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Many insect species have acquired the ability to redirect plant development to form unique organs called galls, which provide these insects with unique, enhanced food and protection from enemies and the elements. Many galls resemble flowers or fruits, suggesting that elements of reproductive development may be involved. We tested this hypothesis using RNA sequencing to quantify the transcriptional responses of wild grapevine ( Vitis riparia ) leaves to a galling parasite, phylloxera ( Daktulosphaira vitifoliae ). If development of reproductive structures is part of gall formation, we expected to find significantly elevated expression of genes involved in flower and/or fruit development in developing galls as opposed to ungalled leaves. We found that reproductive gene ontology categories were significantly enriched in developing galls, and that expression of many candidate genes involved in floral development were significantly increased, particularly in later gall stages. The patterns of gene expression found in galls suggest that phylloxera exploits vascular cambium to provide meristematic tissue and redirects leaf development towards formation of carpels. The phylloxera leaf gall appears to be phenotypically and transcriptionally similar to the carpel, due to the parasite hijacking underlying genetic machinery in the host plant.

Plants are organisms composed of modules connected by xylem and phloem transport streams. Attack by both insects and pathogens elicits sometimes rapid defense responses in the attacked module. We have also known for some time that proteins are often reallocated away from pathogen-infected tissues, while the same infection sites may draw carbohydrates to them. This has been interpreted as a tug of war in which the plant withdraws critical resources to block microbial growth while the microbes attempt to acquire more resources. Sink-source regulated transport among modules of critical resources, particularly carbon and nitrogen, is also altered in response to attack. Insects and jasmonate can increase local sink strength, drawing carbohydrates that support defense production. Shortly after attack, carbohydrates may also be drawn to the root. The rate and direction of movement of photosynthate or signals in phloem in response to attack is subject to constraints that include branching, degree of connection among tissues, distance between sources and sinks, proximity, strength, and number of competing sinks, and phloem loading/unloading regulators. Movement of materials (e.g., amino acids, signals) to or from attack sites in xylem is less well understood but is partly driven by transpiration. The root is an influential sink and may regulate sink-source interactions and transport above and below ground as well as between the plant and the rhizosphere and nearby, connected plants. Research on resource translocation in response to pathogens or herbivores has focused on biochemical mechanisms; whole-plant research is needed to determine which, if any, of these plant behaviors actually influence plant fitness.

We tested the hypothesis that Arabidopsis can recognize and respond differentially to insect species at the transcriptional level using a genome wide microarray. Transcriptional reprogramming was characterized using co-expression analysis in damaged and undamaged leaves at two times in response to mechanical wounding and four insect species. In all, 2778 (10.6%) of annotated genes on the array were differentially expressed in at least one treatment. Responses differed mainly between aphid and caterpillar and sampling times. Responses to aphids and caterpillars shared only 10% of up-regulated and 8% of down-regulated genes. Responses to two caterpillars shared 21 and 12% of up- and down-regulated genes, whereas responses to the two aphids shared only 7 and 4% of up-regulated and down-regulated genes. Overlap in genes expressed between 6 and 24 h was 3-15%, and depended on the insect species. Responses in attacked and unattacked leaves differed at 6 h but converged by 24 h. Genes responding to the insects are also responsive to many stressors and included primary metabolism. Aphids down-regulated amino acid catabolism; caterpillars stimulated production of amino acids involved in glucosinolate synthesis. Co-expression analysis revealed 17 response networks. Transcription factors were a major portion of differentially expressed genes throughout and responsive genes shared most of the known or postulated binding sites. However, cis-element composition of genes down regulated by the aphid M. persicae was unique, as were those of genes down-regulated by caterpillars. As many as 20 cis-elements were over-represented in one or more treatments, including some from well-characterized classes and others as yet uncharacterized. We suggest that transcriptional changes elicited by wounding and insects are heavily influenced by transcription factors and involve both enrichment of a common set of cis-elements and a unique enrichment of a few cis-elements in responding genes.

Plant responses to insects and wounding involve substantial transcriptional reprogramming that integrates hormonal, metabolic, and physiological events. The ability to respond differentially to various stresses, including wounding, generally involves hormone signaling and trans-acting regulatory factors. Evidence of the importance of transcription factors (TFs) in responses to insects is also accumulating. However, the relationships among hormone signaling, TF activity, and ability to respond specifically to different insects are uncertain. We examined transcriptional and hormonal changes in Arabidopsis thaliana after herbivory by larvae of two lepidopteran species, Spodoptera exigua (Hübner) and Pieris rapae L. over a 24-h time course. Transcriptional responses to the two insects differed and were frequently weaker or absent in response to the specialist P. rapae. Using microarray analysis and qRT-PCR, we found 141 TFs, including many AP2/ERFs (Ethylene Response Factors) and selected defense-related genes, to be differentially regulated in response to the two insect species or wounding. Jasmonic Acid (JA), JA-isoleucine (JA-IL), and ethylene production by Arabidopsis plants increased after attack by both insect species. However, the amounts and timing of ethylene production differed between the two herbivory treatments. Our results support the hypothesis that the different responses to these two insects involve modifications of JA-signaling events and activation of different subsets of ERF TFs, resulting in different degrees of divergence from responses to wounding alone.

Plant responses to enemies are coordinated by several interacting signaling systems. Molecular and genetic studies with mutants and exogenous signal application suggest that jasmonate (JA)-, salicylate (SA)-, and ethylene (ET)-mediated pathways modulate expression of portions of the defense phenotype in Arabidopsis (Arabidopsis thaliana), but have not yet linked these observations directly with plant responses to insect attack. We compared the glucosinolate (GS) profiles of rosette leaves of 4-week-old mutant and transgenic Arabidopsis (Columbia) plants compromised in these three major signaling pathways, and characterized responses by those plants to feeding by two phloem-feeding aphids (generalist Myzus persicae and specialist Brevicoryne brassicae) and one generalist caterpillar species (Spodoptera exigua Hubner). Blocked JA signaling in coronatine-insensitive (coi1) and enhanced expression of SA-signaled disease resistance in hypersensitive response-like (hrl1) mutants reduced constitutive GS concentrations, while blocking SA signaling at the mediator protein npr1 mutant (NPR) increased them. There was no significant impact on constitutive GS contents of blocking ET signaling (at ET resistant [etr1]) or reducing SA concentrations (nahG transgene). We found increased GS accumulation in response to insect feeding, which required functional NPR1 and ETR1 but not COI1 or SA. Insect feeding caused increases primarily in short-chain aliphatic methylsulfinyl GS. By contrast, responses to exogenous JA, a frequent experimental surrogate for insect attack, were characterized by an increase in indolyl GS. Insect performance, measured as population increase or weight increase, was negatively related to GS levels, but we found evidence that other, ET-regulated factors may also be influential. Plant resistance to (consumption by) S. exigua was not related to insect growth because some plant chemistries inhibited growth while others inhibited feeding. These major signaling pathways modulate Arabidopsis GS accumulation and response to both phloem-feeding and chewing insects, often antagonistically; NPR appears to be central to these interactions. Our results indicate that exogenous signal application and plant consumption measures may not provide useful measures of plant responses to actual insect feeding.

Plant perception of insect feeding involves integration of the multiple signals involved: wounding, oral secretions, and substrate borne feeding vibrations. Although plant responses to wounding and oral secretions have been studied, little is known about how signals from the rapidly transmitted vibrations caused by chewing insect feeding are integrated to produce effects on plant defenses. In this study, we examined whether 24 h of insect feeding vibrations caused changes in levels of phytohormones and volatile organic compounds (VOCs) produced by leaves of when they were subjected to just feeding vibrations or feeding vibrations and wounding + methyl jasmonate (MeJA), compared to their respective controls of silent sham or wounding + MeJA. We showed that feeding vibrations alone caused a decrease in the concentrations of most phytohormones, compared to those found in control plants receiving no vibrations. When feeding vibrations were combined with wounding and application of MeJA, the results were more complex. For hormones whose levels were induced by wounding and MeJA (jasmonic acid, indole-3-butyric acid), the addition of feeding vibrations caused an even larger response. If the level of hormone was unchanged by wounding and MeJA compared with controls, then the addition of feeding vibrations had little effect. The levels of some VOCs were influenced by the treatments. Feeding vibrations alone caused an increase in β-ionone and decrease in methyl salicylate, and wounding + MeJA alone caused a decrease in benzaldehyde and methyl salicylate. When feeding vibrations were combined with wounding + MeJA, the effects on β-ionone and methyl salicylate were similar to those seen with feeding vibrations alone, and levels of benzaldehyde remained low as seen with wounding + MeJA alone. The widespread downregulation of plant hormones observed in this study is also seen in plant responses to cold, suggesting that membrane fluidity changes and/or downstream signaling may be common to both phenomena.

The induction of systemic responses in plants is associated with the connectivity between damaged and undamaged leaves, as determined by vascular architecture. Despite the widespread appreciation for studying variation in induced plant defense, few studies have characterized spatial variability of induction in the model species, Arabidopsis thaliana. Here we show that plant architecture generates fine scale spatial variation in the systemic induction of invertase and phenolic compounds. We examined whether the arrangement of leaves along the stem (phyllotaxy) produces predictable spatial patterns of cell-wall bound and soluble invertase activities, and downstream phenolic accumulation following feeding by the dietary specialist herbivore, Pieris rapae and the generalist, Spodoptera exigua. Responses were measured in leaves within and outside of the damaged orthostichy (leaves sharing direct vascular connections), and compared to those from plants where source-sink transport was disrupted by source leaf removal and by an insertional mutation in a sucrose transporter gene (suc2-1). Following herbivore damage to a single, middle-aged leaf, induction of cell-wall and soluble invertase was most pronounced in young and old leaves within the damaged orthostichy. The pattern of accumulation of phenolics was also predicted by these vascular connections and was, in part, dependent on the presence of source leaves and intact sucrose transporter function. Induction also occurred in leaves outside of the damaged orthostichy, suggesting that mechanisms may exist to overcome vascular constraints in this system. Our results demonstrate that systemic responses vary widely according to orthostichy, are often herbivore-specific, and partially rely on transport between source and sink leaves. We also provide evidence that patterns of induction are more integrated in A. thaliana than previously described. This work highlights the importance of plant vascular architecture in determining patterns of systemic induction, which is likely to be ecologically important to insect herbivores and plant pathogens.

Feeding by chewing insects induces chemical defenses in plants that are regulated by the jasmonic acid (JA) pathway. Jasmonates are usually quantified by liquid chromatography-mass spectrometry (LC-MS) analysis of precursors and products in the biosynthetic pathway or inferred from the extraction and expression of genes known to respond to elevated levels of JA. Both approaches are costly and time consuming. To address these limitations, we developed a rapid reporter for the synthesis of JA based on the promoter:YFP-PTS1. Yellow fluorescent protein (YFP) fluorescence was increased by mechanical wounding and methyl jasmonate (MeJA) treatment and by caterpillar feeding. To develop an optimal sampling time for a quantitative bioassay, promoter:YFP-PTS1 plants were sampled at 1, 2, 3, and 24 h after treatment with 115 µM MeJA. The first increase in YFP fluorescence was detected at 2 h and remained elevated 3 and 24 h later; as a result, 3 h was chosen as the sampling time for a quantitative bioassay of jasmonate response to insect attack. Feeding by caterpillars induced a 1.8-fold increase in YFP fluorescence, consistent with the known induction of JA production by this insect. We also assessed the utility of this reporter in studies of plant responses to caterpillar feeding vibrations, which are known to potentiate the JA-dependent production of chemical defenses. Pretreatment with feeding vibrations increased expression of the promoter:YFP-PTS1 in response to 14 µM MeJA. Feeding vibrations did not potentiate responses at higher MeJA concentrations, suggesting that potentiating effects of prior treatments can only be detected when plants are below a response threshold to the elicitor. The expression of does not indicate levels of specific downstream jasmonates and quantification of specific jasmonates still requires detailed analysis by LC-MS. However, expression does provide a rapid and inexpensive way to screen large numbers of plants for the involvement of jasmonate signaling in their response to a wide variety of treatments, and to study the induction and expression of .

• Here, we examined the impact of jasmonate (JA) treatment, branching and phloem girdling on 13C and 15N import, invertase activity and polyphenol accumulation in juvenile tissues of unbranched and branched hybrid poplar saplings (Populus nigra × P. deltoides). • The import of 13C to juvenile tissues was positively correlated with invertase activity at the treatment site and enhanced by JA. Both invertase activity and 13C import were greater in shorter, younger branches and smaller, younger leaves. By contrast, JA treatments, branching and girdling had little or no impact on 15N import. • In poplar saplings with multiple lateral branches, we observed almost no 13C movement from subtending source leaves into lateral branches above them, with or without JA treatment. The presence of potentially competing branches, treated with JA or not, girdled or not, had no impact on carbohydrate (CHO) import or polyphenol accumulation in target branches. • We conclude that poplar branches comprise modules that are relatively independent from each other and from the stem below in terms of CHO movement, carbon‐based defence production and response to elicitors. By contrast, branches are closely linked modules in terms of nitrogen movement. This should produce trees that are highly heterogeneous in quality for herbivores.

Plant defenses that respond to the threat of herbivory require accurate sensing of the presence of herbivores. Herbivory cues include mechanical damage, elicitors from insect saliva or eggs, and airborne volatiles emitted by wounded plants. Plants can also respond to the leaf vibrations produced by chewing herbivores. However, previous studies of the influence of feeding vibrations on plant defenses have been limited to single species pairs. In this study we test the hypothesis that chewing vibrations differ among herbivore species, both in their acoustic features and in their potential effect on plant defense responses. We first compare the acoustic traits of larval feeding vibrations in ten species from six families of Lepidoptera and one family of Hymenoptera. We then test responses of Arabidopsis thaliana plants to variation among feeding vibrations of different individuals of one species, and to feeding vibrations of two species, including a pierid butterfly and a noctuid moth. All feeding vibrations consisted of repetitive pulses of vibration associated with leaf tissue removal, although chewing rates varied between species and between large and small individuals within species. The frequency spectra of the vibrations generated by leaf feeding were similar across all ten species. Induced increases in anthocyanins in A. thaliana did not differ when plants were played vibrations from different individuals, or vibrations of two species of herbivores with different chewing rates, when amplitude was held constant. These results suggest that feeding vibrations provide a consistent set of cues for plant recognition of herbivores.