Conclusions and future trends

Considering the relatively low economic importance of most of the crops dealt with here as compared to other major crops (soybean, potato, or tomato) and the fact that the first transgenic plants were generated less than two decades ago, it can be concluded that the research efforts reported are quite significant. In addition, the large number of field trials (Table10.5) performed between 1994 and 1998 are a testimony to the efforts carried out mainly by private companies for the improvement of these so-called secondary or under-exploited species. It can be noticed, however, that field trials on cucurbits represent more than 60% of the total. Progress still remains to be made in a number of areas, including (i) the improvement of transformation protocols, (ii) the search and development of new genes of agronomical interest and (iii) the development of strategies to better meet with public acceptability of transgenic plants. Most of these trends are common to all transgenic plants but some have higher relevance to the less important crops.

In a number of cases, gene transfer methods have been developed that are restricted by variety or genotype. Also, sometimes, the efficiency of the protocols is too low for practical applications that require the generation of a large number of transformation events. Efforts therefore remain to be made in the improvement of the transformation protocols, by increasing the efficiency of the regeneration, choosing the right strain of A. tumefaciens, defining the best conditions for direct transfer via particle bombardment and using an appropriate selectable marker gene. Also, the stability of transgene expression has not always been assessed. Proof of integrative transformation must be sought in genetic (transmission to the progeny, Southern blotting) and phenotypic (expression and effects of the transgene) data.

The biotechnological approaches constitute an important supplement to conventional improvement programmes. A combination of both genetic engineering and traditional breeding techniques is necessary for the genetic improvement of vegetable species. For example, some agronomically important traits such as virus tolerance have been dealt with by biotechnology and conventional breeding. Nevertheless, although dramatic progress has been made in genetic engineering, further efforts are needed to extend the number of species that will be engineered and the number of target genes to be used for protection against a wide range of diseases. This should lead to environmentally safer agricultural practices that will use fewer pesticides and will render genetic engineering better accepted by the consumer. Likewise, improving the nutritional and sensory quality is also a major objective.

Targeting down-regulation or expression of genes at the right time and in the right tissue or organelle is one of the future challenges of biotechnology. Although some tissue-specific promoters are already available, especially for legume seeds, there is a need to develop efficient promoters that will specifically drive gene expression in the part of the plant used for food (fruit, roots, leaves, etc.) or in specific organelles such as the chloroplast. The possible dispersion of antibiotic resistance genes, although theoretical, is of great concern to consumers. Methods are now available for removing selectable marker genes.²⁰⁶ However; so far they have mainly been applied to model plants. There is no doubt that, when extended to commercial products, they will contribute to overcoming public reluctance to accept genetically engineered food.