Conclusion and Future Prospects

Just a decade ago, citrus was recalcitrant to genetic transformation. Nowadays, many citrus species are readily transformed, due to the selection of appropriate vectors, the use of vigorous source plant material com- petent for transformation and regeneration, and the establishment of adequate inocula- tion, co-cultivation and regeneration–selec- tion culture media and conditions. This has permitted initiation of the introduction of

transgenes in citrus with the aim of improv- ing rootstock and variety performance.

Looking ahead, efforts must be made to try to establish genetic transformation sys- tems for other important citrus genotypes, such as mandarins, including clementines and satsumas. Furthermore, desirable traits to incorporate into citrus would include disease and pest resistance, higher toler- ance to abiotic stresses, and improved fruit quality and productivity. Although we and others are producing transgenic citrus plants with genes from other plants and non-plant sources for these purposes, the major obstacle in the improvement of citrus through genetic transformation is the lack of adequate useful genes isolated from the citrus genome. Almost no important traits have been characterized in suffi cient detail to be incorporated into the citrus genome through gene transfer. Therefore, it is nec- essary fi rst to identify and clone genes involved in determining traits of interest. Effi cient genetic transformation procedures will also be an important tool to perform functional genomic analyses. Only then will it be possible to select effi ciently which genes should be inserted to achieve the desired results. In addition, efforts must also focus on understanding in more detail the mechanisms that regulate transgene expression and how the modifi ed genome is affected by the insertion of a foreign gene and its expression, and in investigating tissue-specifi c or inducible promoters from citrus genes. Moreover, research on the environmental impacts of releasing trans- genic citrus plants to the fi eld will also con- stitute a necessary task in the near future.

Acknowledgements

The authors wish to thank C. Ortega, A. Navarro and M. Romero for their excellent technical assistance, and personnel from the greenhouses who took care of the plants. This research was supported by grants SC97-102 and RTA01-120 from the Instituto Nacional de Investigaciones Agrarias, and IFD97-0822 from CICYT.