Anatomical and chemical defenses of conifer bark against bark beetles and other pests.
TL;DR: This review focuses on bark defenses, a front line against organisms trying to reach the nutrient-rich phloem, and questions about their coevolution with bark beetles are discussed.
Abstract: Conifers are long-lived organisms, and part of their success is due to their potent defense mechanisms. This review focuses on bark defenses, a front line against organisms trying to reach the nutrient-rich phloem. A major breach of the bark can lead to tree death, as evidenced by the millions of trees killed every year by specialized bark-invading insects. Different defense strategies have arisen in conifer lineages, but the general strategy is one of overlapping constitutive mechanical and chemical defenses overlaid with the capacity to up-regulate additional defenses. The defense strategy incorporates a graded response from 'repel', through 'defend' and 'kill', to 'compartmentalize', depending upon the advance of the invading organism. Using a combination of toxic and polymer chemistry, anatomical structures and their placement, and inducible defenses, conifers have evolved bark defense mechanisms that work against a variety of pests. However, these can be overcome by strategies including aggregation pheromones of bark beetles and introduction of virulent phytopathogens. The defense structures and chemicals in conifer bark are reviewed and questions about their coevolution with bark beetles are discussed.
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TL;DR: In this article, a conceptual framework using one system as a model, emphasizing interactions across levels of biological hierarchy and spatiotemporal scales is presented, and the dynamics are bidirectional as landscape features influence how lower-scale processes are amplified or buffered.
Abstract: Biome-scaledisturbancesbyeruptiveherbivoresprovidevaluableinsightsintospeciesinteractions,ecosystemfunction,andimpactsof globalchange. We present a conceptual framework using one system as a model, emphasizing interactions across levels of biological hierarchy and spatiotemporal scales. Bark beetles are major natural disturbance agents in western North American forests. However, recent bark beetle population eruptions have exceeded the frequencies, impacts, and ranges documented during the previous 125 years. Extensive host abundance and susceptibility, concentrated beetle density, favorable weather, optimal symbiotic associations, and escape from natural enemies must occur jointly for beetles to surpass a series of thresholds and exert widespread disturbance. Opposing feedbacks determine qualitatively distinct outcomes at junctures at the biochemical through landscape levels. Eruptions occur when key thresholds are surpassed, prior constraints cease to exert influence, and positive feedbacks amplify across scales. These dynamics are bidirectional, as landscape features influence how lower-scale processes are amplified or buffered. Climate change and reduced habitat heterogeneity increase the likelihood that key thresholds will be exceeded, and may cause fundamental regime shifts. Systems in which endogenous feedbacks can dominate after external forces foster the initial breach of thresholds appear particularly sensitive to anthropogenic perturbations.
1,561 citations
Cites background from "Anatomical and chemical defenses of..."
...…of bark beetles and associated microorganisms have selected for sophisticated conifer defenses that integrate physical, chemical, and histological constitutive and induced mechanisms (Bohlmann et al. 2000, Franceschi et al. 2005, Martin and Bohlmann 2005, Raffa et al. 2005, and references therein)....
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...…gene activation within the host tree occur through the 1-deoxy-D-xylulose5-phosphate, mevalonate, and shikimic acid pathways (Martin and Bohlmann 2005, Keeling and Bohlmann 2006), and are regulated by signaling involving jasmonates and ethylene (Franceschi et al. 2005, Martin and Bohlmann 2005)....
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TL;DR: A synthesis of climate change effects on native bark beetles, important mortality agents of conifers in western North America, is provided and a movement of temperature suitability to higher latitudes and elevations is suggested.
Abstract: Climatic changes are predicted to significantly affect the frequency and severity of disturbances that shape forest ecosystems. We provide a synthesis of climate change effects on native bark beetles, important mortality agents of conifers in western North America. Because of differences in temperature-dependent life-history strategies, including cold-induced mortality and developmental timing, responses to warming will differ among and within bark beetle species. The success of bark beetle populations will also be influenced indirectly by the effects of climate on community associates and host-tree vigor, although little information is available to quantify these relationships. We used available population models and climate forecasts to explore the responses of two eruptive bark beetle species. Based on projected warming, increases in thermal regimes conducive to population success are predicted for Dendroctonus rufipennis (Kirby) and Dendroctonus ponderosae Hopkins, although there is considerable spatial and temporal variability. These predictions from population models suggest a movement of temperature suitability to higher latitudes and elevations and identify regions with a high potential for bark beetle outbreaks and associated tree mortality in the coming century.
1,155 citations
Cites background from "Anatomical and chemical defenses of..."
...Once a host is selected, colonization requires overcoming constitutive and inducible tree defenses, which include anatomical, physical, and chemical components (Franceschi et al. 2005)....
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TL;DR: This review focuses on the functions of plant volatiles, their biosynthesis and regulation, and the metabolic engineering of the volatile spectrum, which results in plant defense improvement and changes of scent and aroma properties of flowers and fruits.
Abstract: Volatile compounds act as a language that plants use for their communication and interaction with the surrounding environment. To date, a total of 1700 volatile compounds have been isolated from more than 90 plant families. These volatiles, released from leaves, flowers, and fruits into the atmosphere and from roots into the soil, defend plants against herbivores and pathogens or provide a reproductive advantage by attracting pollinators and seed dispersers. Plant volatiles constitute about 1% of plant secondary metabolites and are mainly represented by terpenoids, phenylpropanoids/benzenoids, fatty acid derivatives, and amino acid derivatives. In this review we focus on the functions of plant volatiles, their biosynthesis and regulation, and the metabolic engineering of the volatile spectrum, which results in plant defense improvement and changes of scent and aroma properties of flowers and fruits.
1,090 citations
Cites background from "Anatomical and chemical defenses of..."
...Terpenoid volatiles in conifers are produced in specialized anatomical structures, resin cells or ducts, that are located within other tissues of stem and needles (Franceschi et al., 2005; Keeling and Bohlmann; 2006) from which they can be released upon disruption by mechanical wounding or herbivory (Miller et al....
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...…in conifers are produced in specialized anatomical structures, resin cells or ducts, that are located within other tissues of stem and needles (Franceschi et al., 2005; Keeling and Bohlmann; 2006) from which they can be released upon disruption by mechanical wounding or herbivory (Miller et…...
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TL;DR: Plant volatiles are lipophilic molecules with high vapor pressure that serve various ecological roles that are synthesized by enzymes that produce multiple products from a single substrate or act on multiple substrates.
Abstract: Plant volatiles (PVs) are lipophilic molecules with high vapor pressure that serve various ecological roles. The synthesis of PVs involves the removal of hydrophilic moieties and oxidation/hydroxylation, reduction, methylation, and acylation reactions. Some PV biosynthetic enzymes produce multiple products from a single substrate or act on multiple substrates. Genes for PV biosynthesis evolve by duplication of genes that direct other aspects of plant metabolism; these duplicated genes then diverge from each other over time. Changes in the preferred substrate or resultant product of PV enzymes may occur through minimal changes of critical residues. Convergent evolution is often responsible for the ability of distally related species to synthesize the same volatile.
792 citations
TL;DR: This review examines what chemicals are produced, the genes and proteins involved in their biosynthesis, how they work, and how they are regulated and how insects and their associated pathogens interact with, elicit, and are affected by conifer-produced terpenoids.
Abstract: Contents
Summary 657
I. Introduction 658
II. Identification and functional characterization of terpenoid pathway genes 658
III. Insect interactions with conifers 667
IV. Conclusions and outlook 670
Acknowledgements 671
References 671
Summary
Insects select their hosts, but trees cannot select which herbivores will feed upon them. Thus, as long-lived stationary organisms, conifers must resist the onslaught of varying and multiple attackers over their lifetime. Arguably, the greatest threats to conifers are herbivorous insects and their associated pathogens. Insects such as bark beetles, stem- and wood-boring insects, shoot-feeding weevils, and foliage-feeding budworms and sawflies are among the most devastating pests of conifer forests. Conifer trees produce a great diversity of compounds, such as an enormous array of terpenoids and phenolics, that may impart resistance to a variety of herbivores and microorganisms. Insects have evolved to specialize in resistance to these chemicals – choosing, feeding upon, and colonizing hosts they perceive to be best suited to reproduction. This review focuses on the plant–insect interactions mediated by conifer-produced terpenoids. To understand the role of terpenoids in conifer–insect interactions, we must understand how conifers produce the wide diversity of terpenoids, as well as understand how these specific compounds affect insect behaviour and physiology. This review examines what chemicals are produced, the genes and proteins involved in their biosynthesis, how they work, and how they are regulated. It also examines how insects and their associated pathogens interact with, elicit, and are affected by conifer-produced terpenoids.
643 citations
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Book•
01 Jan 1997TL;DR: This comprehensive evaluation and synthesis of a rapidly-developing field provides state-of-the-discipline reviews, and highlights areas of research which might be productive, should appeal to a wide variety of theoretical and applied researchers.
Abstract: Plants face a daunting array of creatures which eat them, bore into them and use virtually every plant part for food or shelter. However, plants are far from defenceless under attack. Although they cannot flee their attackers, they can produce defences, such as thorns, and can actively alter their chemistry and physiology in response to damage. For instance, young potato leaves being eaten by potato beetles respond by producing chemicals which inhibit beetle digestive enzymes. Research on these induced responses to herbivory has proceeded since the 1980s, and this comprehensive evaluation and synthesis of a rapidly-developing field provides state-of-the-discipline reviews, and highlights areas of research which might be productive. This overview should appeal to a wide variety of theoretical and applied researchers in ecology, evolutionary biology, plant biology, entomology and agriculture.
2,385 citations
TL;DR: This review addresses the problem of response localization and localization of phenolics relative to the sequential development of stages of disease that lead ultimately to resistance expression and initial demonstrations that phenols are significant components of the host.
Abstract: Antibiotic phenols have been found in all plants investigated to date. Some occur constitutively and are thought to function as preformed inhibitors associated with nonhost resistance (84, 94, 128, 134). Others, which are the subject of this review, are formed in response to the ingress of pathogens, and their appearance is considered as part of an active defense response. Since the first suggestions that phenolic intermediates have a role in the active expres sion of resistance, an underlying problem in ascertaining that such secondary metabolites are of primary (rather than secondary) importance has been the localization and timing of the host response. In this review we address the problem of response localization and localization of phenolics relative to the sequential development of stages of disease that lead ultimately to resistance expression. Initial demonstrations that phenols are significant components of the host
1,352 citations
Journal Article•
1,174 citations
TL;DR: This review focuses on the monoterpene, sesquiterpenes, and diterpene synthases of plant origin that use the corresponding C10, C15, and C20 prenyl diphosphates as substrates to generate the enormous diversity of carbon skeletons characteristic of the terpenoid family of natural products.
Abstract: This review focuses on the monoterpene, sesquiterpene, and diterpene synthases of plant origin that use the corresponding C10, C15, and C20 prenyl diphosphates as substrates to generate the enormous diversity of carbon skeletons characteristic of the terpenoid family of natural products. A description of the enzymology and mechanism of terpenoid cyclization is followed by a discussion of molecular cloning and heterologous expression of terpenoid synthases. Sequence relatedness and phylogenetic reconstruction, based on 33 members of the Tps gene family, are delineated, and comparison of important structural features of these enzymes is provided. The review concludes with an overview of the organization and regulation of terpenoid metabolism, and of the biotechnological applications of terpenoid synthase genes.
1,011 citations
TL;DR: Using a variety of approaches, researchers have begun to unravel the exquisite control mechanisms exerted by cells specialized for CaOx formation that include the machinery for uptake and accumulation of Ca, oxalic acid biosynthetic pathways, and regulation of crystal growth.
Abstract: Calcium oxalate (CaOx) crystals are distributed among all taxonomic levels of photosynthetic organisms from small algae to angiosperms and giant gymnosperms. Accumulation of crystals by these organisms can be substantial. Major functions of CaOx crystal formation in plants include high-capacity calcium (Ca) regulation and protection against herbivory. Ultrastructural and developmental analyses have demonstrated that this biomineralization process is not a simple random physical-chemical precipitation of endogenously synthesized oxalic acid and environmentally derived Ca. Instead, crystals are formed in specific shapes and sizes. Genetic regulation of CaOx formation is indicated by constancy of crystal morphology within species, cell specialization, and the remarkable coordination of crystal growth and cell expansion. Using a variety of approaches, researchers have begun to unravel the exquisite control mechanisms exerted by cells specialized for CaOx formation that include the machinery for uptake and accumulation of Ca, oxalic acid biosynthetic pathways, and regulation of crystal growth.
993 citations