Uncovering the mechanisms of heartwood formation and wood resistance to fungal degradation in the tropical Lauraceae tree Sextonia rubra (Mez.) van der Werff.

Abstract

Heartwood formation is a complex process that contributes to ensuring the integrity of trunks and the longevity of trees. We examined this mechanism in the tropical angiosperm Sextonia rubra in relation to the spatial distribution of specialised metabolites and their functional role at the scale of a mature individual. Heartwood formation was analysed starting from the examination of one of its properties, namely the decay resistance, of the different S. rubra wood tissues (sapwood, heartwood, and pith) using soil bed tests. Annotation and identification of the metabolites present in ethyl acetate extracts were carried out by reverse-phase liquid chromatography coupled to a tandem mass spectrometer (RPLC-ESI-MS/MS) and molecular networks. Following the application of supervised statistical analyses and the use of Glutathione S-transferases enzymatic assays, the specialised metabolites of interest were quantified radially and longitudinally in the different tissues using RPLC-ESI-HRMS system. Heartwood and pith were shown to resist degradation after a ten-months exposure to forest soil, with no effect of the heartwood cambial age. Molecular diversity was specific to each tissue type, with flavonoids and butanolides detected in bark and sapwood, while alkaloids and butyrolactones were identified in heartwood and pith. Supervised analyses and enzyme assays suggested that alkaloids and butyrolactones play a role in the resistance of internal tissues to degradation. Butyrolactone concentrations peaked in the middle heartwood but remained homogeneous longitudinally, while alkaloid concentrations were uniform longitudinally and radially in the heartwood. In conclusion, the resistance of heartwood and pith to fungal degradation was correlated with the accumulation of lactones and alkaloids. While butanolide precursors of butyrolactones have been detected in the sapwood, alkaloids appear to be directly biosynthesised in the heartwood. This suggests that the biosynthesis and accumulation of specialised metabolites during heartwood formation is specific to each molecular family.