Damaged-self recognition in plant herbivore defence

Enhancing plant defensins in a desert shrub: Exploring a regulatory pathway of AnWRKY29

A distinct family of plant-specific WRKY transcription factors plays a crucial role in modulating responses to biotic and abiotic stresses. In this investigation, we unveiled a signaling pathway activated in the desert shrub Ammopiptanthus nanus during feeding by the moth Spodoptera exigua. The process involves a Ca2+ flux that facilitates interaction between the protein kinase AnCIPK12 and AnWRKY29. AnWRKY29 directly interacts with the promoters of two key genes encoding AnPDF1 and AnHsfB1, involved in the biosynthesis of plant defensins. Consequently, AnWRKY29 exerts its transcriptional regulatory function, influencing plant defensins biosynthesis. This discovery implies that A. nanus can bolster resistance against herbivorous insects like S. exigua by utilizing this signaling pathway, providing an effective natural defense mechanism that supports its survival and reproductive success.

Dynamic distress calls: volatile info chemicals induce and regulate defense responses during herbivory

Plants are continuously threatened by a plethora of biotic stresses caused by microbes, pathogens, and pests, which often act as the major constraint in crop productivity. To overcome such attacks, plants have evolved with an array of constitutive and induced defense mechanisms- morphological, biochemical, and molecular. Volatile organic compounds (VOCs) are a class of specialized metabolites that are naturally emitted by plants and play an important role in plant communication and signaling. During herbivory and mechanical damage, plants also emit an exclusive blend of volatiles often referred to as herbivore-induced plant volatiles (HIPVs). The composition of this unique aroma bouquet is dependent upon the plant species, developmental stage, environment, and herbivore species. HIPVs emitted from infested and non-infested plant parts can prime plant defense responses by various mechanisms such as redox, systemic and jasmonate signaling, activation of mitogen-activated protein (MAP) kinases, and transcription factors; mediate histone modifications; and can also modulate the interactions with natural enemies via direct and indirect mechanisms. These specific volatile cues mediate allelopathic interactions leading to altered transcription of defense-related genes, viz., proteinase inhibitors, amylase inhibitors in neighboring plants, and enhanced levels of defense-related secondary metabolites like terpenoids and phenolic compounds. These factors act as deterrents to feeding insects, attract parasitoids, and provoke behavioral changes in plants and their neighboring species. This review presents an overview of the plasticity identified in HIPVs and their role as regulators of plant defense in Solanaceous plants. The selective emission of green leaf volatiles (GLVs) including hexanal and its derivatives, terpenes, methyl salicylate, and methyl jasmonate (MeJa) inducing direct and indirect defense responses during an attack from phloem-sucking and leaf-chewing pests is discussed. Furthermore, we also focus on the recent developments in the field of metabolic engineering focused on modulation of the volatile bouquet to improve plant defenses.

ATM and ATR, two central players of the DNA damage response, are involved in the induction of systemic acquired resistance by extracellular DNA, but not the plant wound response

Background

The plant immune response to DNA is highly self/nonself-specific. Self-DNA triggered stronger responses by early immune signals such as H2O2 formation than nonself-DNA from closely related plant species. Plants lack known DNA receptors. Therefore, we aimed to investigate whether a differential sensing of self-versus nonself DNA fragments as damage- versus pathogen-associated molecular patterns (DAMPs/PAMPs) or an activation of the DNA-damage response (DDR) represents the more promising framework to understand this phenomenon.

Results

We treated Arabidopsis thaliana Col-0 plants with sonicated self-DNA from other individuals of the same ecotype, nonself-DNA from another A. thaliana ecotype, or nonself-DNA from broccoli. We observed a highly self/nonself-DNA-specific induction of H2O2 formation and of jasmonic acid (JA, the hormone controlling the wound response to chewing herbivores) and salicylic acid (SA, the hormone controlling systemic acquired resistance, SAR, to biotrophic pathogens). Mutant lines lacking Ataxia Telangiectasia Mutated (ATM) or ATM AND RAD3-RELATED (ATR) - the two DDR master kinases - retained the differential induction of JA in response to DNA treatments but completely failed to induce H2O2 or SA. Moreover, we observed H2O2 formation in response to in situ-damaged self-DNA from plants that had been treated with bleomycin or SA or infected with virulent bacteria Pseudomonas syringae pv. tomato DC3000 or pv. glycinea carrying effector avrRpt2, but not to DNA from H2O2-treated plants or challenged with non-virulent P. syringae pv. glycinea lacking avrRpt2.

Conclusion

We conclude that both ATM and ATR are required for the complete activation of the plant immune response to extracellular DNA whereas an as-yet unknown mechanism allows for the self/nonself-differential activation of the JA-dependent wound response.

The impact of spring wheat species and sowing density on soil biochemical properties, content of secondary plant metabolites and the presence of Oulema ssp

The physical and chemical properties of the soil are important factors influencing the yield of crops. One of the agrotechnical factors influencing the biochemical properties of soil is sowing density. It affects the yield components, light, moisture and thermal conditions in the canopy and the pressure of pests. Secondary metabolites, many of which are known to act as a defense mechanism against insects, are of importance in the interaction between the crop and abiotic and biotic factors of the habitat. To the best of our knowledge, the studies conducted so far do not sufficiently reveal the impacts of the wheat species and the sowing density, together with the biochemical properties of the soil, on the accumulation of bioactive ingredients in the crop plants, and the subsequent impacts on the occurrence of phytophagic entomofauna in various management systems. Explaining these processes creates an opportunity for more sustainable development of agriculture. The study aimed to determine the effect of wheat species and sowing density on the biochemical properties of the soil, concentrations of biologically active compounds in the plant and the occurrence of insect pests in organic (OPS) and conventional (CPS) production systems. The research was conducted on spring wheat species (Indian dwarf wheat-Triticum sphaerococcum Percival and Persian wheat-Triticum persicum Vavilov) grown in OPS and CPS at sowing densities 400, 500, 600 (seeds m^-2). The following analyzes were performed: (i) soil analysis: the activity of catalases (CAT), dehydrogenases (DEH), peroxidases (PER); (ii) plant analysis: total phenolic compounds (TP), chlorogenic acid (CA), antioxidant capacity (FRAP); (iii) entomological analysis of the number of insects-Oulema spp. adults and larvae. Performing analyzes in such a wide (interdisciplinary) scope will allow for a comprehensive understanding of the soil-plant-insect biological transformation evaluation. Our results showed that an increase in soil enzyme activity caused a decrease in TP contents in the wheat grown the OPS. Despite this, both the content of TP and the anti-oxidative activity of the ferric reducing ability of plasma (FRAP) were higher in these wheats. Bioactive compound contents and FRAP were most favoured by the lowest sowing density. Regardless of the production system, the occurrence of the Oulema spp. adults on T. sphaerococcum was the lowest at a sowing density of 500 seeds m^-2. The occurrence of this pest's larvae was lowest at a sowing density of 400 seeds m^-2. Research on bioactive compounds in plants, biochemical properties of soil and the occurrence of pests make it possible to comprehensively assess the impact of the sowing density of ancient wheat in the ecological and conventional production system, which is necessary for the development of environmentally sustainable agriculture.

Cotton plants as ideal models for teaching and research on inducible direct plant defenses

Cotton (Gossypium hirsutum) stores defensive compounds in glands covering its leaves and other tissues. The density and the chemical filling of these glands increase systematically in developing leaves in response to herbivory on older leaves. Cotton seedlings are known to respond more strongly to actual caterpillar herbivory than to mere physical damage. It is not clear whether this amplified response is linked to insect-derived elicitors or difference in damage properties. To investigate this, we assessed the effect of repeated artificial damage without and with application of regurgitant from Spodoptera exigua caterpillars. Repeated mechanical damage led to a systemic increase of gland density, gland size, and content of defensive terpenes, with no detectable additional elicitation upon regurgitant treatment. Dual choice feeding assays further showed that defense induction triggered by just physical damage made newly developing leaves far less palatable to S. exigua larvae as compared to leaves from undamaged seedlings, whereas they did not distinguish between leaves from damaged plants treated with or without regurgitant. Our study confirms that the systemic induction of cotton glands is an unspecific response to physical damage, although cotton is known to respond to caterpillar-associated elicitors for other defensive traits. Cotton glands induction can be readily visualized under modest magnification, making the experiments described in this study highly suited to teach chemical ecology and aspects of plant defense theory in practical classes.

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

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

The small brown planthopper (Laodelphax striatellus) as a vector of the rice stripe virus

The small brown planthopper, Laodelphax striatellus, is a destructive pest insect found in rice fields. L. striatellus not only directly feeds on the phloem sap of rice but also transmits various viruses, such as rice stripe virus (RSV) and rice black-streaked dwarf virus, resulting in serious loss of rice production. RSV is a rice-infecting virus that is found mainly in Korea, China, and Japan. To develop novel strategies to control L. striatellus and L. striatellus-transmitted viruses, various studies have been conducted, based on vector biology, interactions between vectors and pathogens, and omics, including transcriptomics, proteomics, and metabolomics. In this review, we discuss the roles of saliva proteins during phloem sap-sucking and virus transmission, the diversity and role of the microbial community in L. striatellus, the profile and molecular mechanisms of insecticide resistance, classification of L. striatellus-transmitted RSV, its host range and symptoms, its genome composition and roles of virus-derived proteins, its distribution, interactions with L. striatellus, and resistance and control, to suggest future directions for integrated pest management to control L. striatellus and L. striatellus-transmitted viruses.

Photosynthetic and defensive responses of two Mediterranean oaks to insect leaf herbivory

Insect herbivory is a dominant interaction across virtually all ecosystems globally and has dramatic effects on plant function such as reduced photosynthesis activity and increased levels of defenses. However, most previous work assessing the link between insect herbivory, photosynthesis and plant defenses has been performed on cultivated model plant species, neglecting a full understanding of patterns in natural systems. In this study, we performed a field experiment to investigate the effects of herbivory by a generalist foliar feeding insect (Lymantria dispar) and leaf mechanical damage on multiple leaf traits associated with defense against herbivory and photosynthesis activity on two sympatric oak species with contrasting leaf habit (the evergreen Quercus coccifera L. and the deciduous Quercus pubescens Willd). Our results showed that, although herbivory treatments and oak species did not strongly affect photosynthesis and dark respiration, these two factors exerted interactive effects. Insect herbivory and mechanical damage (vs control) decreased photosynthesis activity for Q. coccifera but not for Q. pubescens. Insect herbivory and mechanical damage tended to increase chemical (increased flavonoid and lignin concentration) defenses, but these effects were stronger for Q. pubescens. Overall, this study shows that two congeneric oak species with contrasting leaf habit differ in their photosynthetic and defensive responses to insect herbivory. While the evergreen oak species followed a more conservative strategy (reduced photosynthesis and higher physical defenses), the deciduous oak species followed a more acquisitive strategy (maintained photosynthesis and higher chemical defenses).

Identification of dynamic microRNA associated with systemic defence against Helicoverpa armigera infestation in Cajanus scarabaeoides

BACKGROUND

Helicoverpa armigera is a major insect pest of several crop plants, including pigeonpea. Resistant gene sources are not available in the cultivated gene pool, but resistance has been observed in its crop wild relative, Cajanus scarabaeoides. Gene regulatory mechanisms governing the systemic immune response of this plant to pod borer infestation have not yet been deciphered. MicroRNA (miRNA) profiles of H. armigera-infested and undamaged adjacent leaves of C. scarabaeoides were compared to gain an insight into the plant-insect interactions and to identify dynamic miRNA molecules potentially acting as mediators of systemic defence responses.

RESULTS

A total of 211 conserved, temporally dynamic miRNA were identified in the unfed adjacent leaves, out of which 98 were found to be differentially expressed in comparison to control leaves. On further analysis, most of the miRNA detected in the adjacent leaves was found to target genes involved in the defence pathways and plant immune response. An overlap of the differentially expressing miRNAs was observed between insect-fed and adjacent unfed leaves, indicating the transmission of signal from the site of infestation to the undamaged parts of the plant, indicative of induction of a systemic defence response.

CONCLUSION

The miRNA response in the unfed leaves had the signatures of induced changes in metabolism and signal transduction for induction of defence pathway genes. This study reveals the participation of miRNAs in imparting pod borer resistance and mounting a systemic defence response against pod borer infestation in C. scarabaeoides. © 2022 Society of Chemical Industry.

The wheat dioxygenase BX6 is involved in the formation of benzoxazinoids in planta and contributes to plant defense against insect herbivores

Benzoxazinoids are plant specialized metabolites with defense properties, highly abundant in wheat (Triticum), one of the world's most important crops. The goal of our study was to characterize dioxygenase BX6 genes in tetraploid and hexaploid wheat genotypes and to elucidate their effects on defense against herbivores. Phylogenetic analysis revealed four BX6 genes in the hexaploid wheat T. aestivum, but only one ortholog was found in the tetraploid (T. turgidum) wild emmer wheat and the cultivated durum wheat. Transcriptome sequencing of durum wheat plants, damaged by either aphids or caterpillars, revealed that several BX genes, including TtBX6, were upregulated upon caterpillar feeding, relative to the undamaged control plants. A virus-induced gene silencing approach was used to reduce the expression of BX6 in T. aestivum plants, which exhibited both reduced transcript levels and reduced accumulation of different benzoxazinoids. To elucidate the effect of BX6 on plant defense, bioassays with different herbivores feeding on BX6-silenced leaves were conducted. The results showed that plants with silenced BX6 were more susceptible to aphids and the two-spotted spider mite than the control. Overall, our study indicates that wheat BX6 is involved in benzoxazinoid formation in planta and contributes to plant resistance against insect herbivores.

Species specific plant‐mediated effects between herbivores converge at high damage intensity

Plants are often exposed to multiple herbivores and densities of these attackers (or corresponding damage intensities) often fluctuate greatly in the field. Plant‐mediated interactions vary among herbivore species and with changing feeding intensity, but little is known about how herbivore identity and density interact to determine plant responses and herbivore fitness. Here, we investigated this question using Triadica sebifera (tallow) and two common and abundant specialist insect herbivores, Bikasha collaris (flea beetle) and Heterapoderopsis bicallosicollis (weevil). By manipulating densities of leaf‐feeding adults of these two herbivore species, we tested how variations in the intensity of leaf damage caused by flea beetle or weevil adults affected the performance of root‐feeding flea beetle larvae and evaluated the potential of induced tallow root traits to predict flea beetle larval performance. We found that weevil adults consistently decreased the survival of flea beetle larvae with increasing leaf damage intensities. In contrast, conspecific flea beetle adults increased their larval survival at low damage then decreased larval survival at high damage, resulting in a unimodal pattern. Chemical analyses showed that increasing leaf damage from weevil adults linearly decreased root carbohydrates and increased root tannin, whereas flea beetle adults had opposite effects as weevil adults at low damage and similar effects as them at high damage. Furthermore, across all feeding treatments, flea beetle larval survival correlated positively with concentrations of carbohydrates and negatively with concentration of tannin, suggesting that root primary and secondary metabolism might underlie the observed effects on flea beetle larvae. Our study demonstrates that herbivore identity and density interact to determine systemic plant responses and plant‐mediated effects on herbivores. In particular, effects are species‐specific at low densities, but converge at high densities. These findings emphasize the importance of considering herbivore identity and density simultaneously when investigating factors driving plant‐mediated interactions between herbivores, which advances our understanding of the structure and composition of herbivore communities and terrestrial food webs.

Characterization of Superoxide Dismutase from the Potato Cyst Nematode, Globodera pallida

Potato cyst nematodes (PCNs), such as Globodera pallida and Globodera rostochiensis, are some of the most agriculturally and economically important pests of potato. Upon nematode infection, a principal component of plant defense is the generation of the reactive oxygen species (ROSs). ROSs are highly toxic molecules that cause damage to pathogens and host alike. To infect the plant, nematodes protect themselves from ROSs by activating their own antioxidant processes and ROS scavenging enzymes. One of these enzymes is a superoxide dismutase (SOD; EC 1.15.1.1), which prevents cellular damage by catalyzing conversion of the superoxide radical (O₂^-·) to hydrogen peroxide (H₂O₂) and molecular oxygen (O₂). We have isolated a putatively secreted isoform of a Cu-Zn SOD (SOD-3) from G. pallida and localized the expression of this gene in the posterior region of the nematode. Furthermore, we studied the expression of the SOD-3 gene during early parasitic stages of infection (24 to 72 h) in the susceptible potato cultivar Desiree, the resistant potato cultivar Innovator, and an immune host, Solanum sisymbriifolium. The SOD-3 gene was significantly upregulated, regardless of the host type; however, the expression pattern differed between the susceptible and the resistant or immune hosts. This finding suggests that SOD-3 gene is responding to infection in plant roots differently depending on whether the nematode is experiencing a compatible or an incompatible interaction.

Damage-Associated Molecular Patterns (DAMPs) in Plant Innate Immunity: Applying the Danger Model and Evolutionary Perspectives

Danger signals trigger immune responses upon perception by a complex surveillance system. Such signals can originate from the infectious nonself or the damaged self, the latter termed damage-associated molecular patterns (DAMPs). Here, we apply Matzinger's danger model to plant innate immunity to discuss the adaptive advantages of DAMPs and their integration into preexisting signaling pathways. Constitutive DAMPs (cDAMPs), e.g., extracellular ATP, histones, and self-DNA, fulfill primary, conserved functions and adopt a signaling role only when cellular damage causes their fragmentation or localization to aberrant compartments. By contrast, immunomodulatory peptides (also known as phytocytokines) exclusively function as signals and, upon damage, are activated as inducible DAMPs (iDAMPs). Dynamic coevolutionary processes between the signals and their emerging receptors and shared co-receptors have likely linked danger recognition to preexisting, conserved downstream pathways.

Caterpillar-Induced Volatile Emissions in Cotton: The Relative Importance of Damage and Insect-Derived Factors

In response to herbivore attack, plants release large amounts of volatiles that can serve as attractants for the natural enemies of the attacking herbivores. Such responses are typically triggered by damage- and insect-associated factors. Cotton plants are somewhat peculiar because they release specific blends of volatiles in two waves in response to caterpillar attack. They first emit constitutively stored volatile compounds, and after about 24 h a second wave that includes various de novo synthesized compounds. The relative importance of damage-associated and insect associated-factors in this induction of cotton volatile emissions is not yet fully clear. We evaluated how cotton plants respond to mechanical damage and to the application of the oral secretion from the generalist lepidopteran pest Spodoptera exigua, by measuring the local and systemic emissions of volatile compounds from their leaves. Our results confirm that cotton plants respond to damage-associated molecular patterns (DAMPs) as well as to herbivore-associated molecular patterns (HAMPs) present in the caterpillars' oral secretion. Interestingly, a stronger response was observed for cotton plants that were treated with oral secretion from cotton-fed caterpillars than those fed on maize. We tested the possibility that volicitin, a common fatty acid-derived elicitor in caterpillar regurgitant plays a role in this difference. Volicitin and volicitin-like compounds were detected in equal amounts in the oral secretion of S. exigua fed on either cotton or maize leaves. We conclude that other elicitors must be involved. The identification of these eliciting cues is expected to contribute to the development of novel strategies to enhance the resistance of cotton plants to insect pests.

Seasonal and elevational variability in the induction of specialized compounds from mountain birch (Betula pubescens var. pumila) by winter moth larvae (Operophtera brumata)

The mountain birch [Betula pubescens var. pumila (L.)] forest in the Subarctic is periodically exposed to insect outbreaks, which are expected to intensify due to climate change. To mitigate abiotic and biotic stresses, plants have evolved chemical defenses, including volatile organic compounds (VOCs) and non-volatile specialized compounds (NVSCs). Constitutive and induced production of these compounds, however, are poorly studied in Subarctic populations of mountain birch. Here, we assessed the joint effects of insect herbivory, elevation and season on foliar VOC emissions and NVSC contents of mountain birch. The VOCs were sampled in situ by an enclosure technique and analyzed by gas chromatography-mass spectrometry. NVSCs were analyzed by liquid chromatography-mass spectrometry using an untargeted approach. At low elevation, experimental herbivory by winter moth larvae (Operophtera brumata) increased emissions of monoterpenes and homoterpenes over the 3-week feeding period, and sesquiterpenes and green leaf volatiles at the end of the feeding period. At high elevation, however, herbivory augmented only homoterpene emissions. The more pronounced herbivory effects at low elevation were likely due to higher herbivory intensity. Of the individual compounds, linalool, ocimene, 4,8-dimethylnona-1,3,7-triene, 2-methyl butanenitrile and benzyl nitrile were among the most responsive compounds in herbivory treatments. Herbivory also altered foliar NVSC profiles at both low and high elevations, with the most responsive compounds likely belonging to fatty acyl glycosides and terpene glycosides. Additionally, VOC emissions from non-infested branches were higher at high than low elevation, particularly during the early season, which was mainly driven by phenological differences. The VOC emissions varied substantially over the season, largely reflecting the seasonal variations in temperature and light levels. Our results suggest that if insect herbivory pressure continues to rise in the mountain birch forest with ongoing climate change, it will significantly increase VOC emissions with important consequences for local trophic interactions and climate.

Plant electrical signals: A multidisciplinary challenge

Plant electrical signals, an early event in the plant-stimulus interaction, rapidly transmit information generated by the stimulus to other organs, and even the whole plant, to promote the corresponding response and trigger a regulatory cascade. In recent years, many promising state-of-the-art technologies applicable to study plant electrophysiology have emerged. Research focused on expression of genes associated with electrical signals has also proliferated. We propose that it is appropriate for plant electrical signals to be considered in the form of a "plant electrophysiological phenotype". This review synthesizes research on plant electrical signals from a novel, interdisciplinary perspective, which is needed to improve the efficient aggregation and use of plant electrical signal data and to expedite interpretation of plant electrical signals.

Light acts as a stressor and influences abiotic and biotic stress responses in plants

Light is important for plants as an energy source and a developmental signal, but it can also cause stress to plants and modulates responses to stress. Excess and fluctuating light result in photoinhibition and reactive oxygen species (ROS) accumulation around photosystems II and I, respectively. Ultraviolet light causes photodamage to DNA and a prolongation of the light period initiates the photoperiod stress syndrome. Changes in light quality and quantity, as well as in light duration are also key factors impacting the outcome of diverse abiotic and biotic stresses. Short day or shady environments enhance thermotolerance and increase cold acclimation. Similarly, shade conditions improve drought stress tolerance in plants. Additionally, the light environment affects the plants' responses to biotic intruders, such as pathogens or insect herbivores, often reducing growth-defence trade-offs. Understanding how plants use light information to modulate stress responses will support breeding strategies to enhance crop stress resilience. This review summarizes the effect of light as a stressor and the impact of the light environment on abiotic and biotic stress responses. There is a special focus on the role of the different light receptors and the crosstalk between light signalling and stress response pathways.

Single-stranded oligodeoxynucleotides induce plant defence in Arabidopsis thaliana

BACKGROUND AND AIMS

Single-stranded DNA oligodeoxynucleotides (ssODNs) have been shown to elicit immune responses in mammals. In plants, RNA and genomic DNA can activate immunity, although the exact mechanism through which they are sensed is not clear. The aim of this work was to study the possible effect of ssODNs on plant immunity.

KEY RESULTS

The ssODNs IMT504 and 2006 increased protection against the pathogens Pseudomonas syringae pv. tomato DC3000 and Botrytis cinerea but not against tobacco mosaic virus-Cg when infiltrated in Arabidopsis thaliana. In addition, ssODNs inhibited root growth and promoted stomatal closure in a concentration-dependent manner, with half-maximal effective concentrations between 0.79 and 2.06 µm. Promotion of stomatal closure by ssODNs was reduced by DNase I treatment. It was also diminished by the NADPH oxidase inhibitor diphenyleneiodonium and by coronatine, a bacterial toxin that inhibits NADPH oxidase-dependent reactive oxygen species (ROS) synthesis in guard cells. In addition it was found that ssODN-mediated stomatal closure was impaired in bak1-5, bak1-5/bkk1, mpk3 and npr1-3 mutants. ssODNs also induced early expression of MPK3, WRKY33, PROPEP1 and FRK1 genes involved in plant defence, an effect that was reduced in bak1-5 and bak1-5/bkk1 mutants.

CONCLUSIONS

ssODNs are capable of inducing protection against pathogens through the activation of defence genes and promotion of stomatal closure through a mechanism similar to that of other elicitors of plant immunity, which involves the BAK1 co-receptor, and ROS synthesis.

Spider Mites Cause More Damage to Tomato in the Dark When Induced Defenses Are Lower

Plants have evolved robust mechanisms to cope with incidental variation (e.g. herbivory) and periodical variation (e.g. light/darkness during the day-night cycle) in their environment. It has been shown that a plant's susceptibility to pathogens can vary during its day-night cycle. We demonstrated earlier that the spider mite Tetranychus urticae induces jasmonate- and salicylate-mediated defenses in tomato plants while the spider mite T. evansi suppresses these defenses probably by secreting salivary effector proteins. Here we compared induction/suppression of plant defenses; the expression of mite-effector genes and the amount of damage due to mite feeding during the day and during the night. T. urticae feeding upregulated the expression of jasmonate and salicylate marker-genes albeit significantly higher under light than under darkness. Some of these marker-genes were also upregulated by T. evansi-feeding albeit to much lower levels than by T. urticae-feeding. The expression of effector 28 was not affected by light or darkness in either mite species. However, the expression of effector 84 was considerably higher under light, especially for T. evansi. Finally, while T. evansi produced overall more feeding damage than T. urticae both mites produced consistently more damage during the dark phase than under light. Our results suggest that induced defenses are subject to diurnal variation possibly causing tomatoes to incur more damage due to mite-feeding during the dark phase. We speculate that mites, but especially T. evansi, may relax effector production during the dark phase because under these conditions the plant's ability to upregulate defenses is reduced.

Prolonged oxygen depletion in microwounded cells of Chara corallina detected with novel oxygen nanosensors

Primary physicochemical steps in microwounding of plants were investigated using electrochemical nano- and microprobes, with a focus on the role of oxygen in the wounding responses of individual plant cells. Electrochemical measurements of cell oxygen content were made with carbon-filled quartz micropipettes with platinum-coated tips (oxygen nanosensors). These novel platinum nanoelectrodes are useful for understanding cell oxygen metabolism and can be employed to study the redox biochemistry and biology of cells, tissues and organisms. We show here that microinjury of Chara corallina internodal cells with the tip of a glass micropipette is associated with a drastic decrease in oxygen concentration at the vicinity of the stimulation site. This decrease is reversible and lasts for up to 40 minutes. Membrane stretching, calcium influx, and cytoskeleton rearrangements were found to be essential for the localized oxygen depletion induced by cell wall microwounding. Inhibition of electron transport in chloroplasts or mitochondria did not affect the magnitude or timing of the observed response. In contrast, the inhibition of NADPH oxidase activity caused a significant reduction in the amplitude of the decrease in oxygen concentration. We suggest that the observed creation of localized anoxic conditions in response to cell wall puncture might be mediated by NADPH oxidase.

Plant-Plant Communication: Is There a Role for Volatile Damage-Associated Molecular Patterns?

Damage-associated molecular patterns (DAMPs) are an ancient form of tissue-derived danger or alarm signals that initiate cellular signaling cascades, which often initiate defined defense responses. A DAMP can be any molecule that is usually not exposed to cells such as cell wall components, peptides, nucleic acid fragments, eATP and other compounds. DAMPs might be revealed upon tissue damage or during attack. Typically, DAMPs are derived from the injured organism. Almost all eukaryotes can generate and respond to DAMPs, including plants. Besides the molecules mentioned, certain volatile organic compounds (VOCs) can be considered as DAMPs. Due to their chemical nature, VOCs are supposed to act not only locally and systemically in the same plant but also between plants. Here, we focus on damage-induced volatiles (DIVs) that might be regarded as DAMPs; we will review their origin, chemical nature, physiochemical properties, biological relevance and putative function in plant-plant communications. Moreover, we discuss the possibility to use such airborne DAMPs as eco-friendly compounds to stimulate natural defenses in agriculture in order to avoid pesticides.

Salivary DNase II from Laodelphax striatellus acts as an effector that suppresses plant defence

Extracellular DNA, released by damaged plant cells, acts as a damage-associated molecular pattern (DAMP). We demonstrated previously that the small brown planthopper (Laodelphax striatellus, SBPH) secreted DNase II when feeding on artificial diets. However, the function of DNase II in insect feeding remained elusive. The influences of DNase II on SBPHs and rice plants were investigated by suppressing expression of DNase II or by application of heterogeneously expressed DNase II. We demonstrated that DNase II is mainly expressed in the salivary gland and is responsible for DNA-degrading activity of saliva. Knocking down the expression of DNase II resulted in decreased performance of SBPH reared on rice plants. The dsDNase II-treated SBPH did not influenced jasmonic acid (JA), salicylic acid (SA), ethylene (ET) pathways, but elicited a higher level of H2 O2 and callose accumulation. Application of heterogeneously expressed DNase II in DNase II-deficient saliva slightly reduced the wound-induced defence response. We propose a DNase II-based invading model for SBPH feeding on host plants, and provide a potential target for pest management.

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.

Pathogen-Mediated Tritrophic Interactions: Baculovirus-Challenged Caterpillars Induce Higher Plant Defenses than Healthy Caterpillars

Although the tritrophic interactions of plants, insect herbivores and their natural enemies have been intensely studied for several decades, the roles of entomopathogens in their indirect modulation of plant-insect relationships is still unclear. Here, we employed a sublethal dose of a baculovirus with a relatively broad host range (AcMNPV) to explore if feeding by baculovirus-challenged Helicoverpa zea caterpillars induces direct defenses in the tomato plant. We examined induction of plant defenses following feeding by H. zea, including tomato plants fed on by healthy caterpillars, AcMNPV-challenged caterpillars, or undamaged controls, and subsequently compared the transcript levels of defense related proteins (i.e., trypsin proteinase inhibitors, peroxidase and polyphenol oxidase) and other defense genes (i.e., proteinase inhibitor II and cysteine proteinase inhibitor) from these plants, in addition to comparing caterpillar relative growth rates. As a result, AcMNPV-challenged caterpillars induced the highest plant anti-herbivore defenses. We examined several elicitors and effectors in the secretions of these caterpillars (i.e., glucose oxidase, phospholipase C, and ATPase hydrolysis), which surprisingly did not differ between treatments. Hence, we suggest that the greater induction of plant defenses by the virus-challenged caterpillars may be due to differences in the amount of these secretions deposited during feeding or to some other unknown factor(s).

Chemical defense responses of upland cotton, Gossypium hirsutum L. to physical wounding

Upland cotton (Gossypium hirsutum L.) produces terpenoid aldehydes (TAs) that protect the plant from microbial and insect infestations. Foliar TAs include plus (+)- and minus (-)-gossypol, hemigossypolone, and heliocides. To examine foliar TAs' response to physical wounding, the four TA derivatives of a fully glanded G. hirsutum variety JACO GL were quantified by ultra-high performance liquid chromatography. The results show that foliar heliocides increased by 1.7-fold in younger leaves after wounding. While the hemigossypolone level was not affected by the physical wounding, the level of heliocides was significantly increased up to 1.8-fold in the younger leaves. Upland cotton accumulates concentrated carbohydrates, amino acids, and fatty acids in foliar extrafloral nectar (EFN) to serve as a nutrient resource, which attracts both beneficial insects and damaging pests. To better understand the nectar physiology, particularly to determine the temporal dynamics of EFN metabolites in response to the wounding, a gas chromatograph-mass spectrometer (GC-MS) was used to perform metabolic profiling analyses of a G. hirsutum variety Deltapine 383 that has fully developed extrafloral nectaries. A total of 301 compounds were monitored, specifically 75 primary metabolites, two secondary metabolites and 224 unidentified compounds. The physical wounding treatment changed the EFN composition and lowered overall production. The accumulation of 30 metabolites was altered in response to the wounding treatment and threonic acid levels increased consistently. GC-MS combined with Kovat's analysis enabled identification of EFN secondary metabolites including furfuryl alcohol and 5-hyrdomethoxyfurfural, which both have antioxidant and antimicrobial properties that may protect the nectar against microbial pathogens. This study provides new insights into the wounding response of cotton plants in terms of cotton metabolites found in leaf glands and extrafloral nectar as well as highlighting some protective functions of secondary metabolites produced in foliar glands and extrafloral nectaries.

Trichoderma atroviride from Predator to Prey: Role of the Mitogen-Activated Protein Kinase Tmk3 in Fungal Chemical Defense against Fungivory by Drosophila melanogaster Larvae

Fungi, like other organisms, have natural predators, including fungivorous nematodes and arthropods that use them as an important food source. Thus, they require mechanisms to detect and respond to injury. Trichoderma atroviride responds to mycelial injury by rapidly regenerating its hyphae and developing asexual reproduction structures. Whether this injury response is associated with attack by fungivorous insects is unknown. Therefore, determining the possible conservation of a defense mechanism to predation in T. atroviride and plants and elucidating the mechanisms involved in the establishment of this response is of major interest. Here, we describe the chemical response of T. atroviride to mechanical injury and fungivory and the role of a MAPK pathway in the regulation of this response.

The response to injury represents an important strategy for animals and plants to survive mechanical damage and predation. Plants respond to injury by activating a defense response that includes the production of an important variety of compounds that help them withstand predator attack and recover from mechanical injury (MI). Similarly, the filamentous fungus Trichoderma atroviride responds to MI by strongly modifying its transcriptional profile and producing asexual reproduction structures (conidia). Here, we analyzed whether the response to MI in T. atroviride is related to a possible predator defense mechanism from a metabolic perspective. We found that the production of specific groups of secondary metabolites increases in response to MI but is reduced after fungivory by Drosophila melanogaster larvae. We further show that fungivory results in repression of the expression of genes putatively involved in the regulation of secondary metabolite production in T. atroviride. Activation of secondary metabolite production appears to depend on the mitogen-activated protein kinase (MAPK) Tmk3. Interestingly, D. melanogaster larvae preferred to feed on a tmk3 gene replacement mutant rather than on the wild-type strain. Consumption of the mutant strain, however, resulted in increased larval mortality.

IMPORTANCE Fungi, like other organisms, have natural predators, including fungivorous nematodes and arthropods that use them as an important food source. Thus, they require mechanisms to detect and respond to injury. Trichoderma atroviride responds to mycelial injury by rapidly regenerating its hyphae and developing asexual reproduction structures. Whether this injury response is associated with attack by fungivorous insects is unknown. Therefore, determining the possible conservation of a defense mechanism to predation in T. atroviride and plants and elucidating the mechanisms involved in the establishment of this response is of major interest. Here, we describe the chemical response of T. atroviride to mechanical injury and fungivory and the role of a MAPK pathway in the regulation of this response.

Nucleic Acid Sensing in Mammals and Plants: Facts and Caveats

The accumulation of nucleic acids in aberrant compartments is a signal of danger: fragments of cytosolic or extracellular self-DNA indicate cellular dysfunctions or disruption, whereas cytosolic fragments of nonself-DNA or RNA indicate infections. Therefore, nucleic acids trigger immunity in mammals and plants. In mammals, endosomal Toll-like receptors (TLRs) sense single-stranded (ss) or double-stranded (ds) RNA or CpG-rich DNA, whereas various cytosolic receptors sense dsDNA. Although a self/nonself discrimination could favor targeted immune responses, no sequence-specific sensing of nucleic acids has been reported for mammals. Specific immune responses to extracellular self-DNA versus DNA from related species were recently reported for plants, but the underlying mechanism remains unknown. The subcellular localization of mammalian receptors can favor self/nonself discrimination based on the localization of DNA fragments. However, autoantibodies and diverse damage-associated molecular patterns (DAMPs) shuttle DNA through membranes, and most of the mammalian receptors share downstream signaling elements such as stimulator of interferon genes (STING) and the master transcription regulators, nuclear factor (NF)-κB, and interferon regulatory factor 3 (IRF3). The resulting type I interferon (IFN) response stimulates innate immunity against multiple threats-from infection to physical injury or endogenous DNA damage-all of which lead to the accumulation of eDNA or cytoplasmatic dsDNA. Therefore, no or only low selective pressures might have favored a strict self/nonself discrimination in nucleic acid sensing. We conclude that the discrimination between self- and nonself-DNA is likely to be less strict-and less important-than assumed originally.

Effects of extracellular ATP on local and systemic responses of bean (Phaseolus vulgaris L) leaves to wounding

Wounding increased the extracellular Adenosine 5'-triphosphate (eATP) level of kidney bean leaves. Treatment with wounding or exogenous ATP increased the hydrogen peroxide (H₂O₂) content, activities of catalase and polyphenol oxidase, and malondialdehyde content in both the treated and systemic leaves. Pre-treatment with ATP-degrading enzyme, apyrase, to the wounded leaves reduced the wound-induced local and systemic increases in H₂O₂ content, activities of catalase and polyphenol oxidase, and malondialdehyde content. Application of dimethylthiourea (DMTU) and diphenylene iodonium (DPI) to the wounded and ATP-treated leaves, respectively, reduced the wound- and ATP-induced local and systemic increases in H₂O₂ content, activities of catalase and polyphenol oxidase, and malondialdehyde content. Moreover, the wound- and ATP-induced systemic increases of these physiological parameters were suppressed when DMTU or DPI applied to leaf petiole of the wounded and ATP-treated leaves. These results suggest that eATP at wounded sites could mediate the wound-induced local and systemic responses by H₂O₂-dependent signal transduction.

Novel Avenues for Plant Protection: Plant Propagation by Somatic Embryogenesis Enhances Resistance to Insect Feeding

Somatic embryogenesis (SE), a clonal propagation method utilizing somatic cells, occurs under conditions that activate plant stress adaptation mechanisms such as production of protective secondary metabolites. Surprisingly, possible differences in susceptibility to insect pests between SE-generated and conventionally cultivated plants have not been previously explored. Here, we recorded frequencies and levels of bark-feeding damage by pine weevils (Hylobius abietis) in two large field trials, consisting of emblings (SE-propagated plants) and seedlings from 50 half-sib Norway spruce (Picea abies) families. We found that emblings were less frequently attacked by pine weevils, and when attacked, they were damaged to a lesser extent than seedlings. Moreover, we detected significant additive genetic variation in damage levels received by plants, indicating a heritable component to differences in resistance to insect herbivory among half-sib families. We present first-time evidence that emblings can be more resistant than seedlings to herbivorous insect damage, thus, SE appears to confer a previously unknown plant protection advantage. This finding indicates novel avenues to explore mechanisms underlying plant resistance and new approaches to develop non-toxic measures against insect pests.

Extracellular self-DNA as a damage-associated molecular pattern (DAMP) that triggers self-specific immunity induction in plants

Mammals sense self or non-self extracellular or extranuclear DNA fragments (hereinafter collectively termed eDNA) as indicators of injury or infection and respond with immunity. We hypothesised that eDNA acts as a damage-associated molecular pattern (DAMP) also in plants and that it contributes to self versus non-self discrimination. Treating plants and suspension-cultured cells of common bean (Phaseolus vulgaris) with fragmented self eDNA (obtained from other plants of the same species) induced early, immunity-related signalling responses such as H₂O₂ generation and MAPK activation, decreased the infection by a bacterial pathogen (Pseudomonas syringae) and increased an indirect defence to herbivores (extrafloral nectar secretion). By contrast, non-self DNA (obtained from lima bean, Phaseolus lunatus, and Acacia farnesiana) had significantly lower or no detectable effects. Only fragments below a size of 700 bp were active, and treating the eDNA preparation DNAse abolished its inducing effects, whereas treatment with RNAse or proteinase had no detectable effect. These findings indicate that DNA fragments, rather than small RNAs, single nucleotides or proteins, accounted for the observed effects. We suggest that eDNA functions a DAMP in plants and that plants discriminate self from non-self at a species-specific level. The immune systems of plants and mammals share multiple central elements, but further work will be required to understand the mechanisms and the selective benefits of an immunity response that is triggered by eDNA in a species-specific manner.

Moth oviposition shapes the species-specific transcriptional and phytohormonal response of Nicotiana attenuata to larval feeding

Oviposition by lepidopteran herbivores on Nicotiana attenuata primes plant defence responses that are induced by the feeding larvae. While oviposition by both the generalist Spodoptera exigua and the specialist Manduca sexta primes the production of defensive phenylpropanoids, their larvae are differentially affected. We investigate here the impact of prior oviposition on the transcriptome and phytohormone levels of plants that were later attacked by larvae to find regulatory signals of this priming. In a full-factorial design, we evaluated the effects of oviposition and herbivory by both species. Oviposition alone had only subtle effects at the transcriptional level. Laval feeding alone induced species-specific plant responses. Larvae of the generalist regulated phytohormones and gene expression stronger than larvae of the specialist. A day after larvae started to feed, we detected no significant alterations of the plant's response to larval feeding due to prior oviposition by conspecific moths. Yet, oviposition by each of the species profoundly influenced the plant's transcriptional and phytohormonal response to feeding larvae of the other species. Remarkably, the species-specific plant responses to larval feeding shifted towards the response normally elicited by larvae of the ovipositing species. Thus, plants may already recognise an insect's identity upon its oviposition.

The responses of photosystem II and intracellular ATP production of Arabidopsis leaves to salt stress are affected by extracellular ATP

Hypertonic salt stress with different concentrations of NaCl increased the levels of extracellular ATP of Arabidopsis leaves. And, hypertonic salt stress decreased the levels of F _v /F _m (the maximal efficiency of photosystem II), Φ _PSII (the photosystem II operating efficiency), qP (photochemical quenching), and intracellular ATP (iATP) production. The treatment with β,γ-methyleneadenosine 5'-triphosphate (AMP-PCP), which can exclude extracellular ATP from its binding sites of extracellular ATP receptors, caused a further decrease in the levels of F _v /F _m , Φ _PSII, qP, and iATP production of the salt-stressed Arabidopsis leaves, while the addition of exogenous ATP rescued the inhibitory effects of AMP-PCP on Φ _PSII , qP, and iATP production under hypertonic salt stress. Under hypertonic salt stress, the values of F _v /F _m , Φ _PSII , qP, and iATP production were lower in the dorn 1-3 mutant than in the wild-type plants. These results indicate that the responses of photosystem II and intracellular ATP production to salt stress could be affected by extracellular ATP.

Plant ion channels and transporters in herbivory-induced signalling

In contrast to many biotic stresses that plants face, feeding by herbivores produces unique mechanical and chemical signatures. Plants have evolved effective systems to recognise these mechanical stimuli and chemical elicitors at the plasma membrane (PM), where this recognition generates ion fluxes, including an influx of Ca2+ that elicits cellular Ca2+ signalling, production of reactive oxygen species (ROS), and variation in transmembrane potential. These signalling events also function in propagation of long-distance signals (Ca2+ waves, ROS waves, and electrical signals), which contribute to rapid, systemic induction of defence responses. Recent studies have identified several candidate channels or transporters that likely produce these ion fluxes at the PM. Here, we describe the important roles of these channels/transporters in transduction or transmission of herbivory-induced early signalling events, long-distance signals, and jasmonic acid and green leaf volatile signalling in plants.

The fructan syndrome: Evolutionary aspects and common themes among plants and microbes

Fructans are multifunctional fructose-based water soluble carbohydrates found in all biological kingdoms but not in animals. Most research has focused on plant and microbial fructans and has received a growing interest because of their practical applications. Nevertheless, the origin of fructan production, the so-called "fructan syndrome," is still unknown. Why fructans only occur in a limited number of plant and microbial species remains unclear. In this review, we provide an overview of plant and microbial fructan research with a focus on fructans as an adaptation to the environment and their role in (a)biotic stress tolerance. The taxonomical and biogeographical distribution of fructans in both kingdoms is discussed and linked (where possible) to environmental factors. Overall, the fructan syndrome may be related to water scarcity and differences in physicochemical properties, for instance, water retaining characteristics, at least partially explain why different fructan types with different branching levels are found in different species. Although a close correlation between environmental stresses and fructan production is quite clear in plants, this link seems to be missing in microbes. We hypothesize that this can be at least partially explained by differential evolutionary timeframes for plants and microbes, combined with potential redundancy effects.

Specificity of herbivore-induced responses in an invasive species, Alternanthera philoxeroides (alligator weed)

Herbivory-induced responses in plants can both negatively affect subsequently colonizing herbivores and mitigate the effect of herbivory on the host. However, it is still less known whether plants exhibit specific responses to specialist and generalist herbivores in non-secondary metabolite traits and how specificity to specialists and generalists differs between invasive and native plant populations. We exposed an invasive plant, Alternanthera philoxeroides, to Agasicles hygrophila (Coleoptera, Chrysomelidae; specialist), Spodoptera litura (Lepidoptera, Noctuidae; generalist), manual clipping, or application of exogenous jasmonic acid and examined both the specificity of elicitation in traits of fitness (e.g., aboveground biomass), morphology (e.g., root:shoot ratio), and chemistry (e.g., C/N ratio and lignin), and specificity of effect on the subsequent performance of A. hygrophila and S. litura. Then, we assessed variation of the specificity between invasive and native populations (USA and Argentina, respectively). The results showed S. litura induced higher branching intensity and specific leaf area but lower C/N ratio than A. hygrophila, whereas A. hygrophila induced higher trichome density than S. litura. The negative effect of induction on subsequent larval growth was greater for S. litura than for A. hygrophila. Invasive populations had a weaker response to S. litura than to A. hygrophila in triterpenoid saponins and C/N ratio, while native populations responded similarly to these two herbivores. The specific effect on the two herbivores feeding on induced plants did not vary between invasive and native populations. Overall, we demonstrate specificity of elicitation to specialist and generalist herbivores in non-secondary metabolite traits, and that the generalist is more susceptible to induction than the specialist. Furthermore, chemical responses specific to specialist and generalist herbivores only exist in the invasive populations, consistent with an evolutionary change in specificity in the invasive populations.

Sensing Danger: Key to Activating Plant Immunity

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

Implication of long-distance cytoplasmic transport into dynamics of local pH on the surface of microinjured Chara cells

Cytoplasmic streaming is essential for intracellular communications but its specific functions are not well known. In Chara corallina internodes, long-distance interactions mediated by cyclosis are clearly evident with microscopy-pulse amplitude modulation (PAM) fluorometer under application of localized light (LL) pulses to a remote cell region. Measurements of LL-induced profiles of chlorophyll fluorescence F' at various distances from the LL source suggest that illuminated chloroplasts release into the streaming cytoplasm excess reducing equivalents that are entrained by the fluid flow and transiently reduce the intersystem electron carriers in chloroplasts of downstream shaded areas. The reducing equivalents propagate to distances up to 4.5 mm from the LL source, with the transport rate nearly equal to the velocity of liquid flow. The F' transients disappeared after the arrest of streaming with cytochalasin D and reappeared upon its recovery in washed cells. The F' responses to a distant LL were used as an indicator for the passage of cytosolic reductants across the analyzed cell area during measurements of cell surface pH (pH_o) in intact and microperforated internodes. In microwounded cell regions, the LL-induced increase in F' occurred synchronously with the increase in pH_o, by contrast to a slight decrease in pH_o observed prior to perforation. The results show that reducing agents transported with the cytoplasmic flow are involved in rapid pH changes on the surface of microinjured cells. A possibility is considered that cytoplasmic reductants are processed by stress-activated plasmalemmal NADPH oxidase carrying electrons to oxygen with the eventual H⁺ consumption on the outer cell side.

Characterization and Comparative Expression Profiling of Browning Response in Medinilla formosana after Cutting

Plant browning is a recalcitrant problem for in vitro culture and often leads to poor growth of explants and even failure of tissue culture. However, the molecular mechanisms underlying browning-induced physiological processes remain unclear. Medinilla is considered one of the most difficult genera for tissue culture owning to its severe browning. In the present study, intact aseptic plantlets of Medinilla formosana Hayata previously obtained by ovary culture, were used to explore the characteristics and molecular mechanism of the browning response. Successive morphological and anatomical observations after cutting showed that the browning of M. formosana was not lethal but adaptive. De novo transcriptome and digital gene expression (DGE) profiling using Illumina high-throughput sequencing were then used to explore molecular regulation after cutting. About 7.5 million tags of de novo transcriptome were obtained and 58,073 unigenes were assembled and annotated. A total of 6,431 differentially expressed genes (DEGs) at three stages after cutting were identified, and the expression patterns of these browning-related genes were clustered and analyzed. A number of putative DEGs involved in signal transduction and secondary metabolism were particularly studied and the potential roles of these cutting-responsive mRNAs in plant defense to diverse abiotic stresses are discussed. The DGE profiling data were also validated by quantitative RT-PCR analysis. The data obtained in this study provide an excellent resource for unraveling the molecular mechanisms of browning processes during in vitro tissue culture, and lay a foundation for future studies to inhibit and eliminate browning damage.

Modulation of plant defense responses to herbivores by simultaneous recognition of different herbivore-associated elicitors in rice

Induced plant defense responses against insect herbivores are triggered by wounding and/or perception of herbivore elicitors from their oral secretions (OS) and/or saliva. In this study, we analyzed OS isolated from two rice chewing herbivores, Mythimna loreyi and Parnara guttata. Both types of crude OS had substantial elicitor activity in rice cell system that allowed rapid detection of early and late defense responses, i.e. accumulation of reactive oxygen species (ROS) and defense secondary metabolites, respectively. While the OS from M. loreyi contained large amounts of previously reported insect elicitors, fatty acid-amino acid conjugates (FACs), the elicitor-active P. guttata's OS contained no detectable FACs. Subsequently, elicitor activity associated with the high molecular mass fraction in OS of both herbivores was identified, and shown to promote ROS and metabolite accumulations in rice cells. Notably, the application of N-linolenoyl-Gln (FAC) alone had only negligible elicitor activity in rice cells; however, the activity of isolated elicitor fraction was substantially promoted by this FAC. Our results reveal that plants integrate various independent signals associated with their insect attackers to modulate their defense responses and reach maximal fitness in nature.

Plant-Mediated Systemic Interactions Between Pathogens, Parasitic Nematodes, and Herbivores Above- and Belowground

Plants are important mediators of interactions between aboveground (AG) and belowground (BG) pathogens, arthropod herbivores, and nematodes (phytophages). We highlight recent progress in our understanding of within- and cross-compartment plant responses to these groups of phytophages in terms of altered resource dynamics and defense signaling and activation. We review studies documenting the outcome of cross-compartment interactions between these phytophage groups and show patterns of cross-compartment facilitation as well as cross-compartment induced resistance. Studies involving soilborne pathogens and foliar nematodes are scant. We further highlight the important role of defense signaling loops between shoots and roots to activate a full resistance complement. Moreover, manipulation of such loops by phytophages affects systemic interactions with other plant feeders. Finally, cross-compartment-induced changes in root defenses and root exudates extend systemic defense loops into the rhizosphere, enhancing or reducing recruitment of microbes that induce systemic resistance but also affecting interactions with root-feeding phytophages.

Sources of specificity in plant damaged-self recognition

Plants perceive injury and herbivore attack via the recognition of damage-associated molecular patterns (DAMPs) and herbivore-associated molecular patterns (HAMPs). Although HAMPs in particular are cues that can indicate the presence of a specific enemy, the application of pure DAMPs or HAMPs frequently activates general downstream responses: membrane depolarization, Ca(2+) influxes, oxidative stress, MAPKinase activation and octadecanoid signaling at the molecular level, and the expression of digestion inhibitors, cell wall modifications and other general defenses at the phenotypic level. We discuss the relative benefits of perceiving the non-self versus the damaged-self and of specific versus non-specific responses and suggest that the perception of a complex mixture of DAMPs and HAMPs triggers fine-tuned plant responses. DAMPs such as extracellular ATP (eATP), cell wall fragments, signaling peptides, herbivore-induced volatile organic compounds (HI-VOCs) and eDNA hold the key for a more complete understanding of how plants perceive that and by whom they are attacked.

Physiological function and ecological aspects of fatty acid-amino acid conjugates in insects†

In tritrophic interactions, plants recognize herbivore-produced elicitors and release a blend of volatile compounds (VOCs), which work as chemical cues for parasitoids or predators to locate their hosts. From detection of elicitors to VOC emissions, plants utilize sophisticated systems that resemble the plant–microbe interaction system. Fatty acid–amino acid conjugates (FACs), a class of insect elicitors, resemble compounds synthesized by microbes in nature. Recent evidence suggests that the recognition of insect elicitors by an ancestral microbe-associated defense system may be the origin of tritrophic interactions mediated by FACs. Here we discuss our findings in light of how plants have customized this defense to be effective against insect herbivores, and how some insects have successfully adapted to these defenses.

Are the metabolomic responses to folivory of closely related plant species linked to macroevolutionary and plant-folivore coevolutionary processes?

The debate whether the coevolution of plants and insects or macroevolutionary processes (phylogeny) is the main driver determining the arsenal of molecular defensive compounds of plants remains unresolved. Attacks by herbivorous insects affect not only the composition of defensive compounds in plants but also the entire metabolome. Metabolomes are the final products of genotypes and are constrained by macroevolutionary processes, so closely related species should have similar metabolomic compositions and may respond in similar ways to attacks by folivores. We analyzed the elemental compositions and metabolomes of needles from three closely related Pinus species with distant coevolutionary histories with the caterpillar of the processionary moth respond similarly to its attack. All pines had different metabolomes and metabolic responses to herbivorous attack. The metabolomic variation among the species and the responses to folivory reflected their macroevolutionary relationships, with P. pinaster having the most divergent metabolome. The concentrations of terpenes were in the attacked trees supporting the hypothesis that herbivores avoid plant individuals with higher concentrations. Our results suggest that macroevolutionary history plays important roles in the metabolomic responses of these pine species to folivory, but plant-insect coevolution probably constrains those responses. Combinations of different evolutionary factors and trade-offs are likely responsible for the different responses of each species to folivory, which is not necessarily exclusively linked to plant-insect coevolution.