Localization of Phenolics in Phloem Parenchyma Cells of Norway Spruce (Picea abies)

Abstract

Norway spruce (Picea abies) bark contains specialized phloem parenchyma cells that swell and change their contents upon attack by the bark beetle Ips typographus and its microbial associate, the blue stain fungus Ceratocystis polonica. These cells exhibit bright autofluorescence after treatment with standard aldehyde fixatives, and so have been postulated to contain phenolic compounds. Laser microdissection of spruce bark sections combined with cryogenic NMR spectroscopy demonstrated significantly higher concentrations of the stilbene glucoside astringin in phloem parenchyma cells than in adjacent sieve cells. After infection by C. polonica, the flavonoid (+)-catechin also appeared in phloem parenchyma cells and there was a decrease in astringin content compared to cells from uninfected trees. Analysis of whole-bark extracts confirmed the results obtained from the cell extracts and revealed a significant increase in dimeric stilbene glucosides, both astringin and isorhapontin derivatives (piceasides A to H), in fungus-infected versus uninfected bark that might explain the reduction in stilbene monomers. Phloem parenchyma cells thus appear to be a principal site of phenolic accumulation in spruce bark.

A Common Fungal Associate of the Spruce Bark Beetle Metabolizes the Stilbene Defenses of Norway Spruce

“…These declines were considered very puzzling because they occurred despite increases in the enzyme activity of STS (Brignolas et al, 1995), which catalyze the formation of the stilbene skeleton. Furthermore, increases in piceatannol concentrations (Viiri et al, 2001) and stilbene dimers (Li et al, 2012) were observed in spruce bark after C. polonica infection. Our results now clearly show that these metabolites originate from the fungal metabolism of host defense compounds and that C. polonica metabolism of stilbenes can override even increases in host stilbene biosynthesis.…”

“…A number of previous studies had reported that the stilbene content of spruce bark declined during fungal attack (Brignolas et al, 1995;Viiri et al, 2001;Li et al, 2012). These declines were considered very puzzling because they occurred despite increases in the enzyme activity of STS (Brignolas et al, 1995), which catalyze the formation of the stilbene skeleton.…”

“…A less well-studied defense mechanism in spruce is the production of phenolic compounds in specialized cells in the bark. These substances are stored in phloem parenchyma cells (Franceschi et al, 2000;Li et al, 2012) that expand during wounding or fungal attack and show major cytological changes.…”

“…Significant increases in astringin were observed after inoculation of Picea glauca saplings with avirulent C. polonica (Hammerbacher et al, 2011). However, stilbene glycoside concentrations in mature Norway spruce were stable or even declined during the course of C. polonica infection (Brignolas et al, 1995;Viiri et al, 2001;Li et al, 2012).…”

Fungitoxic Potential of Extracts of Four Pinus spp. Bark to Inhibit Fungus Trametes versicolor (L. ex. Fr.) Pilát

“…These anatomic changes can be induced by organisms (Franceschi et al 2005). At the same time, active chemical response mechanisms occur in order to repel such threats, and there are changes in quantity and constitution of secondary metabolites such as terpenes (Keeling and Bohlmann 2006;Goodsman et al 2012) and phenolic compounds (Franchesi et al 2005;Li et al 2012). It has been suggested that monoterpenes and terpenoids are more effective against insects (Jost et al 2008;Boulogne et al 2012).…”