Trees are subjected to numerous physical injuries and assaults during their long lifetimes, some abiotic and some biotic. In order to thrive in face of these assorted stresses, trees need multiple and overlapping mechanisms to be able to detect and respond to them in an appropriate and timely manner. It is the intention of this chapter to review briefly what is known about these mechanisms in higher plants in general, how they are related to trees and finally how plant transformation techniques can be used to study them at the functional level.

Emphasis will be given to investigations of defense mechanisms that serve to protect trees against insect attack, and the role and biosynthesis of ter-penoid biomolecules as an example of current work. However, some consideration will also be given to the results obtained when exotic defensive traits have been transformed into trees, such as the toxin genes from Bacillus thuringiensis, and to the use of modern genomic techniques to study the susceptibility and responses of trees to introduced pathogens, which are needed to guide future transformation studies.

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