Ethylene production and peroxidase activity in aphid-infested barley

Physiological and oxidative stress response of carrot (Daucus carota L.) to jumping plant-louse Bactericera trigonica Hodkinson (Hemiptera: Psylloidea) infestation

BACKGROUND

Carrot is an important vegetable crop grown worldwide. The major economic problem in carrot cultivation is yellow disease caused by Bactericera trigonica, which induces biotic stress and has the greatest impact on crop productivity. Comprehensive studies on the mechanism of carrot defense response to biotic stress caused by B. trigonica infestation have yet to be conducted.

METHODS

The changes in photosynthetic pigments, proline, TPC, H2O2 and MDA content, DPPH radical scavenging ability, and antioxidant enzyme activity of SOD, CAT, and POX in carrot leaves in response to insect sex (female and male), rapid response (during the first six hours), and long-term response to B. trigonica infestation were evaluated.

RESULTS

The results of our study strongly suggest that B. trigonica infestation causes significant changes in primary and secondary metabolism and oxidative status of carrot leaves. Photosynthetic pigment content, TPC, and DPPH and CAT activities were significantly reduced in carrot leaves in response to insect infestation. On the other hand, proline, H2O2 content, and the activity of the antioxidant enzymes superoxide dismutase and peroxidase were increased in carrot leaves after B. trigonica infestation. The results indicate that B. trigonica attenuates and delays the oxidative stress responses of carrot, allowing long-term feeding without visible changes in the plant. Carrot responded to long-term B. trigonica infestation with an increase in SOD and POX activity, suggesting that these enzymes may play a key role in plant defense mechanisms.

CONCLUSIONS

This is the first comprehensive study strongly suggesting that B. trigonica infestation causes significant changes in primary and secondary metabolism and an attenuated ROS defense response in carrot leaves that enables long-term insect feeding. The information provides new insights into the mechanisms of carrot protection against B. trigonica infestation.

Bacillus subtilis 26D Triggers Induced Systemic Resistance against Rhopalosiphum padi L. by Regulating the Expression of Genes AGO, DCL and microRNA in Bread Spring Wheat

Bacillus subtilis 26D is a plant growth-promoting endophytic bacteria capable of inducing systemic resistance through the priming mechanism, which includes plant genome reprogramming and the phenomenon of RNA interference (RNAi) and microRNA (miRNAs). The phloem-feeding insect bird cherry-oat aphid Rhopalosiphum padi L. is a serious pest that causes significant damage to crops throughout the world. However, the function of plant miRNAs in the response to aphid infestation remains unclear. The results of this work showed that B. subtilis 26D stimulated aphid resistance in wheat plants, inducing the expression of genes of hormonal signaling pathways ICS, WRKY13, PR1, ACS, EIN3, PR3, and ABI5. In addition, B. subtilis 26D activated the RNAi mechanism and regulated the expression of nine conserved miRNAs through activation of the ethylene, salicylic acid (SA), and abscisic acid (ABA) signaling pathways, which was demonstrated by using treatments with phytohormones. Treatment of plants with SA, ethylene, and ABA acted in a similar manner to B. subtilis 26D on induction of the expression of the AGO4, AGO5 and DCL2, DCL4 genes, as well as the expression of nine conserved miRNAs. Different patterns of miRNA expression were found in aphid-infested plants and in plants treated with B. subtilis 26D or SA, ethylene, and ABA and infested by aphids, suggesting that miRNAs play multiple roles in the plant response to phloem-feeding insects, associated with effects on hormonal signaling pathways, redox metabolism, and the synthesis of secondary metabolites. Our study provides new data to further elucidate the fine mechanisms of bacterial-induced priming. However, further extensive work is needed to fully unravel these mechanisms.

Comparison of Rhizospheric and Endophytic Bacterial Compositions between Netted and Oriental Melons

To elucidate the biological mechanism of formation of the netted pattern in melons, the characteristics of the soil bacterial community structure in the rhizosphere and of the endophytic bacteria in the stems of netted melons were analyzed. High-throughput sequencing technology was used for the analysis of plant stem and soil samples collected from netted melons (NM) and oriental melons (OM). At the phylum level, Acidobacteria, Dependentiae, and Chloroflexi were the dominant endophytic bacteria in the stems of NM only. In addition, at the genus level, the soil bacteria enriched in the rhizospheres of NM and OM were different. Five unique dominant bacterial genera, including Gaiella, Actinoplanes, norank_f__Gemmatimonadaceae, Devosia, and Bradyrhizobium, were the dominant soil bacteria unique to the rhizosphere of NM. In contrast, Mycobacterium and unclassified_f__Acetobacteraceae were the dominant soil bacteria in the rhizosphere of OM. Moreover, Hyphomicrobium, Nocardioides, norank_f__norank_o__Gaiellales, Bryobacter, unclassified_f__Pseudonocardiaceae, Pseudolabrys, norank_f__Micropepsaceae, Ideonella, Mizugakiibacter, norank_f__Vermiphilaceae, unclassified_f__Xanthobacteraceae, Bacillus, and Pseudaminobacter were the dominant endophytic bacteria in the stems of NM. In contrast, Flavobacterium, Stenotrophomonas, unclassified_f__Burkholderiaceae, Paenibacillus, Bordetella, Hephaestia, and Ideonella were the dominant endophytic bacteria in the stems of OM. The specific substances (enzymes, proteins, endogenous hormones, etc.) secreted by unique rhizospheric and endophytic bacteria, such as Bacillus and Bradyrhizobium, may activate the promoters of genes. Therefore, the expression of genes can be regulated by unique rhizospheric and endophytic bacteria for formation or nonformation of netting in melons. IMPORTANCE The study of the differential structures and functions of rhizospheric and endophytic bacterial communities between netted melon and oriental melon treatments is investigated. Our findings make a significant contribution to the literature because they are the first step in coupling the study of rhizospheric and endophytic microbial community structure to reticulation formation in netted melon. Further, we believe that this research appears to be meaningful because it provides new insights into the mechanisms of reticulation formation in netted melon in modern agricultural production.

Additive Effect of the Composition of Endophytic Bacteria Bacillus subtilis on Systemic Resistance of Wheat against Greenbug Aphid Schizaphis graminum Due to Lipopeptides

The use of biocontrol agents based on endophytic bacteria against phloem-feeding insects is limited by a lack of knowledge and understanding of the mechanism of action of the endophyte community that makes up the plant microbiome. In this work, the mechanisms of the additive action of endophytic strains B. subtilis 26D and B. subtilis 11VM on the resistance of bread spring wheat against greenbug aphid Schizaphis graminum, was studied. It was shown that B. subtilis 26D secreted lipopeptide surfactin and phytohormones cytokinins, and B. subtilis 11VM produced iturin and auxins into the cultivation medium. Both strains and their lipopeptide-rich fractions showed direct aphicidal activity against greenbug aphid. For the first time, it was shown that B. subtilis 26D and B. subtilis 11VM in the same manner, as well as their lipopeptide-rich fractions, activated the expression of salicylate- and ethylene-dependent PR genes, and influenced plant redox metabolism, which led to an increase in plant endurance against aphids. The composition of endophytic strains B. subtilis 26D + B. subtilis 11VM had an additive effect on plant resistance to aphids due to an increase in the number of endophytic bacterial cells, and, as well as due to the synergistic effect of their mixture of lipopeptides - surfactin + iturin, both on the aphid mortality and on the expression of PR1 and PR3 genes. All these factors can be the reason for the observed increase in the growth of plants affected by aphids under the influence of B. subtilis 26D and B. subtilis 11VM, individually and in composition. The study demonstrates the possibility of creating in the future an artificial composition to enhance plant microbiome with endophytic bacteria, which combines growth-promoting and plant immunity stimulating properties against phloem-feeding insects. This direction is one of the most promising approaches to green pesticide discovery in the future.

Associational susceptibility of crop plants caused by the invasive weed Canadian goldenrod, Solidago canadensis, via local aphid species

In the present study, field surveys were conducted to detect potential associational susceptibility of crop plants (potato, alfalfa and maize) caused by Canadian goldenrod, Solidago canadensis (L.), one of the most common invasive weeds in Central and Eastern Europe, via plant associated virus vectoring aphids. Assessments were made in two major agricultural land types: crops grown under high input vs. low-input (LIF) conditions, with and without fertilizers and pesticides. The two most frequent aphid species, found both on Canadian goldenrod and crop plants, were the leaf-curling plum aphid, Brachycaudus helichrysi Kaltenbach and the glasshouse-potato aphid, Aulacorthum solani (Kaltenbach). Plant viruses in both weed and crop plants were identified using high-throughput sequencing of small RNAs. Peroxidase (POD) enzyme activity was tested in weed and crop plants to connect aphids feeding processes in weeds and crops. In addition, conceptual modeling was used to detect direct relationships between viruses and other crops. The Canadian goldenrod density was only relevant in the LIF regime. Furthermore, its association with B. helichrysi and associational susceptibility was detected only in LIF. In total, 18 viruses comprising 17 plant and one insect virus were detected in Canadian goldenrod, of which 11 were also detected in potato and alfalfa crops. POD activity was high and correlated with high aphid density in both weed and crop plants, suggesting a direct associational susceptibility between these plants through aphid infestation and viral transmission.

Copper Chaperone for Superoxide Dismutase FoCCS1 in Frankliniella occidentalis May Be Associated with Feeding Adaptation after Host Shifting

Western flower thrips (Frankliniella occidentalis) pose a serious threat to the global vegetable and flower crop production. The regulatory mechanism for superoxide dismutase (SOD) in the feeding adaptation of F. occidentalis after host shifting remains unclear. In this study, the copper chaperone for SOD (CCS) and manganese SOD (MnSOD) genes in F. occidentalis were cloned, and their expression levels at different developmental stages was determined. The mRNA expression of FoCCS1 and FoMnSOD2 in F. occidentalis second-instar larvae and adult females of F₁, F₂, and F₃ generations was analyzed after shifting the thrips to kidney bean and broad bean plants, respectively. The F₂ and F₃ second-instar larvae and F₂ adult females showed significantly upregulated FoCCS1 mRNA expression after shifting to kidney bean plants. The F₁ second-instar larvae and F₂ adult females showed significantly upregulated FoCCS1 mRNA expression after shifting to broad bean plants. The RNA interference significantly downregulated the FoCCS1 mRNA expression levels and adult females showed significantly inhibited SOD activity after shifting to kidney bean and broad bean plants. F. occidentalis adult females subjected to RNA interference and released on kidney bean and broad bean leaves for rearing, respectively, significantly reduced the survival rate and fecundity. These findings suggest that FoCCS1 plays an active role in regulating the feeding adaptation ability of F. occidentalis after host shifting.

Genetic Resources of Cereal Crops for Aphid Resistance

The genetic resources of cereal crops in terms of resistance to aphids are reviewed. Phytosanitary destabilization led to a significant increase in the harmfulness of this group of insects. The breeding of resistant plant genotypes is a radical, the cheapest, and environmentally safe way of pest control. The genetic homogeneity of crops hastens the adaptive microevolution of harmful organisms. Both major and minor aphid resistance genes of cereal plants interact with insects differentially. Therefore, rational breeding envisages the expansion of the genetic diversity of cultivated varieties. The possibilities of replenishing the stock of effective resistance genes by studying the collection of cultivated cereals, introgression, and creating mutant forms are considered. The interaction of insects with plants is subject to the gene-for-gene relationship. Plant resistance genes are characterized by close linkage and multiple allelism. The realizing plant genotype depends on the phytophage biotype. Information about the mechanisms of constitutional and induced plant resistance is discussed. Resistance genes differ in terms of stability of expression. The duration of the period when varieties remain resistant is not related either to its phenotypic manifestation or to the number of resistance genes. One explanation for the phenomenon of durable resistance is the association of the virulence mutation with pest viability.

Plant Secondary Metabolites as Defense Tools against Herbivores for Sustainable Crop Protection

Plants have evolved several adaptive strategies through physiological changes in response to herbivore attacks. Plant secondary metabolites (PSMs) are synthesized to provide defensive functions and regulate defense signaling pathways to safeguard plants against herbivores. Herbivore injury initiates complex reactions which ultimately lead to synthesis and accumulation of PSMs. The biosynthesis of these metabolites is regulated by the interplay of signaling molecules comprising phytohormones. Plant volatile metabolites are released upon herbivore attack and are capable of directly inducing or priming hormonal defense signaling pathways. Secondary metabolites enable plants to quickly detect herbivore attacks and respond in a timely way in a rapidly changing scenario of pest and environment. Several studies have suggested that the potential for adaptation and/or resistance by insect herbivores to secondary metabolites is limited. These metabolites cause direct toxicity to insect pests, stimulate antixenosis mechanisms in plants to insect herbivores, and, by recruiting herbivore natural enemies, indirectly protect the plants. Herbivores adapt to secondary metabolites by the up/down regulation of sensory genes, and sequestration or detoxification of toxic metabolites. PSMs modulate multi-trophic interactions involving host plants, herbivores, natural enemies and pollinators. Although the role of secondary metabolites in plant-pollinator interplay has been little explored, several reports suggest that both plants and pollinators are mutually benefited. Molecular insights into the regulatory proteins and genes involved in the biosynthesis of secondary metabolites will pave the way for the metabolic engineering of biosynthetic pathway intermediates for improving plant tolerance to herbivores. This review throws light on the role of PSMs in modulating multi-trophic interactions, contributing to the knowledge of plant-herbivore interactions to enable their management in an eco-friendly and sustainable manner.

Plant Secondary Metabolites as Defense Tools against Herbivores for Sustainable Crop Protection

Plants have evolved several adaptive strategies through physiological changes in response to herbivore attacks. Plant secondary metabolites (PSMs) are synthesized to provide defensive functions and regulate defense signaling pathways to safeguard plants against herbivores. Herbivore injury initiates complex reactions which ultimately lead to synthesis and accumulation of PSMs. The biosynthesis of these metabolites is regulated by the interplay of signaling molecules comprising phytohormones. Plant volatile metabolites are released upon herbivore attack and are capable of directly inducing or priming hormonal defense signaling pathways. Secondary metabolites enable plants to quickly detect herbivore attacks and respond in a timely way in a rapidly changing scenario of pest and environment. Several studies have suggested that the potential for adaptation and/or resistance by insect herbivores to secondary metabolites is limited. These metabolites cause direct toxicity to insect pests, stimulate antixenosis mechanisms in plants to insect herbivores, and, by recruiting herbivore natural enemies, indirectly protect the plants. Herbivores adapt to secondary metabolites by the up/down regulation of sensory genes, and sequestration or detoxification of toxic metabolites. PSMs modulate multi-trophic interactions involving host plants, herbivores, natural enemies and pollinators. Although the role of secondary metabolites in plant-pollinator interplay has been little explored, several reports suggest that both plants and pollinators are mutually benefited. Molecular insights into the regulatory proteins and genes involved in the biosynthesis of secondary metabolites will pave the way for the metabolic engineering of biosynthetic pathway intermediates for improving plant tolerance to herbivores. This review throws light on the role of PSMs in modulating multi-trophic interactions, contributing to the knowledge of plant-herbivore interactions to enable their management in an eco-friendly and sustainable manner.

Temperature Differentially Influences the Capacity of Trichoderma Species to Induce Plant Defense Responses in Tomato Against Insect Pests

Species of the ecological opportunistic, avirulent fungus, Trichoderma are widely used in agriculture for their ability to protect crops from the attack of pathogenic fungi and for plant growth promotion activity. Recently, it has been shown that they may also have complementary properties that enhance plant defense barriers against insects. However, the use of these fungi is somewhat undermined by their variable level of biocontrol activity, which is influenced by environmental conditions. Understanding the source of this variability is essential for its profitable and wide use in plant protection. Here, we focus on the impact of temperature on Trichoderma afroharzianum T22, Trichoderma atroviride P1, and the defense response induced in tomato by insects. The in vitro development of these two strains was differentially influenced by temperature, and the observed pattern was consistent with temperature-dependent levels of resistance induced by them in tomato plants against the aphid, Macrosiphum euphorbiae, and the noctuid moth, Spodoptera littoralis. Tomato plants treated with T. afroharzianum T22 exhibited enhanced resistance toward both insect pests at 25°C, while T. atroviride P1 proved to be more effective at 20°C. The comparison of plant transcriptomic profiles generated by the two Trichoderma species allowed the identification of specific defense genes involved in the observed response, and a selected group was used to assess, by real-time quantitative reverse transcription PCR (qRT-PCR), the differential gene expression in Trichoderma-treated tomato plants subjected to the two temperature regimens that significantly affected fungal biological performance. These results will help pave the way toward a rational selection of the most suitable Trichoderma isolates for field applications, in order to best face the challenges imposed by local environmental conditions and by extreme climatic shifts due to global warming.

Changes in Aphid-Plant Interactions under Increased Temperature

Thermal stress in living organisms causes an imbalance between the processes of creating and neutralizing reactive oxygen species (ROS). The work aims to explain changes in the aphid-host plant interaction due to an increase in temperature. Tests were carried out at three constant temperatures (20, 25, or 28 °C). Firstly, changes in development of Macrosiphum rosae were determined. Secondly, the activity of enzymatic markers (superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST), β-glucosidase, polyphenol oxidase (PPO), and peroxidase (POD)) in aphid M. rosae tissues and host plant were analyzed at all temperatures. An increase in temperature to 28 °C had a negative effect on the biology of M. rosae by shortening the period of reproduction and longevity, thus reducing the demographic parameters and fecundity. Two stages of the aphid's defensive response to short-term (24-96 h) and long-term (2 weeks) thermal stress were observed. Aphid defense responses varied considerably with temperature and were highest at 28 °C. In turn, for the plants, which were exposed to both abiotic stress caused by elevated temperature and biotic stress caused by aphid feeding, their enzymatic defense was more effective at 20 °C, when enzyme activities at their highest were observed.

Ethylene-Cytokinin Interaction Determines Early Defense Response of Wheat against Stagonospora nodorum Berk

Ethylene, salicylic acid (SA), and jasmonic acid are the key phytohormones involved in plant immunity, and other plant hormones have been demonstrated to interact with them. The classic phytohormone cytokinins are important participants of plant defense signaling. Crosstalk between ethylene and cytokinins has not been sufficiently studied as an aspect of plant immunity and is addressed in the present research. We compared expression of the genes responsible for hormonal metabolism and signaling in wheat cultivars differing in resistance to Stagonospora nodorum in response to their infection with fungal isolates, whose virulence depends on the presence of the necrotrophic effector SnTox3. Furthermore, we studied the action of the exogenous cytokinins, ethephon (2-chloroethylphosphonic acid, ethylene-releasing agent) and 1-methylcyclopropene (1-MCP, inhibitor of ethylene action) on infected plants. Wheat susceptibility was shown to develop due to suppression of reactive oxygen species production and decreased content of active cytokinins brought about by SnTox3-mediated activation of the ethylene signaling pathway. SnTox3 decreased cytokinin content most quickly by its activated glucosylation in an ethylene-dependent manner and, furthermore, by oxidative degradation and inhibition of biosynthesis in ethylene-dependent and ethylene-independent manners. Exogenous zeatin application enhanced wheat resistance against S. nodorum through inhibition of the ethylene signaling pathway and upregulation of SA-dependent genes. Thus, ethylene inhibited triggering of SA-dependent resistance mechanism, at least in part, by suppression of the cytokinin signaling pathway.

Reactive Oxygen Species in Plant Interactions With Aphids

Reactive oxygen species (ROS) such as hydrogen peroxide and superoxide are produced in plants in response to many biotic and abiotic stressors, and they can enhance stress adaptation in certain circumstances or mediate symptom development in others. The roles of ROS in plant-pathogen interactions have been extensively studied, but far less is known about their involvement in plant-insect interactions. A growing body of evidence, however, indicates that ROS accumulate in response to aphids, an economically damaging group of phloem-feeding insects. This review will cover the current state of knowledge about when, where, and how ROS accumulate in response to aphids, which salivary effectors modify ROS levels in plants, and how microbial associates influence ROS induction by aphids. We will also explore the potential adaptive significance of intra- and extracellular oxidative responses to aphid infestation in compatible and incompatible interactions and highlight knowledge gaps that deserve further exploration.

Ethylene Is Not Essential for R-Gene Mediated Resistance but Negatively Regulates Moderate Resistance to Some Aphids in Medicago truncatula

Ethylene is important for plant responses to environmental factors. However, little is known about its role in aphid resistance. Several types of genetic resistance against multiple aphid species, including both moderate and strong resistance mediated by R genes, have been identified in Medicago truncatula. To investigate the potential role of ethylene, a M. truncatula ethylene- insensitive mutant, sickle, was analysed. The sickle mutant occurs in the accession A17 that has moderate resistance to Acyrthosiphon kondoi, A. pisum and Therioaphis trifolii. The sickle mutant resulted in increased antibiosis-mediated resistance against A. kondoi and T. trifolii but had no effect on A. pisum. When sickle was introduced into a genetic background carrying resistance genes, AKR (A. kondoi resistance), APR (A. pisum resistance) and TTR (T. trifolii resistance), it had no effect on the strong aphid resistance mediated by these genes, suggesting that ethylene signaling is not essential for their function. Interestingly, for the moderate aphid resistant accession, the sickle mutant delayed leaf senescence following aphid infestation and reduced the plant biomass losses caused by both A. kondoi and T. trifolii. These results suggest manipulation of the ethylene signaling pathway could provide aphid resistance and enhance plant tolerance against aphid feeding.

Transcriptome analysis reveals rapid defence responses in wheat induced by phytotoxic aphid Schizaphis graminum feeding

BACKGROUND

Schizaphis graminum is one of the most important and devastating cereal aphids worldwide, and its feeding can cause chlorosis and necrosis in wheat. However, little information is available on the wheat defence responses triggered by S. graminum feeding at the molecular level.

RESULTS

Here, we collected and analysed transcriptome sequencing data from leaf tissues of wheat infested with S. graminum at 2, 6, 12, 24 and 48 hpi (hours post infestation). A total of 44,835 genes were either up- or downregulated and differed significantly in response to aphid feeding. The expression levels of a number of genes (9761 genes) were significantly altered within 2 hpi and continued to change during the entire 48 h experiment. Gene Ontology analysis showed that the downregulated DEGs were mainly enriched in photosynthesis and light harvesting, and the total chlorophyll content in wheat leaves was also significantly reduced after S. graminum infestation at 24 and 48 hpi. However, a number of related genes of the salicylic acid (SA)-mediated defence signalling pathway and MAPK-WRKY pathway were significantly upregulated at early feeding time points (2 and 6 hpi). In addition, the gene expression and activity of antioxidant enzymes, such as peroxidase and superoxide dismutase, were rapidly increased at 2, 6 and 12 hpi. DAB staining results showed that S. graminum feeding induced hydrogen peroxide (H2O2) accumulation at the feeding sites at 2 hpi, and increased H2O2 production was detected with the increases in aphid feeding time. Pretreatment with diphenylene iodonium, an NADPH oxidase inhibitor, repressed the H2O2 accumulation and expression levels of SA-associated defence genes in wheat.

CONCLUSIONS

Our transcriptomic analysis revealed that defence-related pathways and oxidative stress in wheat were rapidly induced within hours after the initiation of aphid feeding. Additionally, NADPH oxidase plays an important role in aphid-induced defence responses and H2O2 accumulation in wheat. These results provide valuable insight into the dynamic transcriptomic responses of wheat leaves to phytotoxic aphid feeding and the molecular mechanisms of aphid-plant interactions.

Comparative transcriptome and histological analyses of wheat in response to phytotoxic aphid Schizaphis graminum and non-phytotoxic aphid Sitobion avenae feeding

BACKGROUND

Infestation of the phytotoxic aphid Schizaphis graminum can rapidly induce leaf chlorosis in susceptible plants, but this effect is not observed with the nonphytotoxic aphid Sitobion avenae. However, few studies have attempted to identify the different defence responses induced in wheat by S. graminum and S. avenae feeding and the mechanisms underlying the activation of chlorosis by S. graminum feeding.

RESULTS

S. graminum feeding significantly reduced the chlorophyll content of wheat leaves, and these effects were not observed with S. avenae. A transcriptomic analysis showed that the expression levels of genes involved in the salicylic acid, jasmonic acid and ethylene signalling defence pathways were significantly upregulated by both S. avenae and S. graminum feeding; however, more plant defence genes were activated by S. graminum feeding than S. avenae feeding. The transcript levels of genes encoding cell wall-modifying proteins were significantly increased after S. graminum feeding, but only a few of these genes were induced by S. avenae. Furthermore, various reactive oxygen species-scavenging genes, such as 66 peroxidase (POD) and 8 ascorbate peroxidase (APx) genes, were significantly upregulated after S. graminum feeding, whereas only 15 POD and one APx genes were induced by S. avenae feeding. The activity of four antioxidant enzymes was also significantly upregulated by S. graminum feeding. Cytological examination showed that S. graminum feeding induced substantial hydrogen peroxide (H₂O₂) accumulation in wheat leaves. The chlorosis symptoms and the loss of chlorophyll observed in wheat leaves after S. graminum feeding were reduced and inhibited by the scavenging of H₂O₂ by dimethylthiourea, which indicated that H₂O₂ plays important role in the induction of chlorosis by S. graminum feeding.

CONCLUSIONS

S. graminum and S. avenae feeding induces the JA, SA and ET signalling pathways, but S. graminum activated stronger plant defence responses than S. avenae. S. graminum feeding triggers strong ROS-scavenging activity and massive H₂O₂ production in wheat leaves, and the accumulation of H₂O₂ induced by S. graminum feeding is involved in the activation of chlorosis in wheat leaves. These results enhance our understanding of mechanisms underlying aphid-wheat interactions and provide clues for the development of aphid-resistant wheat varieties.

Comprehensive transcriptome analysis of grafting onto Artemisia scoparia W. to affect the aphid resistance of chrysanthemum (Chrysanthemum morifolium T.)

BACKGROUND

Aphid (Macrosiphoniella sanbourni) stress drastically influences the yield and quality of chrysanthemum, and grafting has been widely used to improve tolerance to biotic and abiotic stresses. However, the effect of grafting on the resistance of chrysanthemum to aphids remains unclear. Therefore, we used the RNA-Seq platform to perform a de novo transcriptome assembly to analyze the self-rooted grafted chrysanthemum (Chrysanthemum morifolium T. 'Hangbaiju') and the grafted Artermisia-chrysanthemum (grafted onto Artemisia scoparia W.) transcription response to aphid stress.

RESULTS

The results showed that there were 1337 differentially expressed genes (DEGs), among which 680 were upregulated and 667 were downregulated, in the grafted Artemisia-chrysanthemum compared to the self-rooted grafted chrysanthemum. These genes were mainly involved in sucrose metabolism, the biosynthesis of secondary metabolites, the plant hormone signaling pathway and the plant-to-pathogen pathway. KEGG and GO enrichment analyses revealed the coordinated upregulation of these genes from numerous functional categories related to aphid stress responses. In addition, we determined the physiological indicators of chrysanthemum under aphid stress, and the results were consistent with the molecular sequencing results. All evidence indicated that grafting chrysanthemum onto A. scoparia W. upregulated aphid stress responses in chrysanthemum.

CONCLUSION

In summary, our study presents a genome-wide transcript profile of the self-rooted grafted chrysanthemum and the grafted Artemisia-chrysanthemum and provides insights into the molecular mechanisms of C. morifolium T. in response to aphid infestation. These data will contribute to further studies of aphid tolerance and the exploration of new candidate genes for chrysanthemum molecular breeding.

Physiological Response of Basil Plants to Twospotted Spider Mite (Acari: Tetranychidae) Infestation

The induction of plant resistance against pests is considered a potential method of controlling mite infestation as it restricts the use of chemical pesticides in herbal crops. Our goal was to investigate whether plant physiological response to mite feeding varied depending on basil cultivar and/or duration of mite infestation. The effect of plant acceptance, mite mortality rate, and changes in physiological parameters: malondialdehyde content (MDA), hydrogen peroxide (H2O2) concentration, and antioxidant enzyme activities, including guaiacol peroxidase (GPX) and catalase (CAT) were examined in this study. Tetranychus urticae Koch (Acari: Tetranychidae) infestation induced oxidative stress in three Ocimum basilicum L. cultivars: 'Sweet basil,' 'Purpurascens,' and 'Fino Verde.' The analysis of mite behavior and alteration in metabolic plant profiles showed different sensitivities of basil cultivars to biotic stress that were dependent on the cultivar and duration of infestation. All basil plants were suitable as host plants for T. urticae, but they varied in the level of susceptibility to mite feeding. O. basilicum 'Fino Verde' was the most suitable host for the twospotted spider mite. In turn, O. basilicum 'Purpurascens' was characterized by the lowest level of susceptibility to T. urticae feeding. The lowest acceptance, the highest mortality of twospotted spider mite individuals as well as decreased levels of H2O2 and MDA, significantly increased GPX activity and low level of CAT activity were recorded in O. basilicum 'Purpurascens' leaves. Research on plant responses to biotic stress can inform breeding cultivars resistant to arthropod attack.

Low and high input agricultural fields have different effects on pest aphid abundance via different invasive alien weed species

We conducted field surveys to detect the population density of the most important invasive weed species and their associated virus vectoring aphids in crops grown under high input field (HIF) vs low-input field (LIF) conditions, with and without fertilizers and pesticides. The most frequent invasive weed species were Stenactisannua, Erigeroncanadensis and Solidagocanadensis. These species were hosts predominantly for the aphids Brachycaudushelichrysi and Aulacorthumsolani in both management systems. The 13% higher coverage of S.annua under LIF conditions resulted in a 30% higher B.helichrysi abundance and ~85% higher A.solani abundance compared with HIF conditions. Host plant quality was assessed by measuring peroxidase enzyme activity. There was a significantly increased POD activity at 10 μmol min−1 mg protein−1 unit in S.annua under LIF conditions, suggesting a higher stress by aphids under this management regime. The high colonization intensity of B.helichrysi on maize, potato and alfalfa crops were detected from both S.annua and E.canadensis. We conclude that new and faster methods need to be used to prevent colonization of such virus vectoring aphids and their host plants, even under low input regimes.

Endosymbionts Differentially Alter Exploratory Probing Behavior of a Nonpersistent Plant Virus Vector

Insect endosymbionts (hereafter, symbionts) can modify plant virus epidemiology by changing the physiology or behavior of vectors, but their role in nonpersistent virus pathosystems remains uninvestigated. Unlike propagative and circulative viruses, nonpersistent plant virus transmission occurs via transient contamination of mouthparts, making direct interaction between symbiont and virus unlikely. Nonpersistent virus transmission occurs during exploratory intracellular punctures with styletiform mouthparts when vectors assess potential host-plant quality prior to phloem feeding. Therefore, we used an electrical penetration graph (EPG) to evaluate plant probing of the cowpea aphid, Aphis craccivora Koch, an important vector of cucurbit viruses, in the presence and absence of two facultative, intracellular symbionts. We tested four isolines of A. craccivora: two isolines were from a clone from black locust (Robinia pseudoacacia L.), one infected with Arsenophonus sp. and one cured, and two derived from a clone from alfalfa (Medicago sativa L.), one infected with Hamiltonella defensa and one cured. We quantified exploratory intracellular punctures, indicated by a waveform potential drop recorded by the EPG, initiation speed and frequency within the initial 15 min on healthy and watermelon mosaic virus-infected pumpkins. Symbiont associations differentially modified exploratory intracellular puncture frequency by aphids, with H. defensa-infected aphids exhibiting depressed probing, and Arsenophonus-infected aphids an increased frequency of probing. Further, there was greater overall aphid probing on virus-infected plants, suggesting that viruses manipulate their vectors to enhance acquisition-transmission rates, independent of symbiont infection. These results suggest facultative symbionts differentially affect plant-host exploration behaviors and potentially nonpersistent virus transmission by vectors.

Arabidopsis Transcription Factor MYB102 Increases Plant Susceptibility to Aphids by Substantial Activation of Ethylene Biosynthesis

Induction of ethylene biosynthesis by aphids increases the susceptibility of several plant species to aphids. Recent studies have indicated that some MYB transcription factors regulate the phloem-based defense against aphid infestation by modulating ethylene (ET) signaling. Arabidopsis MYB102 has previously been shown to be induced by wound signaling and regulate defense response against chewing insects. However, it remains unclear whether ArabidopsisMYB102 takes part in the defense response of plants to aphids. Here, we investigated the function of MYB102 in the response of Arabidopsis to aphid infestation. ArabidopsisMYB102 was primarily expressed in vascular tissues, and its transcription was remarkably induced by green peach aphids (GPA; Myzus persicae). The results of RNA-Sequencing revealed that overexpression of MYB102 in Arabidopsis promoted ET biosynthesis by upregulation of some 1-aminocyclopropane-1-carboxylate synthase (ACS) genes, which are rate-limiting enzymes of the ET-synthetic pathway. Enhanced ET levels led to reduced Arabidopsis resistance to GPA. Furthermore, dominant suppression of MYB102 inhibited aphid-induced increase of ET levels in Arabidopsis. In agreement with a negative regulatory role for ET in aphid defense responses, the MYB102-overexpressing lines were more susceptible to GPA than wild-type (WT) plants. Overexpression of MYB102 in Arabidopsis obviously repressed aphid-induced callose deposition. Conversely, overexpression of MYB102 failed to increase aphid susceptibility in both the ET-insensitive mutants and plants treated with inhibitors of ET signaling pathways, demonstrating that the ET was critical for promoting aphid performance conferred by overexpression of MYB102. Collectively, our findings indicate that the Arabidopsis MYB102 increases host susceptibility to GPA through the ET-dependent signaling pathways.

Nostoc calcicola extract improved the antioxidative response of soybean to cowpea aphid

BACKGROUND

Soybean (Glycine max (L.) Merr. cv. "Nam Dan") is one of the most valuable crops in agricultural production in Nghe An province (Vietnam). Our previous study revealed that extract of the cyanobacterium strain Nostoc calcicola HN9 expressed positive effect on growth and development, and raised soybean productivity (Tran et al. in Proceeding of Vietnam national conference of research on biology, Da Nang, 2016). We hypothesized that N. calcicola HN9 would improve the defense responses of G. max cv. "Nam Dan" to cowpea aphid (Aphis craccivora Koch)-a serious pest of leguminous crops.

RESULTS

Infestation of A. craccivora caused oxidative stress in leaves of G. max cv. "Nam Dan". A strong generation of endogenous reactive oxygen species (ROS) such as superoxide anion radical (O ₂^·- ) and hydrogen peroxide (H₂O₂) resulted in the cellular damages in the aphid-infested leaves through high levels of injury percentage and lipid peroxidation. To protect from aphid attack themselves, soybean plants triggered the antioxidant defense systems, in which, enzymatic antioxidants such as superoxide dismutase (SOD, 1.15.1.1), catalase (CAT, 1.11.1.6) and GPx (EC 1.11.1.9) were strongly accumulated to reduce the toxic effects of ROS. Components of N. calcicola HN9 extract might strengthen the defensive capability of G. max cv. "Nam Dan" to cowpea aphid infestation via establishing the chemical constraints on oxidative stress. Under effect of cyanobacteria extract, generation of O ₂^·- and H₂O₂ was strictly limited, activities of SOD, CAT and GPx were remarkably accumulated in the aphid-infested leaves leading to a significant reduction of oxidative damages.

CONCLUSIONS

Nostoc calcicola HN9 extract probably not only controlled the generation and effects of O ₂^·- and H₂O₂ but also augmented the accumulated activity of SOD, CAT and GPx in soybean leaves that allowed them to control oxidative stress, contributed to increase the resistance of G. max cv. "Nam Dan" to A. craccivora. The improvement of cyanobacteria extract on the antioxidative response of soybean "Nam Dan" to cowpea aphid can be a novel aspect to contribute to current knowledge regarding the soybean-aphid interaction.

Responses of earthworms and microbial communities in their guts to Triclosan

Responses of the earthworm (Eisenia fetida) and compositions of associated microbial communities were determined after exposure to various concentrations of Triclosan (TCS) for 7 d. Concentrations of TCS were greater in intestines than in epidermis of earthworms, which suggested that earthworms accumulate TCS mainly by ingestion rather than by epidermic penetration. Exposure to TCS caused a concentration-dependent increase in activities of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) and in malondialdehyde (MDA) in E. fetida. Analyses of both the bacterial and eukaryotic community by next generation sequencing (NGS), demonstrated that TCS caused a concentration-dependent decrease in sensitive genera. While relative abundances of Pseudomonas, Stenotrophomonas, and Achromobacter were increased. Nine susceptible microbial groups were more sensitive to exposure to TCS, than were activities of enzymes in earthworms. Thus, rapid genomic measures of gut flora can be used as indicators to assess adverse effects of chemicals on earthworms.

The ethylene response factor Pti5 contributes to potato aphid resistance in tomato independent of ethylene signalling

Ethylene response factors (ERFs) comprise a large family of transcription factors that regulate numerous biological processes including growth, development, and response to environmental stresses. Here, we report that Pti5, an ERF in tomato [Solanum lycopersicum (Linnaeus)] was transcriptionally upregulated in response to the potato aphid Macrosiphum euphorbiae (Thomas), and contributed to plant defences that limited the population growth of this phloem-feeding insect. Virus-induced gene silencing of Pti5 enhanced aphid population growth on tomato, both on an aphid-susceptible cultivar and on a near-isogenic genotype that carried the Mi-1.2 resistance (R) gene. These results indicate that Pti5 contributes to basal resistance in susceptible plants and also can synergize with other R gene-mediated defences to limit aphid survival and reproduction. Although Pti5 contains the ERF motif, induction of this gene by aphids was independent of ethylene, since the ACC deaminase (ACD) transgene, which inhibits ethylene synthesis, did not diminish the responsiveness of Pti5 to aphid infestation. Furthermore, experiments with inhibitors of ethylene synthesis revealed that Pti5 and ethylene have distinctly different roles in plant responses to aphids. Whereas Pti5 contributed to antibiotic plant defences that limited aphid survival and reproduction on both resistant (Mi-1.2+) and susceptible (Mi-1.2-) genotypes, ethylene signalling promoted aphid infestation on susceptible plants but contributed to antixenotic defences that deterred the early stages of aphid host selection on resistant plants. These findings suggest that the antixenotic defences that inhibit aphid settling and the antibiotic defences that depress fecundity and promote mortality are regulated through different signalling pathways.

Gene expression profiles responses to aphid feeding in chrysanthemum (Chrysanthemum morifolium)

BACKGROUND

Chrysanthemum is an important ornamental plant all over the world. It is easily attacked by aphid, Macrosiphoniella sanbourni. The molecular mechanisms of plant defense responses to aphid are only partially understood. Here, we investigate the gene expression changes in response to aphid feeding in chrysanthemum leaf by RNA-Seq technology.

RESULTS

Three libraries were generated from pooled leaf tissues of Chrysanthemum morifolium 'nannongxunzhang' that were collected at different time points with (Y) or without (CK) aphid infestations and mock puncture treatment (Z), and sequenced using an Illumina HiSeqTM 2000 platform. A total of 7,363,292, 7,215,860 and 7,319,841 clean reads were obtained in library CK, Y and Z, respectively. The proportion of clean reads was >97.29% in each library. Approximately 76.35% of the clean reads were mapped to a reference gene database including all known chrysanthemum unigene sequences. 1,157, 527 and 340 differentially expressed genes (DEGs) were identified in the comparison of CK-VS-Y, CK-VS-Z and Z-VS-Y, respectively. These DEGs were involved in phytohormone signaling, cell wall biosynthesis, photosynthesis, reactive oxygen species (ROS) pathway and transcription factor regulatory networks, and so on.

CONCLUSIONS

Changes in gene expression induced by aphid feeding are shown to be multifaceted. There are various forms of crosstalk between different pathways those genes belonging to, which would allow plants to fine-tune its defense responses.

Constitutive overexpression of the pollen specific gene SKS13 in leaves reduces aphid performance on Arabidopsis thaliana

BACKGROUND

Plants have developed a variety of mechanisms to counteract aphid attacks. They activate their defences by changing the expression of specific genes. Previously we identified an activation tag mutant of Arabidopsis thaliana on which Myzus persicae population development was reduced. Activation tag mutants are gain-of-function in which the expression of a gene is increased by the insertion of the Cauliflower mosaic virus 35S enhancer that acts on the natural promoter. By further characterizing this previously identified mutant we identified a gene that reduces performance of M. persicae and also provided clues about the mechanism involved.

RESULTS

We show that SKU5 SIMILAR 13 (SKS13), a gene whose expression in wild type plants is restricted to pollen and non-responsive to M. persicae attack, is overexpressed in the A. thaliana mutant showing reduced performance of M. persicae. Monitoring M. persicae feeding behaviour on SKS13 overexpressing plants indicated that M. persicae have difficulties feeding from the phloem. The constitutive expression of SKS13 results in accumulation of reactive oxygen species, which is possibly regulated through the jasmonic acid pathway. The enhanced resistance is not aphid species specific as also the population development of Brevicoryne brassicae was affected.

CONCLUSIONS

We demonstrate that constitutive expression in leaves of the pollen-specific gene SKS13 can enhance plant defence, resulting in a reduction of M. persicae population development and also decreases the transmission of persistent viruses. Overexpression of SKS13 in A. thaliana also affects B. brassicae and possibly other phloem feeding insects as well. Identifying genes that can enhance plant defence against insects will be important to open up new avenues for the development of insect resistant crop plants.

Elicitation of jasmonate-mediated host defense in Brassica juncea (L.) attenuates population growth of mustard aphid Lipaphis erysimi (Kalt.)

The productivity of Brassica oilseeds is severely affected by its major pest: aphids. Unavailability of resistance source within the crossable germplasms has stalled the breeding efforts to derive aphid resistant cultivars. In this study, jasmonate-mediated host defense in Indian mustard Brassica juncea (L.) Czern. was evaluated and compared with regard to its elicitation in response to mustard aphid Lipaphis erysimi (Kalt.) and the defense elicitor methyl jasmonate (MeJ). Identification of jasmonate-induced unigenes in B. juncea revealed that most are orthologous to aphid-responsive genes, identified in taxonomically diverse plant-aphid interactions. The unigenes largely represented genes related to signal transduction, response to biotic and abiotic stimuli and homeostasis of reactive oxygen species (ROS), in addition to genes related to cellular and metabolic processes involved in cell organization, biogenesis, and development. Gene expression studies revealed induction of the key jasmonate biosynthetic genes (LOX, AOC, 12-OPDR), redox genes (CAT3 and GST6), and other downstream defense genes (PAL, ELI3, MYR, and TPI) by several folds, both in response to MeJ and plant-wounding. However, interestingly aphid infestation even after 24 h did not elicit any activation of these genes. In contrast, when the jasmonate-mediated host defense was elicited by exogenous application of MeJ the treated B. juncea plants showed a strong antibiosis effect on the infesting aphids and reduced the growth of aphid populations. The level of redox enzymes CAT, APX, and SOD, involved in ROS homeostasis in defense signaling, and several defense enzymes viz. POD, PPO, and PAL, remained high in treated plants. We conclude that in B. juncea, the jasmonate activated endogenous-defense, which is not effectively activated in response to mustard aphids, has the potential to reduce population growth of mustard aphids.

Differential induction of Pisum sativum defense signaling molecules in response to pea aphid infestation

This study demonstrates the sequence of enhanced generation of signal molecules such as phytohormones, i.e. jasmonic acid (JA), ethylene (ET), salicylic acid (SA), and a relatively stable free radical, nitric oxide (NO), in response of Pisum sativum L. cv. Cysterski seedling leaves to the infestation of pea aphid Acyrthosiphon pisum (Harris) at a varied population size. In time from 0 to 96h after A. pisum infestation these signal molecules accumulated transiently. Moreover, the convergence of these signaling pathways occurred. JA and its methyl derivative MeJA reached the first maximum of generation at 24th hour of infestation. An increase in ET and NO generation was observed at 48th hour of infestation. The increase in SA, JA/MeJA and ET concentrations in aphid-infested leaves occurred from the 72nd to 96th hour. In parallel, an increase was demonstrated for the activities of enzymes engaged in the biosynthesis of SA, such as phenylalanine ammonia-lyase (PAL) and benzoic acid 2-hydroxylase (BA2H). Additionally, a considerable post-infestation accumulation of transcripts for PAL was observed. An increase in the activity of lipoxygenase (LOX), an important enzyme in the biosynthesis of JA was noted. This complex signaling network may contribute to the coordinated regulation of gene expression leading to specific defence responses.

Plant resistance to aphid feeding: behavioral, physiological, genetic and molecular cues regulate aphid host selection and feeding

Aphids damage major world food and fiber crops through direct feeding and transmission of plant viruses. Fortunately, the development of many aphid-resistant crop plants has provided both ecological and economic benefits to food production. Plant characters governing aphid host selection often dictate eventual plant resistance or susceptibility to aphid herbivory, and these phenotypic characters have been successfully used to map aphid resistance genes. Aphid resistance is often inherited as a dominant trait, but is also polygenic and inherited as recessive or incompletely dominant traits. Most aphid-resistant cultivars exhibit constitutively expressed defenses, but some cultivars exhibit dramatic aphid-induced responses, resulting in the overexpression of large ensembles of putative aphid resistance genes. Two aphid resistance genes have been cloned. Mi-1.2, an NBS-LRR gene from wild tomato, confers resistance to potato aphid and three Meloidogyne root-knot nematode species, and Vat, an NBS-LRR gene from melon, controls resistance to the cotton/melon aphid and to some viruses. Virulence to aphid resistance genes of plants occurs in 17 aphid species--more than half of all arthropod biotypes demonstrating virulence. The continual appearance of aphid virulence underscores the need to identify new sources of resistance of diverse sequence and function in order to delay or prevent biotype development.

A dissection of the effects of ethylene, H2O2 and high irradiance on antioxidants and several genes associated with stress and senescence in tobacco leaves

Ethylene and hydrogen peroxide are involved in the modulation of stress responses in plants, but their interrelation is not well understood. This work was designed to find differences between the actions of ethylene and H2O2 on antioxidants and senescence markers. Leaves of Nicotiana tabacum were sprayed with H2O2 or with ethephon (precursor of ethylene). To find the possible modulation of responses to acute abiotic stress, ethephon- and H2O2-sprayed leaves were further subjected to high irradiance (HL). The application of H2O2 strongly stimulated ethylene synthesis (ACC). Ethylene and H2O2, as single factors, stimulated the trolox equivalent antioxidant capacity (TEAC) and the activity of catalase (CAT), in contrast to HL alone (stimulation of nonspecific peroxidases and the total glutathione pool). However, after combined treatments (ethylene+HL and H2O2+HL), the stimulatory action of H2O2 was related to TEAC and CAT activity, while the application of ethylene stimulated the total glutathione pool. Hydrogen peroxide enhanced the expression of the three CAT genes (Cat1, Cat2 and Cat3), in contrast to ethylene (Cat2 and Cat3) and HL (Cat1). In regard to the markers of senescence and pathogenesis the most pronounced difference between the actions of ethylene and H2O2, as single factors, was related to NPR1, whereas when leaf spraying was combined with HL, differences were found at WRKY53 and PR1a. HL reversed the stimulatory effects of H2O2/ethylene-driven enhancements of the expression of several genes (Cat1, Cat2, NPR1, WRKY53). These results show that multiple stressors, as usually encountered by plants in nature, may largely change those expression patterns of genes determined in a single factor analysis. Moreover, the actions of HL (often considered the internal H2O2 trigger) and of exogenous H2O2 on gene expression are clearly different.

Effects of shade treatments on photosynthetic characteristics, chloroplast ultrastructure, and physiology of Anoectochilus roxburghii

Anoectochilus roxburghii was grown under different shade treatments-50%, 30%, 20%, and 5% of natural irradiance-to evaluate its photosynthetic characteristics, chloroplast ultrastructure, and physiology. The highest net photosynthetic rates and stomatal conductance were observed under 30% irradiance, followed in descending order by 20%, 5%, and 50% treatments. As irradiance decreased from 50% to 30%, electron transport rate and photochemical quenching increased, while non-photochemical quenching indexes declined. Reductions in irradiance significantly increased Chl a and Chl b contents and decreased Chl a/b ratios. Chloroplast ultrastructure generally displayed the best development in leaves subjected to 30% irradiance. Under 50% irradiance, leaf protein content remained relatively stable during the first 20 days of treatment, and then increased rapidly. The highest peroxidase and superoxide dismutase levels, and the lowest catalase activities, were observed in plants subjected to the 50% irradiance treatment. Soluble sugar and malondialdehyde contents were positively correlated with irradiance levels. Modulation of chloroplast development, accomplished by increasing the number of thylakoids and grana containing photosynthetic pigments, is an important shade tolerance mechanism in A. roxburghii.

Oxidative stress in pea seedling leaves in response to Acyrthosiphon pisum infestation

In this study we examined whether and to what extent oxidative stress is induced in seedling leaves of Pisum sativum L. cv. Cysterski in response to pea aphid (Acyrthosiphon pisum Harris) infestation. A. pisum caused oxidative stress conditions in pea leaves through enhanced production of the reactive oxygen species (ROS) hydrogen peroxide (H2O2) and superoxide anion radical (O2(·-)). Early, strong generation of H2O2 was observed at 24h in aphid-infested leaves. The highest level of H2O2 at this time point may be related to the functioning of H2O2 as a signaling molecule, triggering defense mechanisms in pea leaves against A. pisum. Additionally, the strong generation and continuous increase of O2(·-) production in aphid-infested leaves from 0 to 96 h enhanced the defense potential to protect against aphid herbivory. Also in the study cytochemical localization of H2O2 and O2(·-) in pea leaves after aphid infestation was determined using the confocal microscope. Relative release of H2O2 and O2(·-) was estimated by staining leaves with specific fluorochromes, i.e. dichlorodihydro-fluorescein diacetate (DCFH-DA) and dihydroethidium (DHE), respectively. DCFH-DA and DHE derived fluorescence was observed to cover a much larger tissue area in aphid-infested leaves, whereas little or no fluorescence was observed in the control leaves. Enhanced activity of the antioxidant enzymes superoxide dismutase (SOD, 1.15.1.1) and catalase (CAT, 1.11.1.6) is one of the most essential elements of defense responses in pea seedling leaves to oxidative stress. Additionally, generation of semiquinones, stable free radicals with g-values of 2.0020 and 2.0035, detected by electron paramagnetic resonance spectroscopy (EPR), was suggested as a protective action of pea that may contribute to build-up of a defensive barrier or activate other defense mechanisms. Concentrations of semiquinone radicals in aphid-infested seedling leaves not only were generally higher than in the control plants but also significantly increased with cultivation time. On the other hand, the small increase in content of thiobarbituric acid reactive substances (TBARS), a product of lipid peroxidation, and the percentage of injury (3-8%) indicated that the cellular damage was caused by oxidative stress. The induced changes in levels of H2O2, O2(·-) and semiquinone radicals as well as activities of antioxidant enzymes in the pea defense responses were proportional to the population size of A. pisum. These findings indicate that the defensive strategies against A. pisum infestation were stimulated in seedling leaves of P. sativum L. cv. Cysterski. Our observations of the enhanced defense responses of P. sativum to infestation by A. pisum reveal some aspects and contribute to current knowledge of regulatory mechanisms in plant-aphid interactions.

Expression of α-DIOXYGENASE 1 in tomato and Arabidopsis contributes to plant defenses against aphids

Plant α-dioxygenases (α-DOX) are fatty acid-hydroperoxidases that contribute to the synthesis of oxylipins, a diverse group of compounds primarily generated through oxidation of linoleic (LA) and linolenic acid (LNA). Oxylipins are implicated in plant signaling against biotic and abiotic stresses. We report here that the potato aphid (Macrosiphum euphorbiae) induces Slα-DOX1 but not Slα-DOX2 expression in tomato (Solanum lycopersicum). Slα-DOX1 upregulation by aphids does not require either jasmonic acid (JA) or salicylic acid (SA) accumulation, since tomato mutants deficient in JA (spr2, acx1) or SA accumulation (NahG) still show Slα-DOX1 induction. Virus-induced gene silencing of Slα-DOX1 enhanced aphid population growth in wild-type (WT) plants, revealing that Slα-DOX1 contributes to basal resistance to aphids. Moreover, an even higher percent increase in aphid numbers occurred when Slα-DOX1 was silenced in spr2, a mutant line characterized by elevated LA levels, decreased LNA, and enhanced aphid resistance as compared with WT. These results suggest that aphid reproduction is influenced by oxylipins synthesized from LA by Slα-DOX1. In agreement with our experiments in tomato, two independent α-dox1 T-DNA insertion mutant lines in Arabidopsis thaliana also showed increased susceptibility to the green peach aphid (Myzus persicae), indicating that the role α-DOX is conserved in other plant-aphid interactions.

Chrysanthemum cutting productivity and rooting ability are improved by grafting

Chrysanthemum has been commercially propagated by rooting of cuttings, whereas the quality will decline over multiple collections from a single plant. Therefore, we compared the vigour, rooting ability, and some physiological parameters between cuttings harvested from nongrafted “Jinba” (non-grafted cuttings) with those collected from grafted “Jinba” plants onto Artemisia scoparia as a rootstock (grafted cuttings). The yield, length, node number, stem diameter, fresh weight, and dry weight of the grafted cuttings were superior to the non-grafted cuttings. Also grafted cuttings “Jinba” rooted 1 day earlier, but showing enhanced rooting quality including number, length, diameter, and dry weight of roots, where compared to the non-grafted. The physiological parameters that indicated contents of soluble protein, peroxidase activity, soluble sugar, and starch, ratios of soluble sugar/nitrogen ratio, and carbohydrate/nitrogen (C/N), as well as contents of indole-3-acetic acid (IAA) and abscisic acid (ABA), and IAA/ABA ratio were significantly increased in the grafted cuttings. This suggested their important parts in mediating rooting ability. Results from this study showed that grafting improved productivity and rooting ability related to an altered physiology, which provide a means to meet the increasing demand.

Arabidopsis thaliana-Myzus persicae interaction: shaping the understanding of plant defense against phloem-feeding aphids

The phloem provides a unique niche for several organisms. Aphids are a large group of Hemipteran insects that utilize stylets present in their mouthparts to pierce sieve elements and drink large volumes of phloem sap. In addition, many aphids also vector viral diseases. Myzus persicae, commonly known as the green peach aphid (GPA), is an important pest of a large variety of plants that includes Arabidopsis thaliana. This review summarizes recent studies that have exploited the compatible interaction between Arabidopsis and GPA to understand the molecular and physiological mechanisms utilized by plants to control aphid infestation, as well as genes and mechanisms that contribute to susceptibility. In addition, recent efforts to identify aphid-delivered elicitors of plant defenses and novel aphid salivary components that facilitate infestation are also discussed.

Mechanisms of plant defense against insect herbivores

Plants respond to herbivory through various morphological, biochemicals, and molecular mechanisms to counter/offset the effects of herbivore attack. The biochemical mechanisms of defense against the herbivores are wide-ranging, highly dynamic, and are mediated both by direct and indirect defenses. The defensive compounds are either produced constitutively or in response to plant damage, and affect feeding, growth, and survival of herbivores. In addition, plants also release volatile organic compounds that attract the natural enemies of the herbivores. These strategies either act independently or in conjunction with each other. However, our understanding of these defensive mechanisms is still limited. Induced resistance could be exploited as an important tool for the pest management to minimize the amounts of insecticides used for pest control. Host plant resistance to insects, particularly, induced resistance, can also be manipulated with the use of chemical elicitors of secondary metabolites, which confer resistance to insects. By understanding the mechanisms of induced resistance, we can predict the herbivores that are likely to be affected by induced responses. The elicitors of induced responses can be sprayed on crop plants to build up the natural defense system against damage caused by herbivores. The induced responses can also be engineered genetically, so that the defensive compounds are constitutively produced in plants against are challenged by the herbivory. Induced resistance can be exploited for developing crop cultivars, which readily produce the inducible response upon mild infestation, and can act as one of components of integrated pest management for sustainable crop production.

Differential responses of double petal and multi petal jasmine to shading: I. Photosynthetic characteristics and chloroplast ultrastructure

A double petal (DP) and a multi petal (MP) type jasmine (Jasminum sambac Ait.) growth and flowering was known largely affected by different levels of irradiance. Here, our objective was to determine the effects of shade on photosynthesis related characteristics and chloroplast ultrastructure of these two types. In both types, net photosynthetic rate (Pn), stomatal conductance (g(s)) and transpiration rate increased with decreasing irradiance from 100% to 20%, while both maximum and variable fluorescence showed a steady increase, and photochemical and nonphotochemical quenching indexes declined. At each conducted time, chlorophyll a, b and carotenoids contents in DP type shaded leaves increased whereas those in MP type decreased at 5% irradiance (considered as extreme shade). The maximum photochemical efficiency of photosystem II of DP plants showed subtle changes but that of MP plants declined by shading thereafter 21 days of treatment. Observation of chloroplast ultrastructure showed its best development in the leaves of DP and MP types mostly from 50% to 20% irradiance (considered as weak and moderate shade, respectively). At each shade treatment, Pn, g(s) and water use efficiency of DP-jasmine were always higher than those of MP-jasmine, thus the shade tolerance ability of the former was higher than that of the latter. The results showed that full sunlight and 5% natural irradiance caused photoinhibition and light deficiency of jasmine plants respectively, and modulating chloroplast development by the more numbers of thylakoids and grana to contain more photosynthetic pigments is an important shade tolerance mechanism of DP type.

Chrysanthemum leaf epidermal surface morphology and antioxidant and defense enzyme activity in response to aphid infestation

Artificial aphid infestation experiments on the three chrysanthemum cultivars 'Keiun', 'Han6' and 'Jinba' showed that the three cultivars vary markedly in their resistance. Of the three cultivars, the most resistant ('Keiun') produced the longest, highest and densest trichomes, the largest and fullest gland cells, and the most wax on the lower leaf epidermis. Superoxide dismutase (EC 1.15.1.1), peroxidase (EC 1.11.1.7), ascorbate peroxidase (EC 1.11.1.11), polyphenol oxidase activity (EC 1.14.18.1) and phenylalanine ammonia lyase (EC 4.3.1.5) were enhanced by aphid herbivory. In the two more resistant cultivars ('Keiun' and 'Han6'), the activity of superoxide dismutase and ascorbate peroxidase enzymes rapidly increased following infestation, and their levels remained high from seventy-two to one hundred and sixty-eight hours after inoculation. We suggest that these two antioxidant enzymes contribute to aphid resistance of these chrysanthemum cultivars. All three cultivars showed quick responses to aphid infestation by increasing polyphenol oxidase and phenylalanine ammonia lyase activities during the early period after inoculation. Activities of these two defense enzymes were higher in the two resistant cultivars after 72h after inoculation, suggesting involvement of these two enzymes in aphid resistance.

Tomato pathogenesis-related protein genes are expressed in response to Trialeurodes vaporariorum and Bemisia tabaci biotype B feeding

The temporal and spatial expression of tomato wound- and defense-response genes to Bemisia tabaci biotype B (the silverleaf whitefly) and Trialeurodes vaporariorum (the greenhouse whitefly) feeding were characterized. Both species of whiteflies evoked similar changes in tomato gene expression. The levels of RNAs for the methyl jasmonic acid (MeJA)- or ethylene-regulated genes that encode the basic β-1,3-glucanase (GluB), basic chitinase (Chi9), and Pathogenesis-related protein-1 (PR-1) were monitored. GluB and Chi9 RNAs were abundant in infested leaves from the time nymphs initiated feeding (day 5). In addition, GluB RNAs accumulated in apical non-infested leaves. PR-1 RNAs also accumulated after whitefly feeding. In contrast, the ethylene- and salicylic acid (SA)-regulated Chi3 and PR-4 genes had RNAs that accumulated at low levels and GluAC RNAs that were undetectable in whitefly-infested tomato leaves. The changes in Phenylalanine ammonia lyase5 (PAL5) were variable; in some, but not all infestations, PAL5 RNAs increased in response to whitefly feeding. PAL5 RNA levels increased in response to MeJA, ethylene, and abscisic acid, and declined in response to SA. Transcripts from the wound-response genes, leucine aminopeptidase (LapA1) and proteinase inhibitor 2 (pin2), were not detected following whitefly feeding. Furthermore, whitefly infestation of transgenic LapA1:GUS tomato plants showed that whitefly feeding did not activate the LapA1 promoter, although crushing of the leaf lamina increased GUS activity up to 40 fold. These studies indicate that tomato plants perceive B. tabaci and T. vaporariorum in a manner similar to baterical pathogens and distinct from tissue-damaging insects.

Aphid feeding activates expression of a transcriptome of oxylipin-based defense signals in wheat involved in resistance to herbivory

Damage by the Russian wheat aphid (RWA), Diuraphis noxia, significantly reduces wheat and barley yields worldwide. In compatible interactions, virulent RWA populations flourish and susceptible plants suffer extensive leaf chlorophyll loss. In incompatible interactions, RWA reproduction and population growth are significantly reduced and RWA-related chlorophyll loss in resistant plants is minor. The objectives of this study were to develop an understanding of the molecular and phytochemical bases of RWA resistance in plants containing the Dnx resistance gene. Microarray, real-time polymerase chain reaction, and phytohormone assays were conducted to identify transcriptome components unique to RWA-infested Dnx plants and susceptible (Dn0) plants, and to identify and characterize putative genes involved in Dnx plant defense responses. We found that RWA-infested Dnx plants upregulated >180 genes related to reactive oxygen species, signaling, pathogen defense, and arthropod allelochemical and physical defense. The expression of several of these genes in RWA-infested Dnx plants increased significantly from 6- to 24-h post infestation (hpi), but their expression in Dn0 plants, when present, was delayed until 48- to 96 hpi. Concentrations of 16- and 18-carbon fatty acids, trans-methyl-12-oxophytodienoic acid, and abscisic acid were significantly greater in Dnx foliage than in Dn0 foliage after RWA infestation, suggesting that Dnx RWA defense and resistance genes may be regulated via the oxylipin pathway. These findings provide a foundation for the elucidation of the molecular basis for compatible- and incompatible plant-aphid interactions.

HORMONOMETER: a tool for discerning transcript signatures of hormone action in the Arabidopsis transcriptome

Plant hormones regulate growth and responses to environmental change. Hormone action ultimately modifies cellular physiological processes and gene activity. To facilitate transcriptome evaluation of novel mutants and environmental responses, there is a need to rapidly assess the possible contribution of hormone action to changes in the levels of gene transcripts. We developed a vector-based algorithm that rapidly compares lists of transcripts yielding correlation values. The application as described here, called HORMONOMETER, was used to analyze hormone-related activity in a transcriptome of Arabidopsis (Arabidopsis thaliana). The veracity of the resultant analysis was established by comparison with cognate and noncognate hormone transcriptomes as well as with mutants and selected plant-environment interactions. The HORMONOMETER accurately predicted correlations between hormone action and biosynthetic mutants for which transcriptome data are available. A high degree of correlation was detected between many hormones, particularly at early time points of hormone action. Unforeseen complexity was detected in the analysis of mutants and in plant-herbivore interactions. The HORMONOMETER provides a diagnostic tool for evaluating the physiological state of being of the plant from the point of view of transcripts regulated by hormones and yields biological insight into the multiple response components that enable plant adaptation to the environment. A Web-based interface has been developed to facilitate external interfacing with this platform.

Gene expression profiling of tolerant barley in response to Diuraphis noxia (Hemiptera: Aphididae) feeding

Aphids are, arguably, the single most damaging group of agricultural insect pests throughout the world. Plant tolerance, which is a plant response to an insect pest, is viewed as an excellent management strategy. Developing testable hypotheses based on genome-wide and more focused methods will help in understanding the molecular underpinnings of plant tolerance to aphid herbivory. As a first step in this process, we undertook transcript profiling with Affymetrix GeneChip Barley Genome arrays using RNA extracted from tissues of tolerant and susceptible genotypes collected at three hours, three days and six days after Diuraphis noxia introduction. Acquired data were compared to identify changes unique to the tolerant barley at each harvest date. Transcript abundance of 4086 genes was differentially changed over the three harvest dates in tolerant and susceptible barley in response to D. noxia feeding. Across the three harvest dates, the greatest number of genes was differentially expressed in both barleys at three days after aphid introduction. A total of 909 genes showed significant levels of change in the tolerant barley in response to D. noxia feeding as compared to susceptible plants infested with aphids. Many of these genes could be assigned to specific metabolic categories, including several associated with plant defense and scavenging of reactive oxygen species (ROS). Interestingly, two peroxidase genes, designated HvPRXA1 and HvPRXA2, were up-regulated to a greater degree in response to D. noxia feeding on tolerant barley plants, indicating that specific peroxidases could be important for the tolerance process. These findings suggest that the ability to elevate and sustain levels of ROS-scavenging enzymes could play an important role in the tolerant response.

Constitutive and induced activities of defense-related enzymes in aphid-resistant and aphid-susceptible cultivars of wheat

Phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), and peroxidase (POD) are considered important biochemical markers in host plant resistance against pest insects. Constitutive activity of these enzymes was analyzed in resistant and susceptible wheat cultivars against cereal aphid Sitobion avenae (F.) at various developmental stages, i.e., tillering, stem elongation, flag leaf, and ear. Following aphid infestation, the activity of these enzymes was determined at the flag leaf and ear stages. Resistant cultivars exhibited greater constitutive PAL activity than susceptible ones at the tillering, stem elongation, and flag leaf stages. Aphid infestation enhanced levels of PAL activity in the flag leaf and ear stages in both resistant and susceptible cultivars. Constitutive PPO activity was higher in the resistant cultivars at all developmental stages. Aphid infestation induced increases in PPO activity in the flag leaf and ear stages of one susceptible cultivar, whereas induction in resistant cultivars was weaker. Resistant cultivars showed greater constitutive POD activity in the tillering, stem elongation, and flag leaf stages, while aphid infestation induced POD activity in all cultivars, especially in susceptible ones. The potential role of PAL, PPO, and POD in wheat defense against aphid infestation is discussed.

Hypersensitive response to Aphis gossypii Glover in melon genotypes carrying the Vat gene

Aphis gossypii Glover causes direct and indirect damage to Cucumis melo L. crops. To decrease the harmful effects of this pest, one of the most economically and environmentally acceptable options is to use genetically resistant melon varieties. To date, several sources of resistance carrying the Vat gene are used in melon breeding programmes that aim to prevent A. gossypii colonization and the subsequent aphid virus transmission. The results suggest that the resistance conferred by this gene is associated with a microscopic hypersensitive response specific against A. gossypii. Soon after aphid infestation, phenol synthesis, deposits of callose and lignin in the cell walls, damage to the plasmalemma, and a micro-oxidative burst were detected in genotypes carrying the Vat gene. According to electrical penetration graph experiments, this response seems to occur after aphid stylets puncture the plant cells and not during intercellular stylet penetration. This type of plant tissue reaction was not detected in melon plants infested with Bemisia tabaci Gennadius nor Myzus persicae Sulzer.

Ethylene contributes to potato aphid susceptibility in a compatible tomato host

Resistance to potato aphid (Macrosiphum euphorbiae) in tomato (Solanum lycopersicum) is conferred by Mi-1. Early during both compatible and incompatible interactions, potato aphid feeding induces the expression of ethylene (ET) biosynthetic genes. Here, we used genetic and pharmacologic approaches to investigate the role of ET signaling in basal defense and Mi-1-mediated resistance to potato aphid in tomato. The effect of potato aphid infestation on ET biosynthesis in susceptible and resistant plants was assessed. Aphid bioassays were performed using plants impaired in ET biosynthesis or perception using virus-induced gene silencing, the Never ripe (Nr) mutant, and 1-methylcyclopropene (MCP) treatment. A burst of ET was observed after aphid feeding in both resistant and susceptible plants, correlated with an increase in the expression of ET biosynthetic genes. However, impairing ET signaling or biosynthesis did not compromise Mi-1-mediated resistance but it did decrease susceptibility to potato aphid in a compatible host. ET may not play a significant role in Mi-1-mediated resistance to potato aphids in tomato but modulates the host basal defense, enhancing its susceptibility to the aphid.

Microarray analysis of the interaction between the aphid Rhopalosiphum padi and host plants reveals both differences and similarities between susceptible and partially resistant barley lines

The bird cherry-oat aphid (Rhopalosiphum padi L.) is an important pest on cereals causing plant growth reduction without specific leaf symptoms. Breeding of barley (Hordeum vulgare L.) for R. padi resistance shows that there are several resistance genes, reducing aphid growth. To identify candidate sequences for resistance-related genes, we performed microarray analysis of gene expression after aphid infestation in two susceptible and two partially resistant barley genotypes. One of the four lines is a descendant of two of the other genotypes. There were large differences in gene induction between the four lines, indicating substantial variation in response even between closely related genotypes. Genes induced in aphid-infested tissue were mainly related to defence, primary metabolism and signalling. Only 24 genes were induced in all lines, none of them related to oxidative stress or secondary metabolism. Few genes were down-regulated, with none being common to all four lines. There were differences in aphid-induced gene regulation between resistant and susceptible lines. Results from control plants without aphids also revealed differences in constitutive gene expression between the two types of lines. Candidate sequences for induced and constitutive resistance factors have been identified, among them a proteinase inhibitor, a serine/threonine kinase and several thionins.

Biochemistry and molecular biology of Arabidopsis-aphid interactions

To ensure their survival in natural habitats, plants must recognize and respond to a wide variety of insect herbivores. Aphids and other Hemiptera pose a particular challenge, because they cause relatively little direct tissue damage when inserting their slender stylets intercellularly to feed from the phloem sieve elements. Plant responses to this unusual feeding strategy almost certainly include recognition of aphid salivary components and the induction of phloem-specific defenses. Due to the excellent genetic and genomic resources that are available for Arabidopsis thaliana (Arabidopsis), this plant was chosen as a model system to study the metabolic and transcriptional responses to infestation by two aphids, Myzus persicae (green peach aphid, a broad generalist) and Brevicoryne brassicae (cabbage aphid, a crucifer-feeding specialist). Future research on Arabidopsis-aphid interactions will lead to the identification of aphid-specific elicitors, components of the defense-signaling pathway, and additional metabolic responses that are induced by aphid infestation.