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.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2005.01436.x

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

Scolytid bark beetles that colonize living conifers are frequently associated with specific fungi that are carried in specialized structures or on the body surface. These fungi are introduced into the tree during the attack process. The continuing association suggests that there is mutual benefit to the fitness of both beetles and fungi. The fungal species may benefit from the association with the beetles by transport to new host trees. Beetle species may benefit from the association with fungi by feeding on the fungi, or by the fungi contributing to the death of the host trees through mycelial penetration of host tissue, toxin release, interactions with preformed and induced conifer defenses, or the combined action of both beetles and fungi during colonization. Extensive research has been directed towards characterizing the interactions of beetle-fungal complexes with live host conifers and determining the ecological advantages for maintaining the associations. However, differences among systems and how species interact under different population and environmental conditions make it difficult to generalize about the importance of the separate biological components in successful host colonization.

/pdf/interactions-among-scolytid-bark-beetles-their-associated-10kyq0zq0s.pdf

Interactions among scolytid bark beetles, their associated fungi, and live host conifers

Metabolism and roles of stilbenes in plants

Wounding of Norway spruce by inoculation with sterile agar, or agar containing the pathogenic fungus Ceratocystis polonica, induced traumatic resin duct formation in the stem. Visible anatomical responses occurred in the cambium 6-9 d post-inoculation. Near the inoculation site cellular proliferation, polyphenolic accumulation, and lignification were induced as a wound reaction to seal the damaged area. Five centimetres from the inoculation site cells in the cambial zone swelled and divided to form clusters. By 18 d post-inoculation, these cells began to differentiate into resin duct epithelial cells surrounding incipient schizogenous lumens. Mature axial traumatic ducts appeared by 36 d as a row of ducts in the xylem centripetal to the cambium. The ducts formed an interconnected network continuous with radial resin ducts. Parenchyma cells surrounding the ducts accumulated polyphenols that disappeared as the cells differentiated into tracheids. These polyphenols appeared to contain fewer sugar residues compared to those accumulating in the secondary phloem, as indicated by the periodic acid-Schiff's staining. The epithelial cells did not accumulate polyphenols but contained immunologically detectable phenylalanine ammonia lyase (EC 4.3.1.5), indicating synthesis of phenolics as a possible resin component. These findings may represent a defense mechanism in Norway spruce against the pathogenic fungus Ceratocystis polonica.

Wound-induced traumatic resin duct development in stems of Norway spruce (Pinaceae): anatomy and cytochemical traits.

Conifer stem pest resistance includes constitutive defenses that discourage invasion and inducible defenses, including phenolic and terpenoid resin synthesis. Recently, methyl jasmonate (MJ) was shown to induce conifer resin and phenolic defenses; however, it is not known if MJ is the direct effector or if there is a downstream signal. Exogenous applications of MJ, methyl salicylate, and ethylene were used to assess inducible defense signaling mechanisms in conifer stems. MJ and ethylene but not methyl salicylate caused enhanced phenolic synthesis in polyphenolic parenchyma cells, early sclereid lignification, and reprogramming of the cambial zone to form traumatic resin ducts in Pseudotsuga menziesii and Sequoiadendron giganteum. Similar responses in internodes above and below treated internodes indicate transport of a signal giving a systemic response. Studies focusing on P. menziesii showed MJ induced ethylene production earlier and 77-fold higher than wounding. Ethylene production was also induced in internodes above the MJ-treated internode. Pretreatment of P. menziesii stems with the ethylene response inhibitor 1-methylcyclopropene inhibited MJ and wound responses. Wounding increased 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase protein, but MJ treatment produced a higher and more rapid ACC oxidase increase. ACC oxidase was most abundant in ray parenchyma cells, followed by cambial zone cells and resin duct epithelia. The data show these MJ-induced defense responses are mediated by ethylene. The cambial zone xylem mother cells are reprogrammed to differentiate into resin-secreting epithelial cells by an MJ-induced ethylene burst, whereas polyphenolic parenchyma cells are activated to increase polyphenol production. The results also indicate a central role of ray parenchyma in ethylene-induced defense.

Methyl jasmonate-induced ethylene production is responsible for conifer phloem defense responses and reprogramming of stem cambial zone for traumatic resin duct formation.

Summary

Changes in soluble phenol contents of Scots-pine (Pinus sylvestris) phloem were studied after inoculation with Ophiostoma brunneo-ciliatum, a fungus associated with Ips sexdentatus. They were compared to the length of the fungus-induced reaction zone of the phloem, taken as an estimation for the trees' response efficiency against aggression.



Five clones consisting of 14 trees were studied, and eight soluble phenolic metabolites were analysed in detail in reaction zones and in unwounded phloem sampled 3 weeks after inoculation.



The stilbenes pinosylvin and pinosylvin monomethylether, and the flavonoid pinocembrin, were detected in reaction zones only. Concentrations of two p-coumaric-acid esters and an acetophenone glycoside decreased after inoculation. Variations of the flavonoids taxifolin and 3' taxifolin glucoside were inconsistent.



One tree had longer reaction zones and a phenolic content different from the others. In unwounded phloem, concentrations of some phenolic compounds and in particular the ratio between the two p-coumaric-acid esters could be markers for trees' response efficiency. In the reaction zone, markers could be the ratios between concentrations of certain preformed compounds and concentrations of newly synthesized pinosylvin and pinocembrin.

Changes in phenolic metabolites of Scots-pine phloem induced by Ophiostoma brunneo-ciliatum, a bark-beetle-associated fungus

Summary

The aim of this study was to investigate the temporal changes in Norway-spruce (Picea abies) phloem phenolics during wound-induced response. Two Norway spruces were inoculated with Ophiostoma polonicum and sterile agar; unwounded phloem was simultaneously tested as control. Both unwounded and inoculated phloem were then sampled 6, 11, 21, 30, and 60 days later. Monophenols were analysed by HPLC, and the tanning capacity of the phloem extracts was estimated from their ability to precipitate proteins. The response to inoculation was characterized by radical changes in the phloem phenolic composition. Initially, an increase in (+)-catechin concentration was observed, concomitant with a slow decrease in both glycoside concentrations and tanning capacity. After day 6, this decrease was accelerated in the event of fungus inoculation. The appearance of aglycones did not take place during the decrease in glycosides, but was delayed. The phenolic response of Norway-spruce phloem to inoculation is modulated by the presence of the fungus and is focused on the formation of insoluble compounds.

F. Brignolas

Papers

Changes in phenolic metabolites of Scots-pine phloem induced by Ophiostoma brunneo-ciliatum, a bark-beetle-associated fungus

Changes in soluble‐phenol content of Norway‐spruce (Picea abies) phloem in response to wounding and inoculation with Ophiostoma polonicum