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Carbon castles and insect invaders: dissecting physical defences in conifer stems.

Conifers are keystone species in many ecosystems, dominating the vast boreal forests of the northern hemisphere as well as many temperate and mountainous forest ecosystems. These forests are critically important for terrestrial carbon fixation, as sources of energy and bio-based products, and for recreation. The conifers are an ancient group of gymnosperms with a phylogenetic history that goes back at least 200 million years. Throughout this period conifers have been challenged by insects and pathogens and have evolved complex defense mechanisms to combat these threats. These defenses, which may be either preformed or inducible in nature, are integrated into a coordinated, multi-purpose defense strategy that has stood the test of time (Franceschi et al. 2005). Conifer defenses are molecular, chemical or physical in nature, but all reside in different cell structures and thus have an anatomical basis. The ultimate purpose of defenses in conifer stems is to maintain the vital functions of the bark, vascular cambium, and sapwood. The first line of defense is made up of preformed (constitutive) defenses. These include components with a physical or mechanical mode of action, such as lignified stone cells in the bark, and components with a chemical mode of action, such as the terpenoid resin stored in specialized resin ducts found throughout the bark and sapwood (Figure 1). The second line of defense consists of inducible defense responses that are activated by an attack or infection. Inducible defenses range in organizational complexity from activation of existing defense structures, such as resin ducts or polyphenolic parenchyma cells, to the formation of entirely new tissues such as traumatic resin ducts and wound periderms (Figure 1). In addition to these short-term inducible defenses, conifers also have the ability to prime or induce additional defenses that increase tree resistance to future attacks (Krokene 2015). The conifers’ ecological successes are the result of their unique and proven defenses, which have allowed them to develop into some of the largest and oldest organisms on earth.