Abstract:
Terpenes and phenolics are important constitutive and inducible conifer defenses
against bark beetles and their associated fungi. In this study, the inducible defenses
of mature Norway spruce (Picea abies) trees with different histories of attack by the
spruce bark beetle, Ips typographus were tested by inoculation with the I. typographusassociated fungus Endoconidiophora polonica. We compared trees that had been under
previous attack with those under current attack and those that had no record of attack.
After fungal inoculation, the concentrations of mono-, sesqui-, and diterpenes in bark
increased 3- to 9-fold. For the phenolics, the flavan-3-ols, catechin, and gallocatechin,
increased significantly by 2- and 5-fold, respectively, while other flavonoids and stilbenes
did not. The magnitudes of these inductions were not influenced by prior bark beetle
attack history for all the major compounds and compound classes measured. Before
fungal inoculation, the total amounts of monoterpenes, diterpenes, and phenolics
(constitutive defenses) were greater in trees that had been previously attacked
compared to those under current attack, possibly a result of previous induction. The
transcript levels of many genes involved in terpene formation (isoprenyl diphosphate
synthases and terpene synthases) and phenolic formation (chalcone synthases) were
significantly enhanced by fungal inoculation suggesting de novo biosynthesis. Similar
inductions were found for the enzymatic activity of isoprenyl diphosphate synthases
and the concentration of their prenyl diphosphate products after fungal inoculation.
Quantification of defense hormones revealed a significant induction of the jasmonate
pathway, but not the salicylic acid pathway after fungal inoculation. Our data highlight the
coordinated induction of terpenes and phenolics in spruce upon infection by E. polonica,
a fungal associate of the bark beetle I. typographus, but provide no evidence for the
priming of these defense responses by prior beetle attack.
