Induction of phenolic glycosides by quaking aspen (Populus tremuloides) leaves in relation to extrafloral nectaries and epidermal leaf mining

A Fungal Endophyte Alters Poplar Leaf Chemistry, Deters Insect Feeding and Shapes Insect Community Assembly

Fungal endophytes of grasses and other herbaceous plants have been known to provide plants with anti-herbivore defence compounds, but there is little information about whether the endophytes of trees also engage in such mutualisms. We investigated the influence of the endophytic fungus Cladosporium sp. on the chemical defences of black poplar (Populus nigra) trees and the consequences for feeding preference and fitness of herbivorous insects and insect community assembly. Endophyte colonisation increased both constitutive- and induced poplar defences. Generalist Lymantria dispar larvae preferred and performed better on uninfected over endophyte-infected poplar leaves, most likely due to higher concentrations of salicinoids in endophyte-inoculated leaves and the endophyte-produced alkaloid stachydrine. Under field conditions, the endophytic fungus shapes insect community assembly i. a. attracting aphids, which can excrete stachydrine. Our results show that endophytic fungi play a crucial role in the defence against insects from different feeding guilds and thereby structuring insect communities.

Dynamic Metabolic Responses of Resistant and Susceptible Poplar Clones Induced by Hyphantria cunea Feeding

Poplar trees are significant for both economic and ecological purposes, and the fall webworm (Hyphantria cunea Drury) poses a major threat to their plantation in China. The preliminary resistance assessment in the previous research indicated that there were differences in resistance to the insect among these varieties, with '2KEN8' being more resistant and 'Nankang' being more susceptible. The present study analyzed the dynamic changes in the defensive enzymes and metabolic profiles of '2KEN8' and 'Nankang' at 24 hours post-infestation (hpi), 48 hpi, and 96 hpi. The results demonstrated that at the same time points, compared to susceptible 'Nankang', the leaf consumption by H. cunea in '2KEN8' was smaller, and the larval weight gain was slower, exhibiting clear resistance to the insect. Biochemical analysis revealed that the increased activity of the defensive enzymes in '2KEN8' triggered by the feeding of H. cunea was significantly higher than that of 'Nankang'. Metabolomics analysis indicated that '2KEN8' initiated an earlier and more intense reprogramming of the metabolic profile post-infestation. In the early stages of infestation, the differential metabolites induced in '2KEN8' primarily included phenolic compounds, flavonoids, and unsaturated fatty acids, which are related to the biosynthesis pathways of phenylpropanoids, flavonoids, unsaturated fatty acids, and jasmonates. The present study is helpful for identifying the metabolic biomarkers for inductive resistance to H. cunea and lays a foundation for the further elucidation of the chemical resistance mechanism of poplar trees against this insect.

Resistance against Leucoptera sinuella (Lepidoptera: Lyonetiidae) among hybrid clones of Populus spp. in central Chile

Leucoptera sinuella (Reutti) (Lepidoptera: Lyonetiidae) is a leaf miner specialist on Salicaceae recently introduced to Chile and Argentina, where it is causing economic damage to poplar plantations. We report a field survey in a poplar nursery naturally infested showing that regardless of the poplar hybrid taxon, high variability in resistance was observed among clones within families for oviposition and leaf-mining damage. A group of susceptible and resistant hybrid poplar clones was then selected for a laboratory evaluation of oviposition (antixenosis) and leaf-mining damage (antibiosis) on potted, rooted shoot cuttings. The concentration of condensed tannins (CTs) and salicinoid phenolic glucosides (SPGs) of the leaves of the selected clones from the laboratory study was also measured. Total oviposited eggs were positively correlated with leaf area, with the lowest oviposition on TMxT 11372 clone. The lowest percentage of mined leaf area was obtained for clones TMxT 11372, TMxT 11463, and TDxD 17574, but surprisingly no correlation between the percentage of mined leaf area and concentration of CTs and SPGs was found. Resistant poplar hybrids of our study could be suitable for breeding programs aimed for L. sinuella integrated pest management.

Detecting Pest-Infested Forest Damage through Multispectral Satellite Imagery and Improved UNet+

Plant pests are the primary biological threats to agricultural and forestry production as well as forest ecosystem. Monitoring forest-pest damage via satellite images is crucial for the development of prevention and control strategies. Previous studies utilizing deep learning to monitor pest-infested damage in satellite imagery adopted RGB images, while multispectral imagery and vegetation indices were not used. Multispectral images and vegetation indices contain a wealth of useful information for detecting plant health, which can improve the precision of pest damage detection. The aim of the study is to further improve forest-pest infestation area segmentation by combining multispectral, vegetation indices and RGB information into deep learning. We also propose a new image segmentation method based on UNet++ with attention mechanism module for detecting forest damage induced by bark beetle and aspen leaf miner in Sentinel-2 images. The ResNeSt101 is used as the feature extraction backbone, and the attention mechanism scSE module is introduced in the decoding phase for improving the image segmentation results. We used Sentinel-2 imagery to produce a dataset based on forest health damage data gathered by the Ministry of Forests, Lands, Natural Resource Operations and Rural Development (FLNRORD) in British Columbia (BC), Canada, during aerial overview surveys (AOS) in 2020. The dataset contains the 11 original Sentinel-2 bands and 13 vegetation indices. The experimental results confirmed that the significance of vegetation indices and multispectral data in enhancing the segmentation effect. The results demonstrated that the proposed method exhibits better segmentation quality and more accurate quantitative indices with overall accuracy of 85.11%, in comparison with the state-of-the-art pest area segmentation methods.

Widespread mortality of trembling aspen (Populus tremuloides) throughout interior Alaskan boreal forests resulting from a novel canker disease

Over the past several decades, growth declines and mortality of trembling aspen throughout western Canada and the United States have been linked to drought, often interacting with outbreaks of insects and fungal pathogens, resulting in a “sudden aspen decline” throughout much of aspen’s range. In 2015, we noticed an aggressive fungal canker causing widespread mortality of aspen throughout interior Alaska and initiated a study to quantify potential drivers for the incidence, virulence, and distribution of the disease. Stand-level infection rates among 88 study sites distributed across 6 Alaska ecoregions ranged from <1 to 69%, with the proportion of trees with canker that were dead averaging 70% across all sites. The disease is most prevalent north of the Alaska Range within the Tanana Kuskokwim ecoregion. Modeling canker probability as a function of ecoregion, stand structure, landscape position, and climate revealed that smaller-diameter trees in older stands with greater aspen basal area have the highest canker incidence and mortality, while younger trees in younger stands appear virtually immune to the disease. Sites with higher summer vapor pressure deficits had significantly higher levels of canker infection and mortality. We believe the combined effects of this novel fungal canker pathogen, drought, and the persistent aspen leaf miner outbreak are triggering feedbacks between carbon starvation and hydraulic failure that are ultimately driving widespread mortality. Warmer early-season temperatures and prolonged late summer drought are leading to larger and more severe wildfires throughout interior Alaska that are favoring a shift from black spruce to forests dominated by Alaska paper birch and aspen. Widespread aspen mortality fostered by this rapidly spreading pathogen has significant implications for successional dynamics, ecosystem function, and feedbacks to disturbance regimes, particularly on sites too dry for Alaska paper birch.

Metabolic Responses of Poplar to Apripona germari (Hope) as Revealed by Metabolite Profiling

Plants have developed biochemical responses to adapt to biotic stress. To characterize the resistance mechanisms in poplar tree against Apripona germari, comprehensive metabolomic changes of poplar bark and xylem in response to A. germari infection were examined by gas chromatography time-of-flight mass spectrometry (GC–TOF/MS). It was found that, four days after feeding (stage I), A. germari infection brought about changes in various metabolites, such as phenolics, amino acids and sugars in both bark and xylem. Quinic acid, epicatechin, epigallocatechin and salicin might play a role in resistance response in bark, while coniferyl alcohol, ferulic acid and salicin contribute resistance in xylem. At feeding stages II when the larvae fed for more than one month, fewer defensive metabolites were induced, but levels of many intermediates of glycolysis and the tricarboxylic acid (TCA) cycle were reduced, especially in xylem. These results suggested that the defense strategies against A. germari might depend mainly on the early defense responses in poplar. In addition, it was found that bark and xylem in infected trees accumulated higher levels of salicylic acid and 4-aminobutyric acid, respectively, these tissues displaying a direct and systemic reaction against A. germari. However, the actual role of the two metabolites in A. germari-induced defense in poplar requires further investigation.

How insects overcome two-component plant chemical defence: plant β-glucosidases as the main target for herbivore adaptation

Insect herbivory is often restricted by glucosylated plant chemical defence compounds that are activated by plant β-glucosidases to release toxic aglucones upon plant tissue damage. Such two-component plant defences are widespread in the plant kingdom and examples of these classes of compounds are alkaloid, benzoxazinoid, cyanogenic and iridoid glucosides as well as glucosinolates and salicinoids. Conversely, many insects have evolved a diversity of counteradaptations to overcome this type of constitutive chemical defence. Here we discuss that such counter-adaptations occur at different time points, before and during feeding as well as during digestion, and at several levels such as the insects’ feeding behaviour, physiology and metabolism. Insect adaptations frequently circumvent or counteract the activity of the plant β-glucosidases, bioactivating enzymes that are a key element in the plant’s two-component chemical defence. These adaptations include host plant choice, non-disruptive feeding guilds and various physiological adaptations as well as metabolic enzymatic strategies of the insect’s digestive system. Furthermore, insect adaptations often act in combination, may exist in both generalists and specialists, and can act on different classes of defence compounds. We discuss how generalist and specialist insects appear to differ in their ability to use these different types of adaptations: in generalists, adaptations are often inducible, whereas in specialists they are often constitutive. Future studies are suggested to investigate in detail how insect adaptations act in combination to overcome plant chemical defences and to allow ecologically relevant conclusions.

Phenolic glycosides of the Salicaceae and their role as anti-herbivore defenses

Since the 19th century the phytochemistry of the Salicaceae has been systematically investigated, initially for pharmaceutical and later for ecological reasons. The result of these efforts is a rich knowledge about the phenolic components, especially a series of glycosylated and esterified derivatives of salicyl alcohol known as "phenolic glycosides". These substances have received extensive attention with regard to their part in plant-herbivore interactions. The negative impact of phenolic glycosides on the performance of many generalist herbivores has been reported in numerous studies. Other more specialized feeders are less susceptible and have even been reported to sequester phenolic glycosides for their own defense. In this review, we attempt to summarize our current knowledge about the role of phenolic glycosides in mediating plant-herbivore interactions. As background, we first review what is known about their basic chemistry and occurrence in plants.