Using the CODIT model to explain secondary metabolites of xylem in defence systems of temperate trees against decay fungi

Abstract

Background In trees, secondary metabolites (SMs) are essential for determining the effectiveness of defence systems against fungi and why defences are sometimes breached. Using the CODIT model (Compartmentalization of Damage/Dysfunction in Trees), we explain defence processes at the cellular level. CODIT is a highly compartmented defence system that relies on the signalling, synthesis and transport of defence compounds through a three-dimensional lattice of parenchyma against the spread of decay fungi in xylem. Scope The model conceptualizes ‘walls’ that are pre-formed, formed during and formed after wounding events. For sapwood, SMs range in molecular size, which directly affects performance and the response times in which they can be produced. When triggered, high-molecular weight SMs such as suberin and lignin are synthesized slowly (phytoalexins), but can also be in place at the time of wounding (phytoanticipins). In contrast, low-molecular weight phenolic compounds such as flavonoids can be manufactured de novo (phytoalexins) rapidly in response to fungal colonization. De novo production of SMs can be regulated in response to fungal pathogenicity levels. The protective nature of heartwood is partly based on the level of accumulated antimicrobial SMs (phytoanticipins) during the transitionary stage into a normally dead substance. Effectiveness against fungal colonization in heartwood is largely determined by the genetics of the host. Conclusion Here we review recent advances in our understanding of the role of SMs in trees in the context of CODIT, with emphasis on the relationship between defence, carbohydrate availability and the hydraulic system.We also raise the limitations of the CODIT model and suggest its modification, encompassing other defence theory concepts. We envisage the development of a new defence system that is modular based and incorporates all components (and organs) of the tree from micro- to macro-scales.

Springtime Bark-Splitting of Acer pseudoplatanus in Germany

“…Apparently, the tree isolated the segment of wood where the ray cells were no longer connected to the phloem. The black border in the wood is generally assumed to be produced in the presence of fungi [20], but in this case it appeared to be a plant response to separate the active and inactive xylem [5,[20][21][22][23]. We found phenolic deposits in ray cells in the absence of fungal hyphae.…”

The Dark Side of Fungal Competition and Resource Capture in Wood: Zone Line Spalting From Science to Application

“…Compartmentalisation boundaries include both constitutive features and induced changes in wood anatomy and physiology including visible reaction zones. Reaction zones are produced as a response to injury and infection and consist of a narrow layer of sapwood parenchyma and that have undergone a physiological shift to produce waterproofing and antimicrobial secondary metabolites [42]. Successful compartmentalisation maintains function in remaining, apparently uninfected sapwood.…”

“…In functional sapwood, K. deusta can only slowly pass or breach reaction zones laid down by the host (Morris et al [9,41]); pits become encrusted with polyphenols, initially blocking hyphal entrance points for the fungus (Fig. 2A in [42]). However, due to the absence of living parenchyma cells (responsible for reaction zone development) in processed timber, members of the Xylariaceae and in particular K. deusta, can form zone lines without obstruction from reaction zones during the inoculation process, a key advantage when using these fungal species to spalt wood.…”

WorldForestID: Addressing the need for standardized wood reference collections to support authentication analysis technologies; a way forward for checking the origin and identity of traded timber

“…Versions that extend further into the trunk are being tested. Although a small area of the cambium is damaged in the process (Figure 1c) this is unlikely to do long‐term harm and is far less damaging than many of the challenges the tree faces throughout its life (Morris et al., 2020). Three cores are collected from each tree, one for retention in the country and two shipped to Kew once properly dried.…”