Abstract
Main conclusionLignan impregnation of the reaction zone wood protects against oxidative degradation by fungi. Traumatic resin canals may play roles in the underlying signal transduction, synthesis, and translocation of defense compounds.Tree defense against xylem pathogens involves both constitutive and induced phenylpropanoids and terpenoids. The induced defenses include compartmentalization of compromised wood with a reaction zone (RZ) characterized by polyphenol deposition, whereas the role of terpenoids has remained poorly understood. To further elucidate the tree–pathogen interaction, we profiled spatial patterns in lignan (low-molecular-weight polyphenols) and terpenoid content in Norway spruce (Picea abies) trees showing heartwood colonization by the pathogenic white-rot fungus Heterobasidion parviporum. There was pronounced variation in the amount and composition of lignans between different xylem tissue zones of diseased and healthy trees. Intact RZ at basal stem regions, where colonization is the oldest, showed the highest level and diversity of these compounds. The antioxidant properties of lignans obviously hinder oxidative degradation of wood: RZ with lignans removed by extraction showed significantly higher mass loss than unextracted RZ when subjected to Fenton degradation. The reduced diversity and amount of lignans in pathogen-compromised RZ and decaying heartwood in comparison to intact RZ and healthy heartwood suggest that α-conindendrin isomer is an intermediate metabolite in lignan decomposition by H. parviporum. Diterpenes and diterpene alcohols constituted above 90% of the terpenes detected in sapwood of healthy and diseased trees. A significant finding was that traumatic resin canals, predominated by monoterpenes, were commonly associated with RZ. The findings clarify the roles and fate of lignan during wood decay and raise questions about the potential roles of terpenoids in signal transduction, synthesis, and translocation of defense compounds upon wood compartmentalization against decay fungi.
Highlights
When considering the longevity, large size, and ecological dominance of trees, one can conclude that these plants have developed highly efficient defense mechanisms against abiotic and biotic stressors
These mean values were standardized and subsequently used for exploratory data analysis by principal component analysis (PCA) and heatmaps combined with hierarchical clustering, using the ‘factoextra’ and ‘gplots’ packages (Kassambara and Mundt 2020; Warnes et al 2020)
It has been demonstrated that both mock inoculation and wounding-assisted artificial stem bark inoculation of Norway spruce with Heterobasidion trigger the formation of traumatic resin canals in the newly forming year ring (Krekling et al 2004). It is quite possible the traumatic resin canals observed in the diseased trees represent an induced defense response triggered already when the pathogen was still confined to roots as these canals must have formed during the year, in which the corresponding year rings were formed
Summary
Large size, and ecological dominance of trees, one can conclude that these plants have developed highly efficient defense mechanisms against abiotic and biotic stressors. Most of the biomass of trees resides in tree stems, which are protected by constitutive structural and chemical barriers. These barriers are coupled with inducible responses, should they become compromised (Franceschi et al 2005; Morris et al 2020). Along with increase in tree stem diameter, the older central part of stem xylem is transformed into heartwood. This process involves the transformation of parenchyma energy reserves into extractives and subsequent death of the parenchyma cells in the heartwood (Hillis 1968; Bamber 1976)
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