Mansonones

Phytoalexins─ the anti-microbial plant compounds synthesized in response to stress or pathogen attack─ are known to reduce the growth of pathogenic invaders {[722]}. Krause et al.{[85]} observed irregular, twisted growth of O. ulmi while growing on resistant U. americana callus tissue. The fungal hyphae showed lipid-like inclusions and increased vacuolation. The hyphal tips were swollen, and free conidia could only rarely be observed. Penetration of the resistant callus was restricted. This effect of the DED resistant elm callus on O. ulmi growth and morphology suggests a phytoalexin-like response {[85]}.

Several authors describe the in vitro/in vivo synthesis of mansonones by members of the genus Ulmus after DED infection {[152],[641],[714-716],[796]}. Mansonones are elm phytoalexins belonging to a family of sesquiterpenoid quinones. So far, seven mansonones (named A, C, D, E, F, G and H) have been isolated from American elm (Fig.3, {[714],[754]}. In vivo, mansonones can be extracted only from elm (sap)wood and root bark, not from green leaves. Parenchyma cells surrounding the xylem vessels may be responsible for the production of these phytoalexins {[201]}. The production of mansonones is found to be correlated with vascular browning symptoms in American elm {[723]}. High concentrations of mansonones are found only in the immediate vicinity of the O. ulmi s.l. infection {[719],[724]}.

Fig. 3:Different mansonones produced by members of the genus Ulmus after infection with O. ulmi s.l.

In addition to a general inhibition of fungal growth, mansonones have been reported to exhibit several effects on O. ulmi s.l. physiology and ultrastructure, e.g., ion leakage, respiration rate reduction, cell wall disruption, aggregation of ribosomes, and the accumulation of electron-dense material in the mitochondria {[717],[720]}. Proctor et al.{[149]} showed that mansonone E inhibits the growth of a number of Ascomycete and Deuteromycete fungi. Although O. novo-ulmi appears to be relatively tolerant to this mansonone, a clear association between phytoalexin tolerance and virulence of the DED fungus could not be found.

There is disagreement about whether mansonones should be considered major determinants of host resistance to DED {[149],[308],[641],[715]}. Duchesne {[718],[719]} reported a correlation between DED resistance in U. pumila and U. americana seedlings and, the rate of mansonone accumulation. Mansonone production reached a maximum two weeks after inoculation of the DED-resistant U. pumila with O. ulmi or O. novo-ulmi. {[718]}. U. americana shows a peak in mansonone production four weeks after inoculation with O. ulmi. In American elm seedlings mansonone accumulation occurs faster after inoculation with the non-aggressive O. ulmi compared to infection with the aggressive O. novo-ulmi (after 6 weeks,{[719]}). However, a relation between accumulation of mansonones and DED resistance is not supported by experiments using elm callus cultures {[152]}. This discrepancy may at least partially be due to cultural and experimental conditions (e.g., growth medium composition, frequency of subculturing) that have been shown to dramatically affect mansonone production and DED resistance in cell and callus cultures {[152],[201],[493]}. Therefore, these in vitro systems appear to be unsuitable for analyzing mansonone production with respect to DED resistance.

Induction of mansonone production is not specific to infection of the elm with O. ulmi s.l. Inoculation of elm seedlings with Verticillium albo-atrum, ethanol, or sulfuric acid also results in mansonone accumulation {[715]}. Meier et al.{[15]} observed that accumulation of mansonones coincides with the initiation of callus formation in callus cultures of U. americana in the absence of a fungal-derived elicitor. This finding suggests a possible relation between mansonone production and the callus wound reaction. Recently, a glycoprotein was purified and characterized from O. ulmi culture filtrate {[184],[641],[725]}. Inoculation of elm trees with this elicitor triggers the synthesis of mansonones within a few hours {[492]}.