Disease resistance

Resistance to post-harvest pathogens is another priority target for genetic engineers but the necessary basic knowledge on the physiology, biochemistry and genetics of the resistance mechanisms is not as advanced as in ripening. Moreover, there is not a common defence mechanism applicable to all pathogens or all crops (as is the case of ethylene in climacteric fruit ripening) which implies that resistance genes need to be found on an individual basis. Nevertheless, there are an increasing number of genes being cloned and the mechanisms underlying the resistance process are being rapidly unravelled. Aside from specific resistance genes, an interesting secondary effect of reducing the production of ethylene during ripening has been recently reported by Cooper et al., (1998). An extensive study of the susceptibility of transgenic tomato plants to Colletotrichum gloeosporioides was reported on two groups of transgenic tomato plants. The first group contained genetically modified plants in which the levels of polygalacturonase had been reduced in transgenic fruits. The second group consisted of genetically modified tomato plants in which antisense constructs had been used to partially silence the ACC oxidase gene and therefore the fruits produced reduced levels of ethylene during ripening. The ripening characteristics of these fruits have been previously discussed in this section and in numerous reports (Hamilton et al., 1990; Pictonet al., 1995; Sheehy et al., 1988; Smithet al., 1988). Wild type and antisense ACO fruits were manually inoculated with C. gloeosporioides and the extent of the infection scored five days after inoculation showing an average infection score of 44.8% and 15.8% respectively. Wild type fruits inoculated with C. gloeosporioides showed a marked increase in ethylene production in response to the infection whereas in transgenic ACO fruits this response was reduced by 96%. Transgenic fruits with reduced levels of PG did not show any noticeable change in behaviour in response to infection or any resistance to fungal infection. Despite the results of this research, it is known that ethylene is an important part of the plant defence mechanism against many pathogens. Therefore impaired ethylene production or insensitivity could result in increased disease susceptibility in many cases.