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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.
