The aim of this chapter has been to give a general view of current concepts of the defence mechanisms in plants and the application of this knowledge to the improvement of natural resistance in fruits. The study of plant defence mechanisms is based on two well-established and interconnected approaches: the physicochemical (classical) and the genetic. The twentieth century began with the discovery that plants can produce specific antifungal substances as a response to fungal attack and ended with the development of disease-resistant transgenic plants. A considerable number of investigations have been conducted on the nature of host-parasite interactions, the identification of the secondary plant metabolites and their specific properties regarding plant health, and on the development of disease-resistant transgenic plants, as has been reviewed here.
However, at the time of writing, crop losses continue to cause reductions of almost 20% in principal crops worldwide. The requirements of modern agriculture are far more restrictive than in the past, namely the inexorable demographic pressure and the need for more environmentally and toxicologically safe pesticides. Although new agrotechnology based on genetic engineering is one of the most dynamic branches of modern biotechnology, the interaction between plants and pathogens is of great complexity and, in many cases, is very specific to a given plant-pathogen combination. Thus, a comprehensive genetic analysis of host-pathogen interactions is in many cases still impractical, such that a more classical phytopathologic approach to the activation of plant defence responses will continue to be used.
The development of new laser-based techniques has had a tremendous impact on plant defence science and consequently on the improvement of natural resistance in fruits. Indeed, the high resolution of these techniques together with their capability to work on-line have made possible plant screening for secondary metabolites with unprecedented sensitivity. This, in turn, has allowed not only the characterisation of genetically modified plants with enhanced resistance to decay, but also the study in real time of the physiology and dynamics underlying the plant-pathogen interaction. Good examples of both types of application have been presented here, namely genetically modified tomatoes which exhibit enhanced antibiotic emission of acetaldehyde and, on the other hand, monitoring of resveratrol in Botrytis infected grapes.
Obviously, such a body of knowledge naturally evolves into the development of new treatments and protocols, which can even be commercialised, to improve the post-harvest health status of fruit. The external application of trans-
resveratrol to grapes and HWRB of various crops are excellent examples of this 'know-how' in post-harvest treatment.
Clearly the interplay between the so-called genetic and physicochemical approach will lead into vigorous developments in modern biology and more specifically in post-harvest science in the near future.
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