Chemical barriers to DED

Accumulating evidence suggests the presence of chemical barriers to DED in elms. These barriers may act either directly on living cells by the induction of non-specific oxidative stress, or indirectly through the formation of impermeable barrier zones (see Compartmentalization of DED infection, Barrier zones (wall 4)) containing such degradation resistant compounds as lignin and suberin.

Phenolic deposits, scopoletin, capric acid, and PR proteins

S. Eshita (M. Mason, personal communication) compared the levels of free t-cinnamic acid, p-coumaric acid, salicylic acid, and benzoic acid in elicited and unelicited cells from DED susceptible and DED-tolerant U. americana. Cells derived from DED-sensitive trees did show a consitutive high level of these early defense response phenolic acids and phenylpropanoid compounds. No changes were observed in these cells upon treatment with the fungal cell wall elicitor.  In contrast, cells from the tolerant elms exhibited low levels of the compounds. However, these low levels raised to high levels comparable to those of the DED-sensitive cells after elicitor treatment. Possibly, the sudden increase in early defense response components serves to trigger further host defense mechanisms. On the other hand obstruction of response reactions downstream of these compounds may result in the accumulation observed in the DED-susceptible American elm cells.

Several authors describe the production of elm compounds with a (putative) anti-fungal activity. Seeds and seedlings of American elm up to 7 months old contain capric acid, a compound known to inhibit O. ulmi spore germination {[721],[726]}. Dumas et al.{[714]} isolated ß-sitosterol and scopoletin from U. americana. Although in this study no anti-fungal activity was found for these compounds, scopoletin was recently shown to negatively affect spore germination and mycelial growth of O. ulmi s.l.in vitro {[14]}. In cultures derived from U. pumila this hydroxycoumarin accumulated rapidly (primarily in the medium, up to 60 µM. Cell cultures from susceptible elm trees showed only a slight increase in scopoletin accumulation after pathogen infection. However, since the concentrations of scopoletin necessary to establish this effect exceed those measured in the cell cultures, the precise role of this phenolic in restricting O. ulmi s.l. spread is yet to be determined {[14]}.

Deposits of phenolics appear to be correlated with the elm's response to DED infection. Rioux et al.{[682]} showed the presence of phenolic compounds in barrier zones of field-grown U. americana after infection with the DED fungus. In addition to extensive vacuolation and plasmolysis, susceptible American elm callus inoculated with O. novo-ulmi shows deposition of phenolic compounds. Callus from resistant elms exhibits reduced fungal growth between the cells after inoculation {[85]}. The heavy and increased accumulation of phenolic-like deposits in resistant callus suggests the involvement of phenolics in DED resistance.

Many plant species have been shown to synthesize ‘pathogenesis-related' proteins (PR proteins) in response to fungal infection. Among them are thaumatin-like proteins, glucanases, peroxidases, and chitinases {[813],[815],[816],[817]}. The anti-fungal activity of chitinases  is based on lysis of the tips of invading fungi which possess chitin as a part of their cell walls {[814]}. Chitinases may act alone or in concert with ß-glucanases. In 1993 the cDNA sequence of a 321 aminoacids chitinase-like polypeptide of U. americana was reported (Accession L 22032). This protein appears to be a class I chitinase that shows 73% identity to a basic chitinase from Elaeagnus umbellata (autumn-olive, Accession AAC16011). The cell wall of the DED fungus does contain chitin {[731]}. Twenty one days after inoculation of U. americana branches with a crude cell wall elicitor or the culture filtrate of O. novo-ulmi genes encoding homologues of PR1, PR4 and PR11 appear to be induced. The PR11-like protein is a class V chitinase that shows no homology to the class chitinase reported by Hajela et al. {[812]}. In elicited cell cultures of American elm cDNAs encoding a P450 mono-oxygenase, an iron-ascorbate oxygenase, a proline-rich protein and a number of additional proteins of unknown function have been identified (M. Mason, personal communication).