later a systemic response in non-infected tissues. Closure of stomata has recently been found to be a local response to bacterial pathogens. Stomata closure is linked to both salicylic acid (SA), an essential hormone in local responses and systemic acquired resistance (SAR), and absisic acid (ABA) a key regulator of drought and other abiotic stresses. SAR reduces the effects of later infections. In this review we discuss recent research elucidating the role of guard cells in local and systemic immune responses, guard cell interactions with abiotic and hormone signals, as well as putative functions and interactions between long-distance SAR signals.

Strengthening Grapevine Resistance by Pseudomonas fluorescens PTA-CT2 Relies on Distinct Defense Pathways in Susceptible and Partially Resistant Genotypes to Downy Mildew and Gray Mold Diseases

Downy mildew caused by the oomycete Plasmopara viticola and gray mold caused by the fungus Botrytis cinerea are among the highly threatening diseases in vineyards. The current strategy to control these diseases relies totally on the application of fungicides. The use of beneficial microbes is arising as a sustainable strategy in controlling various diseases. This can be achieved through the activation of the plants' own immune system, known as induced systemic resistance (ISR). We previously showed that bacteria-mediated ISR in grapevine involves activation of both immune response and priming state upon B. cinerea challenge. However, the effectiveness of beneficial bacteria against the oomycete P. viticola remains unknown, and mechanisms underpinning ISR against pathogens with different lifestyles need to be deciphered. In this study, we focused on the capacity of Pseudomonas fluorescens PTA-CT2 to induce ISR in grapevine against P. viticola and B. cinerea by using two grafted cultivars differing in their susceptibility to downy mildew, Pinot noir as susceptible and Solaris as partially resistant. On the basis of their contrasting phenotypes, we explored mechanisms underlying ISR before and upon pathogen infection. Our results provide evidence that in the absence of pathogen infection, PTA-CT2 does not elicit any consistent change of basal defenses, while it affects hormonal status and enhances photosynthetic efficiency in both genotypes. PTA-CT2 also induces ISR against P. viticola and B. cinerea by priming common and distinct defensive pathways. After P. viticola challenge, PTA-CT2 primes salicylic acid (SA)- and hypersensitive response (HR)-related genes in Solaris, but SA and abscisic acid (ABA) accumulation in Pinot noir. However, ISR against B. cinerea was associated with potentiated ethylene signaling in Pinot noir, but with primed expression of jasmonic acid (JA)- and SA-responsive genes in Solaris, together with downregulation of HR-related gene and accumulation of ABA and phytoalexins.

Identification and characterization of maize ACD6-like gene reveal ZmACD6 as the maize orthologue conferring resistance to Ustilago maydis

Enhancing broad-spectrum resistance is a major goal of crop breeding. However, broad-spectrum resistance has not been thoroughly investigated, and its underlying molecular mechanisms remain elusive. In the model plant Arabidopsis (Arabidopsis thaliana), ACCELERATED CELL DEATH6 (ACD6) is a key component of broad-spectrum resistance that acts in a positive feedback loop with salicylic acid (SA) to regulate multiple pattern recognition receptors. However, the role of ACD6 in disease resistance in crop plants is unclear. Here, we show that the transcript of ANK23, one of the 15 ACD6-like genes in maize (Zea mays), is induced by SA and by infection with the pathogenic fungus Ustilago maydis. Heterologous expression of ANK23 restored disease resistance in the Arabidopsis mutant acd6-2. We show that ANK23 is a maize ortholog of ACD6 and therefore rename ANK23 as ZmACD6. Furthermore, using CRISPR/Cas9, we generated ZmACD6 knockout maize plants, which are more susceptible to U. maydis than wild-type plants. We also identified a maize line (SC-9) with relatively high ZmACD6 expression levels from a diverse natural maize population. SC-9 has increased disease resistance to U. maydis and defense activation, suggesting a practical approach to cultivate elite varieties with enhanced disease resistance.

Recognition of Elicitors in Grapevine: From MAMP and DAMP Perception to Induced Resistance

In a context of a sustainable viticulture, the implementation of innovative eco-friendly strategies, such as elicitor-triggered immunity, requires a deep knowledge of the molecular mechanisms underlying grapevine defense activation, from pathogen perception to resistance induction. During plant-pathogen interaction, the first step of plant defense activation is ensured by the recognition of microbe-associated molecular patterns, which are elicitors directly derived from pathogenic or beneficial microbes. Vitis vinifera, like other plants, can perceive elicitors of different nature, including proteins, amphiphilic glycolipid, and lipopeptide molecules as well as polysaccharides, thanks to their cognate pattern recognition receptors, the discovery of which recently began in this plant species. Furthermore, damage-associated molecular patterns are another class of elicitors perceived by V. vinifera as an invader's hallmark. They are mainly polysaccharides derived from the plant cell wall and are generally released through the activity of cell wall-degrading enzymes secreted by microbes. Elicitor perception and subsequent activation of grapevine immunity end in some cases in efficient grapevine resistance against pathogens. Using complementary approaches, several molecular markers have been identified as hallmarks of this induced resistance stage. This review thus focuses on the recognition of elicitors by Vitis vinifera describing the molecular mechanisms triggered from the elicitor perception to the activation of immune responses. Finally, we discuss the fact that the link between elicitation and induced resistance is not so obvious and that the formulation of resistance inducers remains a key step before their application in vineyards.

Transgenerational effects of ungulates and pre-dispersal seed predators on offspring success and resistance to herbivory

Herbivorous mammals and insect pre-dispersal seed predators are two types of herbivores that, despite their functional and morphological differences, tend to severely impact many plant species, highly decreasing their seed production and even imperiling the performance of their offspring through transgenerational effects. However, how they influence offspring resistance to herbivory remains largely unknown. In this study we experimentally examined the effects of ungulates and pre-dispersal seed predators on seed quality as well as on the emergence, survival and resistance to herbivory of the seedlings of a semiarid herb. We found that ungulates reduced seedling recruitment but increased seedling resistance to leaf miners. These effects were probably a consequence of insufficient carbon provisioning in seeds that reduced seed viability and provoked carbon limitation in seedlings. Pre-dispersal seed predators did not influence seedling recruitment, but seedlings from mothers damaged by ungulates and by pre-dispersal seed predators suffered less herbivory by grasshoppers. Remarkably, intra-individual differences in damage by pre-dispersal seed predators affected the rate of damage underwent by seedlings. That is, seedlings derived from fruits attacked by seed predators were more resistant to herbivores than siblings derived from un-attacked fruits in plant populations exposed to ungulates. To our knowledge, this is the first study reporting variation in transgenerational-induced resistance of seedlings from the same maternal plant. This study is a valuable contribution to the understanding of transgenerational effects of multiple herbivores and their implications for a deeper comprehension of the natural systems in which they co-occur.

Assessment of the impact of PS3-induced resistance to downy mildew on grapevine physiology

Elicitor-induced resistance against diseases is an attractive strategy that could contribute to reduce the use of fungicides for plant protection. However, activation of defenses has an energetic cost that plants have to fuel by a mobilization of their primary metabolism with possible adverse effect on their physiology. In this context, this study was performed to determine whether elicitor-induced resistance of grapevine leaves against downy mildew impacted its development and metabolism. The elicitor PS3 (sulfated β-glucan laminarin) was sprayed on grapevine herbaceous cuttings grown in greenhouses once or three times, and its impact was studied on young and older grapevine leaves, prior to, and after Plasmopara viticola inoculation. PS3 did not affect grapevine development during the time course of the experiment. A metabolomic analysis, mainly focused on primary metabolites, highlighted a leaf age dependent effect of PS3 treatment. Nitrogen compounds, and sugars to a lesser extent, were impacted. The results obtained complete the current knowledge of the impact of elicitor-induced resistance on plant physiology. They will be helpful to guide further experiments required to better determine the costs and benefits of elicitor-induced resistance in plants.

A Polysaccharide Derived from a Trichosporon sp. Culture Strongly Primes Plant Resistance to Viruses

Plant viruses cause devastating diseases in plants, yet no effective viricide is available for agricultural application. We screened cultured filtrates derived from various soil microorganisms cultured in vegetable broth that enhanced plant viral resistance. A cultured filtrate, designated F8 culture filtrate, derived from a fungus belonging to the genus Trichosporon, induced strong resistance to various viruses on different plants. Our inoculation assay found the infection rate of Tobacco mosaic virus (TMV)-inoculated Nicotiana benthamiana with F8 culture filtrate pretreatment may decrease to 0%, whereas salicylic acid (SA)-pretreated N. benthamiana attenuated TMV-caused symptoms but remained 100% infected. Tracking Tobacco mosaic virus tagged with green fluorescence protein in plants revealed pretreatment with F8 culture filtrate affected the initial establishment of the virus infection. From F8 culture filtrate, we identified a previously unknown polysaccharide composed of D-mannose, D-galactose, and D-glucose in the ratio 1.0:1.2:10.0 with a α-D-1,4-glucan linkage to be responsible for the induction of plant resistance against viruses through priming of SA-governed immune-responsive genes. Notably, F8 culture filtrate only triggered local defense but was much more effective than conventional SA-mediated systematic acquired resistance. Our finding revealed that microbial cultured metabolites provided a rich source for identification of potent elicitors in plant defense.

Indole primes plant defense against necrotrophic fungal pathogen infection

Indole is a volatile compound and emitted from plants challenged by insect infestation or mechanic wounding. It has been shown to prime defense against herbivory. Here we identified that indole induced defense either directly or as a priming agent against necrotrophic pathogens Fusarium graminearum and F. moniliforme in maize and Magnaporthe oryzae in rice. With indole pretreatment, smaller lesions were developed in infected leaves, as well as less fungal growth. Indole induced H₂O₂ burst in the priming stage like other priming substances did. Such priming relied on mitogen-activated protein kinase (MAPK) cascade, which potentially activated downstream defense signaling. In addition, indole priming resulted in earlier and stronger defensive gene expression upon pathogen infection, including genes of jasmonate and phytoalexin biosynthesis, pathogenesis-related proteins (PRs) and anti-oxidant enzymes, which enhanced plant resistance. Meanwhile, H₂O₂ was also identified as the priming agent to induce plant defense. Taken together, indole exhibited priming function not only against herbivory but also necrotrophic pathogens. The common emission of indole in plants suggests that it plays important roles as the universal and endogenous priming substance in plant defense.

Recurrent water deficit causes alterations in the profile of redox proteins in citrus plants

Plant acclimation to recurrent stress involves profound alterations in multiple genetic, metabolic and physiological processes. Stressful conditions usually implicate imbalance in reactive oxygen species (ROS) production and removal rates, which may lead to oxidative stress. However, the primary cellular targets of oxidative stress and their relevance in plant acclimation to abiotic stresses remains poorly characterized. By comparing redox proteomic and sugar profiles in citrus Valencia (VO) scions grafted onto two rootstocks with different soil water extraction capacities - Rangpur Lime (RL) and Sunki Maravilha (SM) - here we demonstrate that both ROS-mediated post-translational protein modification and changes in sugar composition are associated with acclimation to recurrent drought in citrus. The redox proteomic analysis of the distinct scion/rootstock combinations exposed to one (WD1), two (WD2) or three (WD3) water deficit episodes revealed a total of 32 and 55 redox protein spots present in VO/RL and VO/SM plants, respectively. Mass spectrometry analysis of these protein spots revealed essential targets of ROS-mediated posttranslational protein modification in citrus plants challenged by recurrent drought. The oxidation of cysteine thiol groups into glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was shown to increase in WD3 samples of the VO/RL combination, whereas the opposite was observed for the VO/SM combination. Similarly, recurrent drought promoted the oxidation of catalase thiol groups in VO/SM, but not in VO/RL. Carbohydrate profiling revealed that glucose, fructose and galactose may also contribute to the phenotypic differences observed between the citrus genotypes exposed to drought. These findings reveal for the first time that recurrent drought differentially affects the profile of redox proteomics of citrus, suggesting that this alteration may be part of the stress memory in perennial plants.

Receptors and Signaling Pathways for Recognition of Bacteria in Livestock and Crops: Prospects for Beneficial Microbes in Healthy Growth Strategies

Modern animal and crop production practices are associated with the regular use of antimicrobials, potentially increasing selection pressure on bacteria to become resistant. Alternative approaches are needed in order to satisfy the demands of the growing human population without the indiscriminate use of antimicrobials. Researchers have brought a different perspective to solve this problem and have emphasized the exploitation of animal- and plant-associated microorganisms that are beneficial to their hosts through the modulation of the innate immune system. There is increasing evidence that plants and animals employ microbial perception and defense pathways that closely resemble each other. Formation of pattern recognition receptor (PRR) complexes involving leucine-rich repeat (LRR)-containing proteins, mitogen-activated protein kinase (MAPK)-mediated activation of immune response genes, and subsequent production of antimicrobial products and reactive oxygen species (ROS) and nitric oxide (NO) to improve defenses against pathogens, add to the list of similarities between both systems. Recent pioneering work has identified that animal and plant cells use similar receptors for sensing beneficial commensal microbes that are important for the maintenance of the host's health. Here, we reviewed the current knowledge about the molecular mechanisms involved in the recognition of pathogenic and commensal microbes by the innate immune systems of animal and plants highlighting their differences and similarities. In addition, we discuss the idea of using beneficial microbes to modulate animal and plant immune systems in order to improve the resistance to infections and reduce the use of antimicrobial compounds.

Plant and animal PR1 family members inhibit programmed cell death and suppress bacterial pathogens in plant tissues

A role for programmed cell death (PCD) has been established as the basis for plant-microbe interactions. A functional plant-based cDNA library screen identified possible anti-PCD genes, including one member of the PR1 family, designated P14a, from tomato. Members of the PR1 family have been subject to extensive research in view of their possible role in resistance against pathogens. The PR1 family is represented in every plant species studied to date and homologues have been found in animals, fungi and insects. However, the biological function of the PR1 protein from plants has remained elusive in spite of extensive research regarding a role in the response of plants to disease. Constitutive expression of P14a in transgenic tomato roots protected the roots against PCD triggered by Fumonisin B1, as did the human orthologue GLIPR1, indicating a kingdom crossing function for PR1. Tobacco plants transformed with a P14a-GFP fusion construct and inoculated with Pseudomonas syringae pv. tabaci revealed that the mRNA was abundant throughout the leaves, but the fusion protein was restricted to the lesion margins, where cell death and bacterial spread were arrested. Vitus vinifera grapes expressing the PR1 homologue P14a as a transgene were protected against the cell death symptoms of Pierce's disease. A pull-down assay identified putative PR1-interacting proteins, including members of the Rac1 immune complex, known to function in innate immunity in rice and animal systems. The findings herein are consistent with a role of PR1 in the suppression of cell death-dependent disease symptoms and a possible mode of action.