Application of plant phytoalexins

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As indicated earlier, at the time of writing tens of thousands of secondary plant metabolites have been identified and there is a growing evidence that most of these compounds are involved in the defence mechanisms of plants, representing a large reservoir of natural pesticides to be used for pest control.159 In the particular case of vine plants, one of the most important mechanisms for their resistance to fungal diseases involves the synthesis of trans-resveratrol as a response to the infection.160-162

Most of the investigations on the fungitoxic character of resveratrol have been carried out on its role against Botrytis cinerea, but resveratrol has also shown to enhance the resistance of vine plants to other species such as Rhizopus stonifer,155 Plasmopara viticola163 and Phomopsis viticola.164 This rather unspecific antifungal characteristic and the selective accumulation of trans-resveratrol in grape skin makes it a good candidate as a 'natural pesticide' against pathogen attack and therefore for improving the natural resistance of grape to fungal infection.

To demonstrate this possibility several grape bunches were immersed for 5 s in a water solution of resveratrol (1.6 x 10-4 M). A similar number of bunches were immersed in bidistilled water for the same time period. After this short treatment, the fruits were kept in open air at room temperature. The results obtained with white grapes (Aledo variety) are shown in Fig. 12.11.165,166 The picture was obtained ten days after treatment and significant differences can be noticed in the two set of bunches; while the resveratrol-treated bunches still maintained a physical aspect with no sign of losses or deterioration, the untreated ones were not only dehydrated but clearly infected and had deteriorated with local development of fungi.

This result opened the way to subsequent investigations of other fruits. In fact, the phytopathogenic fungus B. cinerea can infect a huge range of host plants with no apparent specialisation (berry fruits, horticultural vegetables, monocotyledons, bulbs, ornamentals, etc.), being able to infect more than 235 identified plant species.167 In addition the grapevine genes encoding for the resveratrol synthase have been transferred to plants which usually do not produce this compound like tobacco,135,136 rice137 and tomato138 with satisfactory results; the antifungal activity of trans-resveratrol was transferred to the transgenic lines, obtaining more resistant plants.

In order to demonstrate the capabilities of resveratrol as a natural pesticide, subsequent work was conducted on the application of resveratrol to fruits other than grapes with similar results, except for the decay time. The main results were obtained for apples, tomatoes, avocado fruit and peppers. This interesting finding opens the way to maintaining the post-harvest fruit quality by exogenous application of resveratrol.

Fig. 12.11 (a) Bunch of Aledo grapes immersed for 5 s in bidistilled water after 10 days of storage at room temperature. (b) Bunch of Aledo grapes immersed for 5 s in a 1.6 x 10-4M of resveratrol and stored under the same conditions. Their different health status is evident.

Fig. 12.11 (a) Bunch of Aledo grapes immersed for 5 s in bidistilled water after 10 days of storage at room temperature. (b) Bunch of Aledo grapes immersed for 5 s in a 1.6 x 10-4M of resveratrol and stored under the same conditions. Their different health status is evident.

It is interesting to note that although some authors have claimed that the risks to human health related to the consumption of natural chemicals in foods are even greater than the risks from pesticide residues,168 169 the lack of toxicity of resveratrol has been already demonstrated. Effectively, one of the main stages in the development of new natural pesticides is the study of the toxicological and environmental properties of the compound to be used.159 Biological control agents are some of the more interesting alternatives to the use of harmful chemical pesticides, but they have to be demonstrated to be safe for human consumption. In the case of resveratrol a considerable number of investigations are currently focused on the health benefits of resveratrol consumption (see references 170-173 for reviews on this subject) giving it an added value as a natural pesticide.

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