Trees are large perennial plants, which often live in dense stands that we call forests, and have the potential to accumulate large populations of pests and diseases. Ecological mechanisms normally prevent this, but even occasional mass pest or disease outbreaks can threaten entire forests. The environmental and economic damage that such outbreaks can cause is not unlike that of a forest fire, in that many years of accumulated growth can be lost in a short time (Raffa 1989).
The sheer longevity and size of trees are the critical determinants of their ecological survival strategies in response to pests and diseases. The
Max Planck Institute for Chemical Ecology, Biochemistry Department, Beutenburg Campus, Jena, Germany
M.Fladung and D.Ewald (Eds.)
Tree Transgenesis: Recent Developments
© Springer-Verlag Berlin Heidelberg 2006
importance of this issue cannot be over-emphasized, because most studies into plant survival and defense traits have been performed with short lived model plants and/or crop species, which are of uncertain relevance when studying the defense and ecological responses of trees. The success of efforts to breed faster growing or better trees may be severely undermined if they are inadvertently rendered more susceptible to biotic stresses, so this matter needs to be given serious consideration at the beginning of any tree improvement program and not treated as an afterthought.
Crop plants have been subjected to intensive breeding by humans for millennia for our own objectives, and farmers are principally concerned about the overall ability of their crops to compete at the field level rather than for individual plants. It is therefore to be expected that the defense and stress responses of crop plants will have been heavily disrupted as compared to wild plants, just as the scents of ornamental flowers are often impoverished compared to their wild relatives (Pichersky and Gershenzon 2002; E. Pichersky, personal communication).
In addition, pests and stresses that afflict crop plants, which naturally attract the attention of agriculturalists, often owe more to the details of agricultural practice and the inadvertent transfer of herbivores pests during human migrations than to what the ancestors of these plants may have had to face in wild situations. As a result, considerable caution is needed when comparing the extensive work on the diseases and stress responses of crop plants, to the results that may be obtained with wild plants of any kind.
However, caution is still needed when comparing results with wild annual or herbaceous plant systems to trees, due to the literally very large differences in their life habits. It is most likely that all the basic primary physiological and biochemical mechanisms of trees will be homologous to those of easier to study model plants such as arabidopsis (Kirst et al. 2003; Groover 2005), but the details of their secondary metabolic processes and ecological survival strategies are likely to be as divergent from those of smaller plants as any other aspect of their appearance.
In particular, because trees are large and long lived, it is clear that they cannot hope to evade herbivores or diseases, because when such organisms locate a suitable host tree, the only requirement for them to find abundant food for many generations thereafter is to stay there. This is quite unlike the situation facing the natural herbivores of annual plants, which have to contend with an intermittent food supply that re-appears somewhere else every year. It might be expected, therefore, that trees would accumulate more and more herbivores and pathogens over time, and dense stands of trees of the same species especially so, to the point where one might logically expect their survival to be at risk.
A reliance on direct defenses alone is therefore unlikely to be sufficient to ensure a trees' survival in the face of such biotic threats, as the much more rapid generation times of the pest species would enable them to quickly overcome such brute force defensive strategies, immediately followed by a mass infestation once this occurred (Raffa 1989, 2004). That this does not usually happen indicates that other defensive mechanisms may be highly developed in tree species, possibly involving a combination of sophisticated defense mechanisms and ecological partnerships that serve to keep tissue losses due to herbivory down to tolerable levels (Whitham et al. 1991; Haukioja 2005).
Identifying what these 'tree specific' defense or ecological mechanisms are, and how they function at the biochemical and genetic levels, is a formidable challenge that is only just beginning to be addressed. It is the aim of this chapter to review briefly the uses and limitations of plant transformation for studying these issues, as well as how this effort is being complimented by other newly developed technologies.
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