Plant defence mechanisms ethylene phytoalexins and other compounds

At the time of writing, probably one of the most studied plant defence molecules is ethylene, a plant hormone that plays an important role in the regulation of many environmentally and developmentally induced processes such as pathogen infection responses, stress resistance, seed germination, pollination and wilting of flowers, fruit ripening and degreening, senescence, leaf and fruits abscission, and so on.1-6 Although the emission of ethylene shows a huge variation from one organ to other and among different species, it has been widely demonstrated that the chances of survival of a stressed plant strongly depend on its ability to initiate ethylene-related reponses.7,8

A second group of important secondary metabolites are the so-called 'phytoalexins', antipathogenic compounds produced by plants after infection or elici-tation by abiotic agents.9 Phytoalexins were widely studied during the second half of the twentieth century, involving many areas of plant science, including biosynthesis, chemosystematics, natural products chemistry, molecular biology, pharmacology or fungal genetics.10-13 In general, they are non-volatile compounds with low molecular weight (below 1000 amu), that is, pathogenesis-related peptides and proteins produced by the plants are not included in this category. Phytoalexins present a great chemical diversity and, while many plant families produce phytoalexins with similar chemical structures, a plant can produce a phytoalexin totally unrelated to the ones produced by another plant of the same family. Selected examples of compounds with demonstrated phytoalexinic character are: flavonoid and isoflavonoid derivatives, stilbenes, sesquiterpenes, phenylpropanoid derivatives and polyketides.

In addition to this chemical variety, there are many other difficulties in determining whether a given compound is a phytoalexin or not. Although the production of phytoalexins after infection suggests that some pathogen compound or some of the products arising from the host-pathogen interaction (known as elicitors) trigger the phytoalexin biosynthesis, the biosynthetic pathway is not always easy to elucidate. Thus, some compounds can act as a preformed antifungal constitutive compound in one family and phytoalexin in another; even more, in the case of rice, it has been shown that momilactone A is a constitutive compound in husks and stems, but it is a phytoalexin in leaves.

Thus, although difficult to define through their chemical structure or their synthetic pathway, phytoalexins are well defined by the dynamics of their biosynthesis and their functions within the plant. It is already clear that the induction of phytoalexins is not just a response to infection, but it is one of the main strategies of the defence mechanism of plants against their pathogens.14

Besides phytoalexins and signalling substances like ethylene, other defence molecules induced in plants by the action of biotic or abiotic elicitors are classified as pathogenesis-related proteins, cellular barriers (lignins, extensins, callose) and antioxidative systems. In all cases it is necessary for there to be present in the plant some receptor for these elicitors, which are responsible for the initial

Ellicitor -

->• Pathogenesis-related proteins ->• Cellular barriers

->■ Antioxidative systems

Fig. 12.1 Main plant defence mechanisms induced by biotic and/or abiotic elicitors (adapted from Sandermann et al.18).

signal (in many cases activated oxygen species) that provokes the production of a specific defence molecule.1517 The general scheme for the action of such elicitors18 is summarised in Fig. 12.1.

The main objective of this chapter is to deal with the basic question of how our current knowledge of plant defence mechanism, including the huge variety of types of chemical warfare on pathogens, can be exploited to increase resistance in fruits. Several pertinent questions are related to this basic one. What level of resolution and sensitivity can be reached by modern techniques to monitor the health status of fruits? Can the internal fruit concentration of these 'natural' pesticides be increased so as to enhance their resistance to spoilage? Can these natural pesticides be externally applied to improve the shelf-life of plants and fruits? If so, can they be biological, ecological and commercially acceptable? What can be learned from the plant defence physiology which, ultimately, could even be commercially used to maintain the post-harvest fruit quality?

Progress made in answering these questions, together with a discussion of the new methods developed in this interesting field is the subject of this chapter. To this end, sections 12.3 and 12.4 deal with the application of highly sensitive analytical methods for the detection and monitoring of natural defence compounds in plants, particularly ethylene and the phytoalexin resveratrol. Sections 12.5 to 12.9 present selected examples of different approaches to improving the natural resistance of plants by using the plant's own defence molecules; thus, the likely future major areas of research devoted to improving the natural resistance in fruits is given. Finally the main sources of further information and advice are listed.

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