Anatomical and chemical defenses of conifer bark against bark beetles and other pests

Abstract

Contents Summary 1 I. Introduction 2 II. Integrating defense strategies 2 III. Constitutive defense systems: first tier of defense 4 IV. Inducible defense systems: second tier of defense 11 V. Genetics and physiology of bark defense 13 VI. Bark beetles: diminutive but deadly 15 VII. The arms race: coevolution of conifer defense and bark beetle strategies 15 VIII. Bark‐beetle‐vectored blue‐stain fungi: multiple attacks against multiple defenses 17 IX. Conclusions 18 Acknowledgements 19 References 19 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.

Tissue carbon concentration of 175 Mexican forest species

“…The high values of C (ca. 50%) found in bark and cones of Pinophyta in this study, could be the result of C-rich lignin and suberin in these structures (Franceschi et al 2005).…”

Strong Induction of Minor Terpenes in Italian Cypress, Cupressus sempervirens, in Response to Infection by the Fungus Seiridium cardinale

“…Conifers make use of chemical defenses, primarily terpenes and phenols (Franceschi et al 2005;Phillips and Croteau 1999) that ward against pathogenic fungi and other threats. Terpenes are used in conifers as constitutive defenses (a first line of defense against any enemy) but also as induced defenses against pathogens; increases in absolute amounts, proportional changes, phytoalexin production, and general or specific responses to an antagonist can appear at different time points following infection (Michelozzi 1999).…”

Effect of Water Stress and Fungal Inoculation on Monoterpene Emission from an Historical and a New Pine Host of the Mountain Pine Beetle

“…Climate change is expected to produce longer and more frequent droughts in many regions of the world (Breshears et al, 2009) that may influence the ability of trees to defend themselves against invading insects and diseases (reviewed in Franceschi et al, 2005). For plant species that are conservative water users, prolonged droughts will decrease carbon dioxide uptake and associated photosynthetic carbon assimilation due to stomata closure.…”