Proanthocyanidin production in Picea abies during infection by the bark beetle-associated fungus Ceratocystis polonica

Norway spruce (Picea abies Karst.) is an important forest and landscape tree endemic to Europe and Northern Asia. Due to its high yields and hardiness, it is a major source of fibre and wood. However, P. abies is susceptible to attack by the European bark beetle, Ips typographus and its fungal associate Ceratocystis polonica, which can destroy large tracts of forest within a single growing season. Based on anatomical studies, the production of phenolics, especially proanthocyanidins, is considered to be one of the major defensive mechanisms of P. abies against the Ips-Ceratocystis pathocomplex. The objective of our research was to study the chemical, biochemical and molecular changes in proanthocyanidin production in fungal-infected spruce bark. In order to determine the degree of polymerization of proanthocyanidins, novel analytical procedures were developed for the quantification of proanthocyanidin polymers. In addition, genes involved in the committed steps of proanthocyanidin biosynthesis were identified and functionally characterized. The transcript levels of these genes in C. polonica infected spruce saplings were quantified, and significant increases were observed. Levels of catechin and gallocatechin also increased during fungal infection. The quantity of proanthocyanidin monomers decreased significantly during later stages of infection leading to increased levels of polymers.