Physiological Response of Pedunculate Oak Trees to Gall-Inducing Cynipidae

Comparative transcriptome reprogramming in oak galls containing asexual or sexual generations of gall wasps

Oak gall wasps have evolved strategies to manipulate the developmental pathways of their host to induce gall formation. This provides shelter and nutrients for the developing larva. Galls are entirely host tissue; however, the initiation, development, and physical appearance are controlled by the inducer. The underlying molecular mechanisms of gall formation, by which one or a small number of cells are reprogrammed and commit to a novel developmental path, are poorly understood. In this study, we sought a deeper insight into the molecular underpinnings of this process. Oak gall wasps have two generations each year, one sexual, and one asexual. Galls formed by these two generations exhibit a markedly different appearance. We sequenced transcriptomes of both the asexual and sexual generations of Neuroterus quercusbaccarum and Neuroterus numismalis. We then deployed Nanopore sequencing to generate long-read sequences to test the hypothesis that gall wasps introduce DNA insertions to determine gall development. We detected potential genome rearrangements but did not uncover any non-host DNA insertions. Transcriptome analysis revealed that transcriptomes of the sexual generations of distinct species of wasp are more similar than inter-generational comparisons from the same species of wasp. Our results highlight the intricate interplay between the host leaves and gall development, suggesting that season and requirements of the gall structure play a larger role than species in controlling gall development and structure.

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.

Isolation, Identification, and Analysis of Potential Functions of Culturable Bacteria Associated with an Invasive Gall Wasp, Leptocybe invasa

Symbioses between invasive insects and bacteria are one of the key drivers of insect invasion success. Gall-inducing insects stimulate host plants to produce galls, which affects the normal growth of plants. Leptocybe invasa Fisher et La Salle, an invasive gall-inducing wasp, mainly damages Eucalyptus plantations in Southern China, but little is known about its associated bacteria. The aim of this study was to assess the diversity of bacterial communities at different developmental stages of L. invasa and to identify possible ecological functions of the associated bacteria. Bacteria associated with L. invasa were isolated using culture-dependent methods and their taxonomic statuses were determined by sequencing the 16S rRNA gene. A total of 88 species belonging to four phyla, 27 families, and 44 genera were identified by phylogenetic analysis. The four phyla were Proteobacteria, Actinobacteria, Firmicutes, and Bacteroidetes, mainly from the genera Pantoea, Enterobacter, Pseudomonas, Bacillus, Acinetobacter, Curtobacterium, Sphingobium, Klebsiella, and Rhizobium. Among them, 72 species were isolated in the insect gall stage and 46 species were isolated from the adult stage. The most abundant bacterial species were γ-Proteobacteria. We found significant differences in total bacterial counts and community compositions at different developmental stages, and identified possible ecological roles of L. invasa-associated bacteria. This study is the first to systematically investigate the associated bacteria of L. invasa using culture-dependent methods, and provides a reference for other gall-inducing insects and associated bacteria.

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.

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.

Effect of Neuroterus quercusbaccarum (L.) galls on physiological and biochemical response of Quercus robur leaves

Gall formation is associated with multiple changes in plant cells, which still requires a better understanding. In this study, galls caused by sexual generation (♀♂) of Neuroterus quercusbaccarum (L.) (Hymenoptera: Cynipidae) on pedunculate oak trees (Quercus robur L.) were used as a model. Cytoplasmic membrane condition, concentration of hydrogen peroxide (H2O2), the activity of antioxidant enzymes and amino acid decarboxylase as well as chlorophyll fluorescence parameters were determined. Changes in physiological and biochemical parameters were analyzed in foliar tissues with galls and gall tissues themselves and compared to control. The presence of galls on oak leaves caused an increase of lipid peroxidation level. A significant decline in H2O2 and TBARS content with the reduction of guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) activity were observed in gall tissues. The activity amino acid decarboxylase, i.e., LDC, ODC and TyDC varied between samples, which may affect the content of amino acids. The presence of N. quercusbaccarum galls caused an insignificant increase of the chlorophylls, carotenoids and anthocyanin contents, while the content of pigments and their ratios in gall tissues was extremely low. Moreover, photosynthetic parameters (F0, Fm, Fv/Fm, Y, qP) were significantly decreased. Data generated in this study indicate that the development of N. quercusbaccarum galls on pedunculate oak leaves has a negative effect on host plant related to the disruption of cell membrane integrity, disturbance of photosynthesis and reduction of the antioxidant potential of the host plant.

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.