Page 94 - University of Pretoria Research Review 2017

92
Chemical interactions between trees, pests and diseases
Almuth Hammerbacher, Department of Zoology and Entomology, and FABI
Dr Almuth Hammerbacher in the Department of Zoology and Entomology and FABI has a specific interest in the chemical interactions of forest trees with insects and microorganisms. Her research combines methodologies from diverse fields of specialisation
and, in partnership with researchers at the Max Planck Institute for Chemical Ecology in Germany, she has been investigating the molecular and chemical responses
of trees against single and simultaneous attacks
by different pests and diseases, and the ecological consequences of such attacks. To understand tree defence mechanisms, the research team has used a multidisciplinary research approach: analysing the chemical changes in trees by using both liquid and gas chromatography, coupled to mass spectrometry. Where there are significant chemical changes in resistant trees, they have also investigated the degree of toxicity of individual chemical compounds.
The research team is also interested in the signalling networks and genes involved in mediating the
trees’ defence responses, and to this end have used transcriptional analyses and enzymology to pinpoint networks and genes involved in the survival of trees. By manipulating environmental conditions, they have extended their analyses to understand how global change might affect interactions between attacker and trees and, finally, to determine the ecological impact of tree responses to specific combinations of attack, and how chemical changes in trees may affect the environment.
Their latest research, published in the journals New Phytologist and Plant Physiology in 2017, illustrates that multiple attacks by different organisms on the
Cross-sections of poplar leaves. Tannins accumulate more in resistant varieties (blue staining) compared to susceptible varieties.
same tree have complex ecological consequences.
For example, simultaneous attacks by gypsy moth caterpillars and a rust fungus on poplar trees result in increased feeding by caterpillars due to their preference for the smell of fungal spores. Fungus infection reduces the amount of defence chemicals trees produce as
a defence against the caterpillar. Fungus infection,
on the other hand, results in increased levels of complex phenolic compounds known as tannins, which generally inhibit fungal growth. Tannins also render leaf litter recalcitrant to decomposition by saprophytic microorganisms, thereby affecting the microbial community on the forest floor. Finally, altered levels of volatile compounds emitted by fungus-infected trees in response to caterpillar feeding reduces the risk of predation for caterpillars and therefore negatively affects higher levels in the food chain.
Due to their longevity, trees are prone to attack by an ever- increasing number of pests and diseases. To survive under constant attack, they defend themselves by deploying an impressive array of chemical, molecular and enzymatic defence mechanisms that kill or retard their attackers’ development.