Abstract:
Conifer forests worldwide are becoming increasingly vulnerable to attacks by
bark beetles and their fungal associates due to the effects of global warming.
Attack by the bark beetle Ips typographus and the blue-stain fungus it vectors
(Endoconidiophora polonica) on Norway spruce (Picea abies) is well known to induce
increased production of terpene oleoresin and polyphenolic compounds. However,
it is not clear whether specific compounds are important in resisting attack. In this
study, we observed a significant increase in dihydroflavonol and flavan-3-ol content
after inoculating Norway spruce with the bark beetle vectored fungus. A bioassay
revealed that the dihydroflavonol taxifolin and the flavan-3-ol catechin negatively
affected both I. typographus and E. polonica. The biosynthesis of flavan-3-ols is
well studied in Norway spruce, but little is known about dihydroflavonol formation
in this species. A flavanone-3-hydroxylase (F3H) was identified that catalyzed the
conversion of eriodictyol to taxifolin and was highly expressed after E. polonica infection.
Down-regulating F3H gene expression by RNA interference in transgenic Norway spruce
resulted in significantly lower levels of both dihydroflavonols and flavan-3-ols. Therefore
F3H plays a key role in the biosynthesis of defense compounds in Norway spruce that
act against the bark beetle-fungus complex. This enzyme forms a defensive product,
taxifolin, which is also a metabolic precursor of another defensive product, catechin,
which in turn synergizes the toxicity of taxifolin to the bark beetle associated fungus.
