The imbalance of redox homeostasis in arthropod-induced plant galls: Mechanisms of stress generation and dissipation

Ontogenetic differences in sun and shade galls of Clinodiplosis profusa on Eugenia uniflora leaves and the cytological antioxidant mechanisms in gall cells

The gall-host Eugenia uniflora (Myrtaceae) is adaptable to different light conditions, enabling leaf production and survival in both sun and shade. Leaves of E. uniflora in shaded environments have more mesophyll layers, and galls of Clinodiplosis profusa (Cecidomyiidae) are larger and wider. Based on these previous observations, this study investigated the morphogenesis of galls induced by C. profusa on leaves of E. uniflora in different light conditions, revealing if the galls have a potential for acclimation, as observed with leaves. For this purpose, we compared the anatomical, histometric, and histochemical development of leaves and galls at different stages of development in sun and shade environments. Additionally, we analyzed the cytological features of the tissues composing the mature gall walls. Cells of shade galls expanded more toward the end of the developmental phase, which may explain the larger volume found for shade galls in a previous study. However, during the mature phase, these galls showed no significant differences in tissue thickness and final cell elongation in the contrasting light conditions. In the ultrastructural analyses, mature galls showed a gradient distinguishing the outer and inner parenchyma cells. The inner parenchyma had nutritive cells, with dense cytoplasm and abundant organelles. A higher accumulation of starch grains in nutritive cells, with evidence of hydrolysis of starch grains detected in the innermost layers leads to the accumulation of reducing sugars, which, with the presence of plastoglobules and protein bodies, are important mechanisms of oxidative stress dissipation in the cells in contact with the gall inducer.

Jasmonic Acid and Salicylic Acid improved resistance against Spodoptera frugiperda Infestation in maize by modulating growth and regulating redox homeostasis

Exploring host plant resistance and elevating plant defense mechanisms through the application of exogenous elicitors stands as a promising strategy for integrated pest management. The fall armyworm, a pernicious menace to grain crops in tropical and subtropical regions, stands as a formidable threat due to its capacity for devastation and a wide-ranging spectrum of host plants. There is no literature regarding artificially induced resistance in maize against fall armyworm (Spodoptera frugiperda) by exogenous application of phytohormones. The present investigation was performed to evaluate the role of jasmonic acid (JA) and salicylic acid (SA) on two maize hybrids namely FH-1046 and YH-1898 against fall armyworm. Results showed that plant height, biomass and lengths, fresh and dry weight of root shoot which decreased with armyworm infestation improved with phytohormonal application. JA treatment resulted in a higher increase in all attributes as compared to SA treatment. Improvement in relative water contents, photosynthetic pigments and pronounced levels of phenol and proline accumulation were observed in infested plants after JA treatment. Infested plants recovered from oxidative stress as JA application activated and increased the antioxidant enzyme activity of superoxide dismutase, peroxidase and polyphenol oxidase activity in both FH-1046 and YH-1898 . The oxidative stress reduction in infested plants after JA treatment was also evident from a fair decrease in MDA and H2O2 in both varieties. The SA and JA mediated genes expression was studied and it was found that in FH1046 maize cultivar, JA dependent genes, particularly marker genes PR1 and Lox5 were highly expressed along with TPS10 and BBT12. Whereas SPI, WRKY28, ICS and PAL were shown to be activated upon SA application. Evidently, both JA and SA elicited a robust defensive response within the maize plants against the voracious S. frugiperda, which in consequence exerted a discernible influence over the pest's developmental trajectory and physiological dynamics. A decrease in detoxification enzyme activity of the insects was observed after feeding on treated plants. Moreover, it was recorded that the survival and weight gain of FAW feeding on phytohormone treated maize plants also decelerated. In conclusion, FH-1046 was found to be more tolerant than YH-1898 against fall armyworm infestation and 1 mM JA was more effective than 1 mM SA for alleviation of fall armyworm stress. Therefore, it was inferred that phytohormones regulated redox homeostasis to circumvent oxidative damage and mediate essential metabolic events in maize under stress. To our current understanding, this study is the very first presentation of induced resistance in maize against S. frugiperda with the phytohormonal application (JA and SA).

Soil-plant-gall relationships: from gall development to ecological patterns

The adaptive nature of the galler habit has been tentatively explained by the nutrition, microenvironment, and enemy hypotheses. Soil attributes have direct relationships with these three hypotheses at the cellular and macroecological scales, but their influence has been restricted previously to effects on the nutritional status of the host plant on gall richness and abundance. Herein, we discuss the ionome patterns within gall tissues and their significance for gall development, physiology, structure, and for the nutrition of the gallers. Previous ecological and chemical quantification focused extensively on nitrogen and carbon contents, evoking the carbon-nutrient defence hypothesis as an explanation for establishing the plant-gall interaction. Different elements are involved in cell wall composition dynamics, antioxidant activity, and regulation of plant-gall water dynamics. An overview of the different soil-plant-gall relationships highlights the complexity of the nutritional requirements of gallers, which are strongly influenced by environmental soil traits. Soil and plant chemical profiles interact to determine the outcome of plant-herbivore interactions and need to be addressed by considering not only the soil features and galler nutrition but also the host plant's physiological traits. The quantitative and qualitative results for iron metabolism in gall tissues, as well as the roles of iron as an essential element in the physiology and reproduction of gallers suggest that it may represent a key nutritional resource, aligning with the nutrition hypothesis, and providing an integrative explanation for higher gall diversity in iron-rich soils.

Nutritive tissue rich in reserves in the cell wall and protoplast: the case of Manihot esculenta (Euphorbiaceae) galls induced by Iatrophobia brasiliensis (Diptera, Cecidomyiidae)

The galls can offer shelter, protection, and an adequate diet for the gall-inducing organisms. Herein, we evaluated the structure of Manihot esculenta leaves and galls induced by Iatrophobia brasiliensis in order to identify metabolic and cell wall composition changes. We expected to find a complex gall with high primary metabolism in a typical nutritive tissue. Non-galled leaves and galls were subjected to anatomical, histochemical, and immunocytochemical analyses to evaluate the structural features, primary and secondary metabolites, and glycoproteins, pectins, and hemicelluloses in the cell wall. The gall is cylindric, with a uniseriate epidermis, a larval chamber, and a parenchymatic cortex divided into outer and inner compartments. The outer compartment has large cells with intercellular spaces and stocks starch and is designated as storage tissue. Reducing sugars, proteins, phenolic compounds, and alkaloids were detected in the protoplast of inner tissue cells of galls, named nutritive tissue, which presents five layers of compact small cells. Cell walls with esterified homogalacturonans (HGs) occurred in some cells of the galls indicating the continuous biosynthesis of HGs. For both non-galled leaves and galls, galactans and xyloglucans were broadly labeled on the cell walls, indicating a cell growth capacity and cell wall stiffness, respectively. The cell wall of the nutritive tissue had wide labeling for glycoproteins, HGs, heteroxylans, and xyloglucans, which can be used as source for the diet of the galling insect. Manihot esculenta galls have compartments specialized in the protection and feeding of the galling insect, structured by nutritive tissue rich in resource compounds, in the cell walls and protoplast.

Hemiptera-induced galls of Sapium glandulosum have histological and cytological compartmentalization created with a large amount of carbohydrate

Gall formation impacts the development of plant species by altering the structure and mobilization of reserves, and the functional and physiological patterns of the host organ. The current study aimed to evaluate the impact generated by the Neolithus fasciatus galling insect (Hemiptera: Triozidae) in Sapium glandulosum leaves (Euphorbiaceae) at the cytological, histological, histochemical, and biochemical levels. Non-galled leaves and galls in the young, mature, and senescent stages were evaluated. The non-galled leaf has a uniseriate epidermis, stomata only on the abaxial side, a dorsiventral mesophyll, and parenchyma cells with thin primary walls containing chloroplasts with plastoglobules. The gall has a parenchymatous compartmentalized cortex. The young and mature galls already have a dense cytoplasm, especially in the inner cells of the cortex, with chloroplasts, mitochondria, Golgi complex, and large and evident nuclei. In senescent galls, there are signs of organelle degradation and cell digestion. Carbohydrates occur in greater amounts in the mature gall, mainly in the starch grain form, while proteins and lipids predominate in non-galled leaves. Secondary metabolites occur mainly in the young gall and may be related to its protection and to the signaling of its development. Sapium glandulosum galls have histological and cytological compartmentalization of the cortex with a large amount of carbohydrates, which supply energy to maintain the development of the structure.

Vascular implications of Dasineura sp. galls' establishment on Peumus boldus stems

Some chewing larvae are capable of inducing galls in the host vascular cylinder, e.g. Dasineura sp. (Cecidomyiidae) on Peumus boldus stems. Due to the medicinal and economic importance of P. boldus, the anatomical and functional implications of establishment of Dasineura sp. on P. boldus stems were investigated. We asked if establishment of Dasineura sp. in P. boldus stems induces abnormalities at the cellular and organizational level of the vascular system that increase during gall development in favour of the hydric status of the gall. Anatomical alterations induced in the stems during gall development were determined. Cytohistometric analyses in mature galls were compared to non-galled stems, and water potential and leaf area of non-galled stems were compared with galled stems. Dasineura sp. establishes in the vascular cambium, leading to delignification and rupture of xylem cells, inhibiting formation of phloem and perivascular sclerenchyma. Gall diameter increases together with larval feeding activity, producing a large larval chamber and numerous layers of nutritive tissue, vascular parenchyma, and sclerenchyma. These anatomical alterations do not affect the leaf area of galled stems but favour increased water flow towards these stems. The anatomical alterations induced by Dasineura sp. in P. boldus stems guarantee water and nutrient supply to the gall and larva. After the inducer exits stems, some host branches no longer have vascular connections with the plant body.

Epidermal and subepidermal changes during the formation of hairy galls induced by Eriophyidae on Avicennia schaueriana leaves

Leaf-galling Eriophyidae (Acarina) may promote simple or complex alterations in the organs of their host plants, such as an increase in indumentum density or the reorganization of epidermis and ground system tissue patterns. To test if hairy galls of Eriophyidae on Avicennia schaueriana (Acanthaceae) are related to complex changes, leaf galls in distinct developmental phases were compared to non-galled leaves using anatomical, histochemical, and histometric analyses. Quantitative comparisons of preferential gall induction sites and gall area according to distinct leaf portions were made to evaluate if the impacts of gall formation can be related to the distinct potentialities of leaf microsites. The apical portion of the leaves and leaf margins were the sites with the highest occurrence of galls, but no relationship was detected between gall area and induction site. The gall anatomy revealed that epidermal features are influenced the most with the development of abnormal stomata and projected or sunken salt glands. The most striking change is the neoformation of elongated filiform trichomes on the abaxial surface (where the mites occur) that accumulate reducing sugars and proteins. The filiform trichomes may protect the inducers against abiotic stressors and enemies, and the primary metabolites that accumulate are important foods for mites. The mesophyll has simple alterations, only in the spongy parenchyma. Complex alterations occur only in abaxial epidermal cells close to feeding sites of the inducer. The number of inducers per gall seems to be the most important influence on gall size, since gall area is not related to the position in the leaves.

Structural patterns of Lepidoptera galls and the case of Andescecidium parrai (Cecidosidae) galls on Schinus polygama (Anacardiaceae)

Gall anatomical and metabolic peculiarities are determined by the feeding habit of the gall inducer, but develop under the constraints of the host plants. The chewing habit of the Lepidoptera larvae imposes a high impact on the host plant cells, and supposedly drives peculiar structural and histochemical patterns. So, our starting point was the search of such patterns in literature, and the test of these traits on the Andescecidium parrai (Cecidosidae)-Schinus polygama (Anacardiaceae) system, as a case study in Chilean flora. The literature on the structure of lepidopteran galls in the temperate and tropical regions comprises 13 works, describing stems as the most frequent host organs, followed by leaves, buds, and flowers. As common structural traits of Lepidoptera galls, the literature converge in describing the processes of cell hypertrophy and hyperplasia, resulting in a variable number of common storage parenchyma layers, interspersed by the redifferentiated sclerenchyma, vascular, and typical nutritive cells around the larval chamber. These nutritive cells accumulate lipids and proteins, which support the lepidopteran larvae nutrition. As expected, the A. parrai galls follow the patterns herein described for the lepidoptera-induced galls, but with peculiarities associated with its host organ. Even though the Lepidoptera galls have destructive mouthparts and can induce large and complex galls, they cannot alter important conservative features of their hosts' organs.

Parasitoid impairment on the galling Lopesia sp. activity reflects on the cytological and histochemical profiles of the globoid bivalve-shaped gall on Mimosa gemmulata

Gall cytological and histochemical features established by the constant feeding activity of the associated gall inducer may be changed due to the attack of parasitoids. We accessed two tri-trophic systems involving the globoid bivalve-shaped gall on Mimosa gemmulata Barneby (Fabaceae) and its galling undescribed species of Lopesia (Diptera: Cecidomyiidae), which may be ectoparasitized by Torymus sp. (Hymenoptera: Torymidae) or endoparasitized by a polyembryonic Platygastridae (Hymenoptera), as models of study. The ectoparasitoid species paralyzes and kills Lopesia sp. larva, which stops the feeding stimuli, while the endoparasitoid larvae feed in Lopesia sp. larva body and keep it alive for a certain time. Our hypothesis is that the time lapse of Lopesia sp. feeding impairment by the two parasitoids will cause distinct cytological and histochemical responses in the ecto- and endoparasitized galls compared to the non-parasitized condition. In both parasitoidism cases, the impairment of the feeding activity of the galling Lopesia sp. directs the common storage and nutritive cells toward a similar process of induced cell death, involving cell collapse and loss of membrane integrity. The cell metabolism is maintained mainly by mitochondria, and by the translocation of lipids from the common storage tissue, via plasmodesmata, through the living sclereids of the mechanical zone toward the nutritive tissue. Accordingly, the parasitoid impairment on the feeding activity of Lopesia sp. larvae causes precocious senescence, but similar cytological alterations, and no impact over the histochemical profiles, regarding lipids, reactive oxygen species, and secondary metabolites, which support gall metabolism along the parasitoid cycles.

Anatomical profiles validate gall morphospecies under similar morphotypes

Plant galls are generated by the stimuli of gall-inducing organisms on their hosts, creating gall morphotypes that vary in color, shape, size, and tissue organization. Herein, we propose to compare the structural features of gall morphotypes on the superhost Croton floribundus (Euphorbiaceae) in order to recognize gall morphospecies, i.e., galls with similar shapes but different internal structures. Non-galled leaves and galls were analyzed macroscopically, histologically, and histochemically for the detection of primary metabolites, and the results obtained were used for statistical analyses of similarity. Among the eight gall morphospecies, four are globoid, two are lenticular, one is fusiform and one is marginal leaf rolling. Stomatal differentiation and the occurrence of different types of trichomes were impaired in some gall morphospecies. Three patterns of organization of the ground system are recognized, ranging from the maintenance of mesophyll cells that differentiate into palisade and spongy cells dorsiventrally to the formation of a complex cortex with three morphofunctional layers. The marginal leaf rolling galls have the simplest anatomical structures, quite similar to those of the non-galled host leaf, while lenticular, globoid (types I to IV), and fusiform galls are anatomically more complex. Herein, we report on eight gall morphospecies occurring on C. floribundus, which are distinguished by morpho-anatomical attributes and show the disruption of the morphogenetic patterns of the host leaf toward the morphogenesis of unique gall features.

Cell death and changes in primary metabolism: the onset of defence in Eucalyptus in the war against Leptocybe invasa

BACKGROUND

Here, we investigated changes in primary metabolism and cell death around oviposition sites in two hybrid clones of Eucalyptus with different degrees of resistance to Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae), as well as tolerance to water deficiency.

RESULTS

We showed that apices of the resistant clone with oviposition had a higher content of amino acids, organic acids and the compound putrescine compared with those of the susceptible clone with oviposition. By contrast, apices of the resistant clone with oviposition had lower sugar and pyruvate organic acid content than those of the susceptible clone with oviposition. Small areas of necrosis were induced around the oviposition sites in the stem apices of Eucalyptus 24 h after infestation. The resistant clone developed larger necrotic areas that showed progressive increases 24-72 h after infestation compared with the susceptible clone, in which cell death was significantly lower and no changes were observed in necrotic area over time. Thus, the programmed death of cells around the egg, modulated by several amino acids, is likely the first defence response of Eucalyptus against L. invasa.

CONCLUSION

Our results serve as the basis for the early identification of key metabolites produced in plants in defence against galling insects. © 2022 Society of Chemical Industry.

Complex meristematic activity induced by Eucecidoses minutanus on Schinus engleri turns shoots into galls

PREMISE

Gall-inducing organisms change the development of their host plant organs, resulting in ontogenetic patterns not observed in the non-galled plants. Distinct taxa induce galls on Schinus spp., manipulating meristematic patterns in the host plant in distinct ways. Here we report ontogenetic novelties induced in the lateral buds of S. engleri by Eucecidoses minutanus, a Cecidosidae, whose galls have been poorly understood.

METHODS

The anatomy, histochemistry, and histometry of galls in distinct phases of development, non-galled buds, and stems of Schinus engleri were analyzed in parallel with the instars of E. minutanus to detail the morphogenetic changes in the host with each larval stage.

RESULTS

Ontogenetic phases of the galls were intricately associated with larval development. First and second-instar larvae induced pericycle and pith cells to dedifferentiate into the gall inner meristem, where hyperplasia and cell hypertrophy characterized the growth and development phase of the gall. The innermost layers were lipid-rich nutritive cells that lined the larval chamber. Additional vascular bundle rows were produced in young galls. Third and fourth instar-larvae were associated with the gall maturation phase: centripetal lignification of the outer parenchyma cell layers, epidermal stratification, and activation of a cambium-like meristem (CLM). The CLM activity resulted in new layers of nutritive cells that differentiated inward as the first layers of nutritive cells were consumed by E. minutanus larvae, and, also, in more parenchyma cell layers that formed outward. All tissues between the innermost layer of nutritive tissue that surround the gall chamber and the outermost layer of the dermal system that externally covers the gall form the gall wall, and increased in thickness until the end of gall maturation.

CONCLUSIONS

E. minutanus induces a structurally complex globoid stem gall, modifying all host plant tissues and stimulating a novel meristematic pattern in S. engleri. The gall developmental stages are each related to specific gall-inducing instars, as gall development progresses according to the development of E. minutanus.

The Variation of Selected Physiological Parameters in Elm Leaves (Ulmus glabra Huds.) Infested by Gall Inducing Aphids

Three aphid species, Eriosoma ulmi (L.), Colopha compressa (Koch) and Tetraneura ulmi (L.) induce distinct gall morphotypes on Ulmus glabra Huds.; opened and closed galls. Because the trophic relationship of aphids and their galls shows that throughout the gall formation aphids can elicit multiple physiological regulations, we evaluated the changes of hydrogen peroxide content (H₂O₂), cytoplasmic membrane condition, expressed as electrolyte leakage (E_L) and concentration of thiobarbituric acid reactive substances (TBARS), as well as, the activity of catalase (CAT), guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) in gall tissues, as well as, in damaged and undamaged parts of galled leaves. All aphid species increased E_L from gall tissues and significantly upregulated APX activity in galls and galled leaves. Alterations in H₂O₂ and TBARS concentrations, as well as GPX and CAT activities, were aphid- and tissue-dependent. The development of pseudo- and closed galls on elm leaves did not have a clear effect on the direction and intensity of the host plant’s physiological response. The different modes of changes in H₂O₂, TBARS, CAT and GPX were found in true galls of C. compressa and T. ulmi. Generally, physiological alterations in new plant tissues were quite different compared to other tissues and could be considered beneficial to galling aphids.

High antioxidant activity of phenolic compounds dampens oxidative stress in Espinosa nothofagi galls induced on Nothofagus obliqua buds

Reactive oxygen species (ROS) are considered the first signaling molecules involved in gall development, linked to the establishment of cyto-histological gradients leading to gall tissue redifferentiation. ROS overproduction induces the failure of gall establishment or its premature senescence. Galls could therefore have efficient mechanisms of ROS dissipation and maintenance of homeostasis, such as polyphenol synthesis. The co-occurrence of ROS and polyphenols in the Espinosa nothofagi galls induced on Nothofagus obliqua buds was explored and was related to the antioxidant capacity of the inner (IC) and outer (OC) gall compartments. We hypothesize that: (i) ROS are produced and accumulated in both tissue compartments of E. nothofagi galls in co-occurrence with polyphenolic, flavonols, and lignin, conferring high antioxidant activity to inner and outer gall tissue compartment; (ii) antioxidant activity is higher in IC related to a higher polyphenol concentration in this compartment. The results show that ROS and polyphenols, mainly flavonols, are produced and accumulated in IC and OC, while lignin accumulated mainly in the IC. In both gall compartments, polyphenols mediate ROS elimination, confirmed by histochemical and spectrophotometry techniques. The IC extract has the highest antioxidant capacity, probably due to lignin deposition and a higher polyphenol concentration in this compartment.

Physiological response of Populus nigra 'Italica' to galling aphids feeding

Although gall-inducing aphids, namely Pemphigus bursarius (L.) and P. phenax Börner et Blunck, are widely known as serious pests for their secondary hosts (lettuce and carrot, respectively), the physiological mechanism of their gall induction on Populus L. trees still requires a better understanding. In this study, we compared physiological parameters, i.e. H₂ O₂ , electrolyte leakage (E_L ), MDA, APX and GPOD. Changes in physiological parameters were analysed in foliar tissues with galls and in the gall tissues themselves and compared to leaves without galls. Significantly higher H₂ O₂ levels were observed in P. phenax galls when compared to leaves with galls. In turn, the highest E_L of cells and MDA content was in P. bursarius galls. Other samples had lower or similar oxidative stress marker levels to leaves without galls. APX and GPOD had similar activity profiles in galls of both aphid species. Their activity decreased significantly in gall tissues, where it was even ten-fold lower than in leaves without galls. Data generated in this study indicate that patterns of the physiological features, e.g. ROS accumulation and cell membrane integrity, of Populus leaves with galls and galls alone depends on the gall-inducing aphid species. Where decreased APX and GPOD activity, especially in gall tissues, indicated a reduction in the antioxidant potential of these neoformed structures.

Structural and Nutritional Peculiarities Related to Lifespan Differences on Four Lopesia Induced Bivalve-Shaped Galls on the Single Super-Host Mimosa gemmulata

Super-host plants are elegant models to evaluate the peculiarities of gall structural and nutritional profiles due to the stimuli of distinct gall inducers in temporal and spatial perspectives. Galls induced by congeneric insects, Lopesia spp. (Diptera, Cecidomyiidae) on the same host plant, Mimosa gemmulata Barneby (Fabaceae) were analyzed to estimate if variations of 1 or 2 months in gall lifespans may result in differences over the accumulation of nutritional resources, and their compartmentalization both in cell walls and protoplasm. Mimosa gemmulata hosts four Lopesia-induced galls: the lenticular bivalve-shaped gall (LG) with a 2-month life cycle, the brown lanceolate bivalve-shaped gall (BLG) and the green lanceolate bivalve-shaped gall (GLG) with 3 month-life cycles, and the globoid bivalve-shaped gall (GG) with a 4 month-life cycle. The comparisons among the four Lopesia galls, using anatomical, histometric, histochemical, and immunocytochemical tools, have demonstrated that the longest lifespan of the GG related to its highest increment in structural and nutritional traits compared with the LG, GLG, and BLG. The differences among the tissue stratification and cell wall thickness of the galls with the 2-month and the 3-month lifespans were subtle. However, the GG had thicker cell walls and higher stratification of the common storage tissue, schlerenchymatic layers and typical nutritive tissue than the other three gall morphospecies. The higher tissue thickness of the GG was followed by the formation of a bidirectional gradient of carbohydrates in the protoplasm, and the detection of xyloglucans in cell walls. Current data supported the presumption that the longest the lifespan, the highest the impact over the structural and nutritional metabolism of the Lopesia galls associated to M. gemmulata.

Insect egg-induced physiological changes and transcriptional reprogramming leading to gall formation

Gall-inducing insects and their hosts present some of the most intricate plant-herbivore interactions. Oviposition on the host is often the first cue of future herbivory and events at this early time point can affect later life stages. Many gallers are devastating plant pests, yet little information regarding the plant-insect molecular interplay exists, particularly following egg deposition. We studied the physiological and transcriptional responses of Eucalyptus following oviposition by the gall-inducing wasp, Leptocybe invasa, to explore potential mechanisms governing defence responses and gall development. RNA sequencing and microscopy were used to explore a susceptible Eucalyptus-L. invasa interaction. Infested and control material was compared over time (1-3, 7 and 90 days post oviposition) to examine the transcriptional and morphological changes. Oviposition induces accumulation of reactive oxygen species and phenolics which is reflected in the transcriptome analysis. Gene expression supports phytohormones and 10 transcription factor subfamilies as key regulators. The egg and oviposition fluid stimulate cell division resulting in gall development. Eucalyptus responses to oviposition are apparent within 24 hr. Putative defences include the oxidative burst and barrier reinforcement. However, egg and oviposition fluid stimuli may redirect these responses towards gall development.

Differential development times of galls induced by Leptocybe invasa (Hymenoptera: Eulophidae) reveal differences in susceptibility between two Eucalyptus clones

BACKGROUND

The significance of morphological responses of hosts on susceptibility against gall-inducing insects is relatively unknown, especially in planted forests. Here, we investigate the temporal morphological responses (gall development) induced by the invasive gall wasp Leptocybe invasa and the subsequent insect development in two Eucalyptus clones.

RESULTS

Our results identified a novel stage of gall development, not previously reported, termed here 'brownish ring'. In both hosts similar gall development stages were observed. Although L. invasa oviposited in both clones, comparison of external morphological traits of galls over time revealed a differential response in the number of galls between clones. Comparison of the developmental time of each gall and insect stage between clones suggests that plant defense mechanisms against L. invasa are activated shortly after oviposition by the wasp, yet before gall formation.

CONCLUSION

Gall number is an important parameter that should be used to measure host susceptibility among Eucalyptus clones. To the best of our knowledge, this is the first study showing differential morphological responses induced by a galling insect, even before gall formation, revealing differences in susceptibility between different plant hosts. These findings provide insight into the use of early stages of gall formation by L. invasa to prevent invasion and establishment of this pest.

Hemicelluloses and associated compounds determine gall functional traits

The intriguing questions concerning gall development refer to the processes of the remodelling of the host plant organ. Such processes involve the restructuring of cell walls and can be influenced by phenolics, indole-3-acetic acid (IAA) and reactive oxygen species (ROS). Alterations in cell walls demand the interference in the coupling of cellulose fibrils and hemicelluloses (xyloglucans) at specific stages of gall development. In addition to cell wall remodelling, hemicelluloses, such as the, xyloglucans and heteromannans can act as reserve carbohydrates, while xylans provide rigidity to the secondary cell walls. Developmental traits of the lenticular, fusiform and globoid galls on Inga ingoides (Fabaceae) were analysed using anatomical, cytometric, histochemical and immunocytochemical tools. Phenolics, IAA and ROS accumulated in similar gall tissue compartments, and may have influenced the restructuring of hemicelluloses and pectins. Contrary to expectations, cell wall flexibility regarding the dynamics of xyloglucans and cellulose fibrils does not relate to a temporal scale. The detection of xyloglucans in nutritive cell walls relate to carbohydrate nutritional resources to the galling insect, while xylans were associated to the lignified cell walls. Heteromanans were not detected, either in non-galled or galled tissues. The patterns of cell expansion during gall development relied on the relationship among phenolics, ROS and IAA with the hemicelluloses (xyloglucans and xylans) and cellulose fibrils. Although cell wall dynamics is specific to each gall morphotype in I. ingoides, the xyloglucans function as carbohydrate reserve to the gall inducers, which constitutes a functional trait common to the three morphotypes.

Pseudophacopteron longicaudatum (Hemiptera) induces intralaminar leaf galls on Aspidosperma tomentosum (Apocynaceae): a qualitative and quantitative structural overview

The structural complexity of galls depends on species-specific interaction driven by the galling taxa. However, the host plant and environment stressors can impose limits on gall developmental patterns and impact the establishment of gall morphology. Herein, we employed qualitative and quantitative approaches in order to elucidate how cell divisions, elongation patterns, and tissue organization are determinant for the development of intralaminar gall morphology induced by Pseudophacopteron longicaudatum Malenovský, Burckhardt, Queiroz, Isaias & Oliveira (Hemiptera: Psylloidea: Phacopteronidae) on leaves of Aspidosperma tomentosum Mart. (Apocynaceae). In addition, we aimed to determine which anatomical process can discriminate the stages of gall development, plus, examine the histochemical and cytological profiles of the galls. The differentiated structures, mainly abaxial epidermis and spongy parenchyma, are associated with gall closure, with hyperplastic events concentrated in the young phase of the galls. Thus, epidermis and spongy parenchyma hypertrophy and are responsible for the determination of the nymphal chamber formation and gall shape. The mature galls do not differentiate into a typical nutritive cells and do not develop a histochemical gradient in their tissues. The cytological features of galls such as plastoglobules and multivesicular bodies are related to ROS scavenging mechanisms due the high oxidative stress.

Apoplast-symplast compartmentalization and functional traits of iron and aluminum in promeristematic tissues of nematode induced galls on Miconia spp

The nutritive tissues of galls induced by Ditylenchus gallaeformans (Nematoda) have promeristematic capacity, which may turn these galls into sinks of Al on their Melastomataceae Al-accumulating hosts. Such a sink of Al may affect gall growth and mineral nutrient intake. Based on the fact that galls are good models for plant developmental studies, we aimed to understand how Al-accumulating host plants in the Cerrado environment deal with Al toxicity in subcellular levels. Here, we used the ICP-OES method to check the variations on mineral nutrients, and the morin, hematoxylin, and Prussian blue stainings for Al and Fe histolocalization in galls induced on four Miconia species of the Brazilian Cerrado. We confirmed the new Al-accumulating feature for two Miconia species of the Cerrado environment. Furthermore, we found that Al accumulates in lesser concentrations in gall tissues than in non-galled tissues of the Miconia hosts. Staining methods indicated that the polyphenols avoid Al-binding to the apoplast and the nucleolus of the promeristematic cells, and mediated its binding to parenchyma cell walls. As well, we inferred that Fe³⁺ is transported by xylem and stored in gall parenchyma, where it is reduced to Fe²⁺, being available in gall nutritive cells. Our results demonstrated an Al compartmentalization between the apoplast and symplast of the inner cell layers in galls, as well as indicated the phenolics action against Al-toxicity and toward Fe availability for the diet of Ditylenchus gallaeformans.

Poplar Tree Response to Feeding by the Petiole Gall Aphid Pemphigus spyrothecae Pass

Pemphigus spyrothecae Pass. which is a member of the subfamily Pemphiginae is one of the gall-inducing aphids that occurs on poplar trees. Phloem feeding of a founding mother on leaf petiole results in the formation of a new organ, i.e., the spiral gall. This study documents aphid development inside the galls during the growing season and the effect of their feeding on leaf architecture and physiology of the host plant. In particular, leaf length, width, and area were measured, as well as hydrogen peroxide (H₂O₂) content, electrolyte leakage (E_L), malondialdehyde (MDA) concentration, and the activity of ascorbate (APX) and guaiacol peroxidase (GPX) were determined in galls and galled leaves. The presence of petiole galls significantly decreased the length, width, and leaf area. Aphid activity increased H₂O₂ concentration in galls and E_L from galls and leaf tissues, which was accompanied by a strong decrease in MDA content and both peroxidase activities, especially in gall tissues. It can be suggested that P. spyrothecae can manipulate physiological machinery of the host plant for its own benefit.

Phytophages of linden under the conditions of Grodno Ponemany (Belarus) and Krasnodar (Russia)

Comparative studies of linden phytophages in the urban landscapes of Grodno Ponemany (Belarus) and Krasnodar (Russia) have been carried out. 22 arthropod species were revealed, including 6 species of herbivorous ticks and 16 species of insects. Among these organisms, 18 species are representatives of the native fauna, 4 are invasive, while the invaders in the Grodno Ponemany and in Krasnodar are different (two species each). In Grodno Ponemany, oligophages prevail, in Krasnodar, the proportion of polyphages is high. The invasive species Phyllonorycter issikii (Kumata) and native species of gall mites are the most harmful in Belarus. The significance of the invaders in Krasnodar is much lower, and Metcalfa pruinosa Say, and Hyphantria cunea Drury cause significant damage to linden only in the years of sharp increase in number. In Krasnodar, linden spider mite – Schizotetranychus tiliarium (Hermann) is the most harmful, populating 100 % of linden trees older than 10 years. Almost all polyphages, with the exception of the invaders identified for Krasnodar, exhibit moderate or low harmfulness.

Spatiotemporal variation in phenolic levels in galls of calophyids on Schinus polygama (Anacardiaceae)

The expression of plant secondary metabolism is strongly controlled by plant both in time and space. Although the variation of secondary metabolites, such as soluble and structural phenolics (e.g., lignins), has been largely observed in gall-inducing insects, and compared to their non-galled host organs, only a few datasets recording such variation are available. Accordingly, the relative importance of spatiotemporal variability in phenolic contents, and the influence of gall developmental stages on the original composition of host organs are poorly discussed. To address this knowledge gap, we histochemically determined the sites of polyphenol and lignin accumulation, and the polyphenol contents in three developmental stages of two calophyid galls and their correspondent host organs. Current results indicate that the compartmentalization of phenolics and lignins on Schinus polygama (Cav.) Cabrera follows a similar pattern in the two-calophyid galls, accumulating in the outer (the external tissue layers) and in the inner tissue compartments (the cell layers in contact with the gall chamber). The non-accumulation in the median compartment (median parenchyma layers of gall wall with vascular bundles, where gall inducer feeds) is important for the inducer, because its mouth apparatus enter in contact with the cells of this compartment. Also, the concentration of phenolics has opposite dynamics, decreasing in leaf galls and increasing in stem galls, in temporal scale, i.e., from maturation toward senescence. The concentration of phenolics in non-galled host organs, and in both galls indicated the extended phenotype of Calophya rubra (Blanchard) and C. mammifex Burckhardt & Basset (Hemiptera: Sternorrhyncha: Psylloidea: Calophyidae) over the same host plant metabolic potentiality.

Red galls: the different stories of two gall types on the same host

Several studies have suggested reasons why galls have conspicuous colours, but none of the ideas have been confirmed. However, what if the vibrant colours of some galls are explained simply by the effect of light exposure? This may lead to anthocyanin accumulation, functioning as a defence mechanism against the effects of high light. We studied the globoid galls induced by Cecidomyiidae (Diptera) on Qualea parviflora (Vochysiaceae), relating anthocyanin accumulation and chlorophyll fluorescence parameters to light incidence in abaxial and adaxial galls. We also tested if the anthocyanin accumulation patterns apply to another Cecidomyiidae-induced gall morphotype (intralaminar) within the same plant. Adaxial galls are exposed to higher incident light, with more anthocyanin accumulation and therefore red coloration. In galls from angled leaves, the greater the angle of the leaf, the higher the difference between anthocyanins on the sun and shade sides of galls. Photosynthetic pigment concentrations did not differ between abaxial and adaxial galls. However, we found higher (F_m ' - F')/F_m ' and F_v /F_m in the abaxial galls. Conversely, NPQ and R_fd were higher in adaxial galls. Finally, the pattern of anthocyanin accumulation was not found in the intralaminar gall. Anthocyanin accumulation in galls functions as a photoprotective strategy, maintaining tissue vitality in regions exposed to high light conditions. However, this mechanism may vary even among galls within the same host, indicating idiosyncrasy when it comes to coloration in galls. To date, this is the first study to demonstrate quantitatively why the galls of a specific species may be coloured: the variation in light regimes creates differential anthocyanin accumulation, influencing coloration.

Structural and Chemical Profiles of Myrcia splendens (Myrtaceae) Leaves Under the Influence of the Galling Nexothrips sp. (Thysanoptera)

Thysanoptera-induced galls commonly culminate in simple folding or rolling leaf gall morphotypes. Most of these galls are induced by members of the suborder Tubulifera, with only a few species of the suborder Terebrantia being reported as gall inducers. The Terebrantia, as most of the gall inducers, manipulates the host plant cellular communication system, and induces anatomical and biochemical changes in its host plant. In an effort to keep its homeostasis, the host plant reacts to the stimuli of the galling insect and triggers chemical signaling processes. In contrast to free-living herbivores, the signaling processes involving galling herbivores and their host plants are practically unknown. Current investigation was performed into two steps: first, we set the structural profile of non-galled and galled leaves, and looked forward to find potential alterations due to gall induction by an undescribed species of Nexothrips (suborder Terebrantia) on Myrcia splendens. Once oil glands had been altered in size and number, the second step was the investigation of the chemical profile of three tissue samples: (1) non-galled leaves of a control individual, (2) non-galled leaves of galled plants, and (3) galls. This third sample was divided into two groups: (3.1) galls from which the inducing thrips were manually removed and (3.2) galls macerated with the inducing thrips inside. The chemical profile was performed by gas chromatography/ mass spectrometric detector after headspace solid-phase extraction. The galling activity of the Nexothrips sp. on M. splendens culminates in mesophyll compactness interspersed to diminutive hypersensitive spots, development of air cavities, and the increase in size and number of the secretory glands. Seventy-two compounds were completely identified in the volatile profile of the three samples, from which, sesquiterpenes and aldehydes, pertaining to the "green leaf volatile" (GLVs) class, are the most abundant. The rare event of gall induction by a Terebrantia revealed discrete alterations toward leaf rolling, and indicated quantitative differences related to the plant bioactivity manipulated by the galling thrips. Also, the content of methyl salicylate has varied and has been considered a potential biomarker of plant resistance stimulated as a long-distance effect on M. splendens individuals.

Antioxidant metabolism in galls due to the extended phenotypes of the associated organisms

Animal-induced galls are considered extended phenotypes of their inducers, and therefore plant morphogenesis and metabolism may vary according to the species of gall inducers. The alterations in vacuolar and apoplastic polyphenols, carotenoids, chlorophyll fluorescence rates, PSII quantum yield, and phospholipid peroxidation were studied in galls induced by Ditylenchus gallaeformans (Nematoda) on Miconia albicans and M. ibaguensis (Melastomataceae), and by an unidentified Eriophyidae (Acarina) on M. ibaguensis. The focus currently addressed is gall metabolism as the extended phenotype of the gall inducers, and the neglected determination of gall functionalities over host plant peculiarities. Galls induced by D. gallaeformans on M. albicans and by the Eriophyidae on M. ibaguensis have increased accumulation of apoplastic and vacuolar phenolics, which is related to the control of phospholipid peroxidation and photoprotection. The galls induced by D. gallaeformans on M. ibaguensis have higher carotenoid and vacuolar polyphenol contents, which are related to excessive sunlight energy dissipation as heat, and photoprotection. Accordingly, antioxidant strategies varied according to the gall-inducing species and to the host plant species. The distinctive investments in carotenoid and/or in polyphenol concentrations in the studied galls seemed to be peculiar mechanisms to maintain oxidative homeostasis. These mechanisms were determined both by the stimuli of the gall-inducing organism and by the intrinsic physiological features of the host plant species. Therefore, the roles of both associated organisms in host plant-galling organisms systems over gall metabolism is attested.

Metabolic and functional distinction of the Smicronyx sp. galls on Cuscuta campestris

The weevil gall contains two distinct regions, differing in hydrolytic and antioxidant enzymes activity and profiles, which is also functionally distinct from the non-infected Cuscuta stems. Weevils of the genus Smicronyx are gall-forming insects, widely distributed on parasitic flowering plants of the genus Cuscuta. Thus, they are considered epiparasites and potential method for biological control of their agriculturally harmful hosts. Although several reports on gall formation in Cuscuta spp. exist, the metabolic and functional changes, occurring in the gall, remained largely unknown. Smicronyx sp. galls, collected from a wild Cuscuta campestris population, were dissected into two distinct regions, inner and outer cortex, defined by the higher chlorophyll content of the inner cortex. Based on hydrolytic and antioxidant enzymes activity and isoenzymatic profiles as analyzed after electrophoretic separation, we suggested that the gall differs in its metabolic activity from the non-infected plant tissue. While the outer cortex serves as a region of nutrient storage and mobilization, the inner cortex is directly involved in larvae nutrition. The increase in metabolic activity resulted in significantly increased superoxide dismutase activity in the gall, while several other antioxidant enzymes diminished. The present research offers new insights into the functionally differing regions of Smicronyx galls and the metabolic changes, induced in C. campestris in result of the gall formation.

Tetraneura ulmi (Hemiptera: Eriosomatinae) Induces Oxidative Stress and Alters Antioxidant Enzyme Activities in Elm Leaves

Gall formation is induced by an insect, which changes normal plant development and results in the formation of a new organ, following distinct stages of metabolic and developmental alterations. Research on mechanisms of recognition and responses to biotic stress may help to understand the interactions between galling aphids and their host plants. In this study, Tetraneura ulmi L. (Hemiptera: Eriosomatinae) galls and Ulmus pumila L. (Rosales: Ulmaceae) leaves were used as a model. Concentrations of hydrogen peroxide (H2O2) and thiobarbituric acid reactive substances, electrolyte leakage, as well as the activity of ascorbate peroxidase, guaiacol peroxidase, and catalase (CAT) were determined in galls and two parts of galled leaves (with and without visible damage). Biochemical analyses were performed at three stages of gall development: initial, fully developed, and mature galls. A slight increment in H2O2 content with a strong enhancement of ascorbate peroxidase and CAT activities were observed in galls and galled leaves in the first stage. In subsequent stages of gall development, a progressing increase in H2O2 production and cell membrane damage was associated with declining antioxidant enzyme activities, especially in gall tissues. The stages of gall development are likely to be part of cell death triggered by aphid feeding. It seems that the gall is the result of a biochemical struggle between the host plant and the gall inducer.

Physiological Response of Pedunculate Oak Trees to Gall-Inducing Cynipidae

Gall-inducing Cynipidae (Hymenoptera) manipulate the leaves of their host plants and induce local resistance, resulting in a diversity of physiological changes. In this study, three gall morphotypes caused by the asexual generation of Cynips quercusfolii L., Neuroterus numismalis (Fourc.) and Neuroterus quercusbaccarum L. (Hymenoptera: Cynipidae) on pedunculate oaks (Quercus robur L. (Fagales: Fagaceae)), were used as a model to examine physiological alterations in galls and foliar tissues, compared to non-galled tissues. Our goal was to investigate whether plant physiological response to insect feeding on the same host plant varies depending on gall-wasp species. In particular, the cytoplasmic membrane condition, hydrogen peroxide (H2O2) concentration and changes in antioxidative enzyme activities, including guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) were examined in this study. All cynipid species increased H2O2 levels in the leaves with galls, while the level of H2O2 in galls depended on the species. The presence of galls of all species on oak leaves caused an increase of electrolyte leakage and lipid peroxidation level. A significant induction of GPX activity was observed in the leaves with galls of all species, indicating stress induction. Conversely, the decrease in APX activity in both leaves with galls and galled tissues exposed to feeding of all cynipid species.

Preventing False Negatives for Histochemical Detection of Phenolics and Lignins in PEG-Embedded Plant Tissues

Polyethylene glycol (PEG) is a low-cost and advantageous embedding medium, which maintains the majority of cell contents unaltered during the embedding process. Some hard or complex plant materials are better embedded in PEG than in other usual embedding media. However, the histochemical tests for phenolics and lignins in PEG-embedded plant tissues commonly result in false negatives. We hypothesize that these false negatives should be prevented by the use of distinct fixatives, which should avoid the bonds between PEG and phenols. Novel protocols for phenolics and flavanols detection are efficiently tested, with fixation of the samples in ferrous sulfate and formalin or in caffeine and sodium benzoate, respectively. The differentiation of lignin types is possible in safranin-stained sections observed under fluorescence. The Maule's test faultlessly distinguishes syringyl-rich from guaiacyl- and hydroxyphenyl-rich lignins in PEG-embedded material under light microscopy. Current hypothesis is corroborated, that is, the adequate fixation solves the false-negative results, and the new proposed protocols fill up some gaps on the detection of phenolics and lignins.

Reacquisition of New Meristematic Sites Determines the Development of a New Organ, the Cecidomyiidae Gall on Copaifera langsdorffii Desf. (Fabaceae)

The development of gall shapes has been attributed to the feeding behavior of the galling insects and how the host tissues react to galling stimuli, which ultimately culminate in a variable set of structural responses. A superhost of galling herbivores, Copaifera langsdorffii, hosts a bizarre "horn-shaped" leaflet gall morphotype induced by an unidentified species of Diptera: Cecidomyiidae. By studying the development of this gall morphotype under the anatomical and physiological perspectives, we demonstrate the symptoms of the Cecidomyiidae manipulation over plant tissues, toward the cell redifferentiation and tissue neoformation. The most prominent feature of this gall is the shifting in shape from growth and development phase toward maturation, which imply in metabolites accumulation detected by histochemical tests in meristem-like group of cells within gall structure. We hypothesize that the development of complex galls, such as the horn-shaped demands the reacquisition of cell meristematic competence. Also, as mature galls are green, their photosynthetic activity should be sufficient for their oxygenation, thus compensating the low gas diffusion through the compacted gall parenchyma. We currently conclude that the galling Cecidomyiidae triggers the establishment of new sites of meristematic tissues, which are ultimately responsible for shifting from the young conical to the mature horn-shaped gall morphotype. Accordingly, the conservative photosynthesis activity in gall site maintains tissue homeostasis by avoiding hypoxia and hipercarbia in the highly compacted gall tissues.

Sink Status and Photosynthetic Rate of the Leaflet Galls Induced by Bystracoccus mataybae (Eriococcidae) on Matayba guianensis (Sapindaceae)

The galling insect Bystracoccus mataybae (Eriococcidae) induces green and intralaminar galls on leaflets of Matayba guianensis (Sapindaceae), and promotes a high oxidative stress in host plant tissues. This biotic stress is assumed by the histochemical detection of hydrogen peroxide, a reactive oxygen species (ROS), whose production alters gall physiology. Thus, we hypothesize that high levels of nutrients are accumulated during gall development in response to a local maintenance of photosynthesis and to the galling insect activity. Moreover, the maintenance of low levels of photosynthesis may guarantee O₂ production and CO₂ consumption, as well as may avoid hypoxia and hypercarbia in gall tissues. To access the photosynthesis performance, the distribution of chlorophyllous tissues and the photochemical and carboxylation rates in gall tissues were analyzed. In addition, histochemical tests for hydrogen peroxide and phenolic derivatives were performed to confirm the biotic stress, and set the possible sites where stress dissipation occurs. The contents of sugars and nitrogen were evaluated to quantify the gall sink. Currently, we assume that the homeostasis in gall tissues is ruptured by the oxidative stress promoted by the galling insect activity. Thus, to supply the demands of gall metabolism, the levels of water-soluble polysaccharides and starch increase in gall tissues. The low values of maximum quantum efficiency of PSII (F_v/F_m) indicate a low photosynthetic performance in gall tissues. In addition, the decrease of PSII operating efficiency, (F'm-F')/F'm, and Rfd (instantaneous fluorescence decline ratio in light, to measure tissue vitality) demonstrate that the tissues of B. mataybae galls are more susceptible to damage caused by stressors than the non-galled tissues. Thus, the high oxidative stress in gall developmental sites is dissipated not only by the accumulation of phenolic derivatives in the protoplast, but also of lignins in the walls of neoformed sclereids.

Manipulation of host plant cells and tissues by gall-inducing insects and adaptive strategies used by different feeding guilds

Biologists who study insect-induced plant galls are faced with the overwhelming diversity of plant forms and insect species. A challenge is to find common themes amidst this diversity. We discuss common themes that have emerged from our cytological and histochemical studies of diverse neotropical insect-induced galls. Gall initiation begins with recognition of reactive plant tissues by gall inducers, with subsequent feeding and/or oviposition triggering a cascade of events. Besides, to induce the gall structure insects have to synchronize their life cycle with plant host phenology. We predict that reactive oxygen species (ROS) play a role in gall induction, development and histochemical gradient formation. Controlled levels of ROS mediate the accumulation of (poly)phenols, and phytohormones (such as auxin) at gall sites, which contributes to the new cell developmental pathways and biochemical alterations that lead to gall formation. The classical idea of an insect-induced gall is a chamber lined with a nutritive tissue that is occupied by an insect that directly harvests nutrients from nutritive cells via its mouthparts, which function mechanically and/or as a delivery system for salivary secretions. By studying diverse gall-inducing insects we have discovered that insects with needle-like sucking mouthparts may also induce a nutritive tissue, whose nutrients are indirectly harvested as the gall-inducing insects feeds on adjacent vascular tissues. Activity of carbohydrate-related enzymes across diverse galls corroborates this hypothesis. Our research points to the importance of cytological and histochemical studies for elucidating mechanisms of induced susceptibility and induced resistance.