Anatomical and Chemical Defenses of Conifer Bark against Bark Beetles and Other Pests. Franceschi, V. R., Krokene, P., Christiansen, E., & Krekling, T. New Phytologist, 167(2):353–376, May, 2005.
doi  abstract   bibtex   
[Summary] 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. [Excerpt] Generating a defensive capacity that can deal with the diversity of potential attackers and their varying means of attack requires a wide range of mechanisms. As will be described here, the basic defense strategy of conifer stems involves multiple structures and chemicals that are overlapping in space and time. Because defense is a costly venture, not all defenses are expressed constitutively under normal growth. Investment in constitutive defenses provides for inhibition of an initial attack, whereas inducible defense mechanisms help to ensure that an initial invasion of tissues is both perceived and defended against actively and vigorously. Thus, two basic types of defense strategies can be discussed: constitutive defenses that are present in the tree without any challenge, and inducible defenses that are generated upon perception of a foreign challenge. A third strategy, acquired or systemic defense, can be considered to be a variation of inducible defenses but at some distance from the attack, temporally displaced with respect to the initial event, and with persistent properties. The 'choice' of strategy has been hypothesized to vary with the type of challenge (cf. Matson & Hain, 1985; Christiansen et al., 1987; Bonello et al., 2001). Here we review the nature of the basic bark defenses and how they are integrated. Questions concerning their evolution and coevolution with pests are discussed, focusing on scolytid bark beetles because of their dominating role in forest ecology and the large amount of research done with these organisms. [::Basic defense strategies] The active phloem and cambium of stems represent a relatively small amount of tissue that can be easily damaged or destroyed, resulting in death of part, or all, of the tree. The basic function of bark defenses is to protect the nutrient- and energy-rich phloem, the vital meristematic region of the vascular cambium, and the transpiration stream in the sapwood. There are four basic steps or phases of defense systems in plants that are independent of the attacking organism. The first is an effective constitutive defense that can repel or inhibit invasion of tissues. If this is not effective, the next stage is to kill or compartmentalize the invading organism. A third phase of defense is to seal and repair damage incurred so that the plant can continue to function normally, and so that opportunistic infections are prevented. Finally, acquired or systemic resistance can be induced so that future attacks are more easily defended against. In addition, once an invading organism is identified, more specialized inducible defense responses can be elicited, such as gene-for-gene (R-gene) responses. The combination of constitutive and inducible systems provides a potent defense against attack. [...] [::Conclusions] During their long lives, conifer trees meet challenges from a wide variety of organisms, among the most serious being bark beetles and their associated phytopathogenic fungi. Conifer defenses against stem infesting insects and pathogens can be classified as constitutive or induced. The constitutive defense system includes resin accumulating cells and channels in the phloem and wood, cells in the phloem that store toxic substances (e.g. phenolics), and mechanical properties of the bark such as suberized and lignified cell layers, stone cells and calcium oxalate crystals. The induced defense system involves de novo synthesis or activation of a wide range of defense chemicals, including terpenoids, phenolics, PR proteins and enzymes. The induced defense system may act against a current infection (the hypersensitive response and local resistance) or against future infections or bark beetle attacks (acquired resistance). These multiple overlapping defense structures and systems provide a formidable defense against a wide range of possible attacking organisms. However, conifers are still susceptible to certain organisms that have evolved strategies to overcome the defenses or avoid them. A number of challenges remain in improving our understanding of these defenses and the interaction of invading organisms with conifer defense strategies, as well as our understanding of the coevolution of conifer defense mechanisms and bark infesting organisms. Defensive chemicals such as the terpenoid resins and phenolics involve complex pathways that can lead to a large range of different chemical components, and characterization of the pathways operating in diverse species (and their regulation) will help determine the importance of various compounds singly and in combinations. Another key to understanding these defenses is to identify the signaling mechanisms involved in establishing patterns of constitutive defenses and in each phase of the induced defense responses. Another perspective that needs to be integrated with our knowledge of defense systems is the nature of the tree-beetle interaction. The mechanisms by which bark beetles are attracted to host trees and decide on the suitability of a tree are not well established, but evidence of the interaction of otherwise defensive compounds from the bark with beetle attraction or deterrence is particularly intriguing and merits further intensive study. [\n] The remarkable longevity of various conifer species is a testament to the success of their defense strategies, some of which have been reviewed here, and the persistence of minute beetles capable of overcoming these defenses indicates a fascinating coevolutionary history. Understanding bark defenses and these interactions is critical in predicting the effect of ecological disturbances on tree-pest dynamics as well as in developing management and genetic strategies for improving the health of conifer forests or conifer production.
@article{franceschiAnatomicalChemicalDefenses2005,
  title = {Anatomical and Chemical Defenses of Conifer Bark against Bark Beetles and Other Pests},
  author = {Franceschi, Vincent R. and Krokene, Paal and Christiansen, Erik and Krekling, Trygve},
  year = {2005},
  month = may,
  volume = {167},
  pages = {353--376},
  issn = {0028-646X},
  doi = {10.1111/j.1469-8137.2005.01436.x},
  abstract = {[Summary] 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.

[Excerpt] Generating a defensive capacity that can deal with the diversity of potential attackers and their varying means of attack requires a wide range of mechanisms. As will be described here, the basic defense strategy of conifer stems involves multiple structures and chemicals that are overlapping in space and time. Because defense is a costly venture, not all defenses are expressed constitutively under normal growth. Investment in constitutive defenses provides for inhibition of an initial attack, whereas inducible defense mechanisms help to ensure that an initial invasion of tissues is both perceived and defended against actively and vigorously. Thus, two basic types of defense strategies can be discussed: constitutive defenses that are present in the tree without any challenge, and inducible defenses that are generated upon perception of a foreign challenge. A third strategy, acquired or systemic defense, can be considered to be a variation of inducible defenses but at some distance from the attack, temporally displaced with respect to the initial event, and with persistent properties. The 'choice' of strategy has been hypothesized to vary with the type of challenge (cf. Matson \& Hain, 1985; Christiansen et al., 1987; Bonello et al., 2001). Here we review the nature of the basic bark defenses and how they are integrated. Questions concerning their evolution and coevolution with pests are discussed, focusing on scolytid bark beetles because of their dominating role in forest ecology and the large amount of research done with these organisms. 

[::Basic defense strategies]
The active phloem and cambium of stems represent a relatively small amount of tissue that can be easily damaged or destroyed, resulting in death of part, or all, of the tree. The basic function of bark defenses is to protect the nutrient- and energy-rich phloem, the vital meristematic region of the vascular cambium, and the transpiration stream in the sapwood. There are four basic steps or phases of defense systems in plants that are independent of the attacking organism. The first is an effective constitutive defense that can repel or inhibit invasion of tissues. If this is not effective, the next stage is to kill or compartmentalize the invading organism. A third phase of defense is to seal and repair damage incurred so that the plant can continue to function normally, and so that opportunistic infections are prevented. Finally, acquired or systemic resistance can be induced so that future attacks are more easily defended against. In addition, once an invading organism is identified, more specialized inducible defense responses can be elicited, such as gene-for-gene (R-gene) responses. The combination of constitutive and inducible systems provides a potent defense against attack. [...]

[::Conclusions] During their long lives, conifer trees meet challenges from a wide variety of organisms, among the most serious being bark beetles and their associated phytopathogenic fungi. Conifer defenses against stem infesting insects and pathogens can be classified as constitutive or induced. The constitutive defense system includes resin accumulating cells and channels in the phloem and wood, cells in the phloem that store toxic substances (e.g. phenolics), and mechanical properties of the bark such as suberized and lignified cell layers, stone cells and calcium oxalate crystals. The induced defense system involves de novo synthesis or activation of a wide range of defense chemicals, including terpenoids, phenolics, PR proteins and enzymes. The induced defense system may act against a current infection (the hypersensitive response and local resistance) or against future infections or bark beetle attacks (acquired resistance). These multiple overlapping defense structures and systems provide a formidable defense against a wide range of possible attacking organisms. However, conifers are still susceptible to certain organisms that have evolved strategies to overcome the defenses or avoid them. A number of challenges remain in improving our understanding of these defenses and the interaction of invading organisms with conifer defense strategies, as well as our understanding of the coevolution of conifer defense mechanisms and bark infesting organisms. Defensive chemicals such as the terpenoid resins and phenolics involve complex pathways that can lead to a large range of different chemical components, and characterization of the pathways operating in diverse species (and their regulation) will help determine the importance of various compounds singly and in combinations. Another key to understanding these defenses is to identify the signaling mechanisms involved in establishing patterns of constitutive defenses and in each phase of the induced defense responses. Another perspective that needs to be integrated with our knowledge of defense systems is the nature of the tree-beetle interaction. The mechanisms by which bark beetles are attracted to host trees and decide on the suitability of a tree are not well established, but evidence of the interaction of otherwise defensive compounds from the bark with beetle attraction or deterrence is particularly intriguing and merits further intensive study.

[\textbackslash n] The remarkable longevity of various conifer species is a testament to the success of their defense strategies, some of which have been reviewed here, and the persistence of minute beetles capable of overcoming these defenses indicates a fascinating coevolutionary history. Understanding bark defenses and these interactions is critical in predicting the effect of ecological disturbances on tree-pest dynamics as well as in developing management and genetic strategies for improving the health of conifer forests or conifer production.},
  journal = {New Phytologist},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-246698,bark-beetle,co-evolution,conifers,ecology,evolution,forest-pests,forest-resources,opportunistic-plant-pests,plant-self-defense},
  lccn = {INRMM-MiD:c-246698},
  number = {2}
}
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