Item

Abstract

One of the best-studied defense responses to fungal infection in Norway spruce (Picea abies) is the biosynthesis of flavan-3-ols, which accumulate as monomers or polymers known as proanthocyanidins. The individual flavan-3-ol units consist of compounds with a 30,40-dihydroxylated B ring [2,3-(trans)- (þ)-catechin or 2,3-(cis)-()-epicatechin] and compounds with a 30,40,50-trihydroxylated B ring [2,3 (trans)-(þ)-gallocatechin or 2,3-(cis)-()-epigallocatechin]. While much is known about the biosynthesis and biological activity of catechin in Norway spruce, there is little comparable information about gallocatechin or epigallocatechin.We found that there was a significant increase in the gallocatechin content of Norway spruce bark and wood after inoculation with the bark beetle-associated sap-staining fungus Endoconidiophora polonica. Gallocatechins increased proportionally more than catechins as both monomers and units of polymers. A flavonoid 30,50-hydroxylase gene identified in Norway spruce was shown by heterologous expression in Nicotiana benthamiana to be involved in the conversion of 2,3 (trans)-(þ)-catechin to 2,3 (trans)-(þ)-gallocatechin. The formation of the trihydroxylated B ring in Norway spruce occurs at the level of flavan-3-ols, rather than at the level of dihydroflavonols as in many angiosperms. The transcript abundance of the flavonoid 30,50- hydroxylase gene also increased significantly during fungal infection underlining its importance in gallocatechin biosynthesis. Comparisons of the effect of 2,3 (trans)-(þ)-catechin and 2,3 (trans)- (þ)-gallocatechin on fungal growth revealed that 2,3 (trans)-(þ)-catechin is a stronger inhibitor of fungal growth, while 2,3 (trans)-(þ)-gallocatechin is a stronger inhibitor of melanin biosynthesis.