Future Prospects

As indicated at the outset of the chapter, the science of genomics, driven by advances in the fi eld of human genomics and the rap- idly expanding capability of technology, is revolutionizing the entire fi eld of biology and genetics. These developments have already impacted the citrus genetic improvement community worldwide. However, it is expected that the rate of progress toward greater understanding of the genetic control of agriculturally impor- tant traits, and the ability to manipulate and modify citrus genomes to improve plant performance, will greatly accelerate. As some of the examples illustrated above reveal, much is possible in achieving quan- tum improvements both in the ability to make meaningful genetic changes and in the nature of the changes that are possible. It is important to recognize, though, that most of the work described above took more than ten years to accomplish. The long path toward cloning the CTV resistance gene, for example, began in the mid-1990s, and it has not yet been completed. This is not for lack of diligent effort, but more clearly it is related to attempting to achieve the goal with technology that was not as mature as that which exists currently. Since the inception of the Ctv cloning project, many new technological developments have been seen. These include the ability to sequence comprehensive expressed sequence tag (EST) libraries and even complete genomes, microarray technology to study global gene expression, bioinformatics capabilities that enable processing volumes of informative data that would have been unimaginable just a decade ago, and high-throughput marker systems for mapping projects that can yield high density maps containing thousands of markers. These new abilities

will clearly hasten progress into the future. There remain some impediments that have plagued citrus breeders over the course of time, and some new impediments as well, that may temper the rate of accelerated progress. Citrus will remain a plant for which the fundamental underlying genetic studies are diffi cult to conduct. The diffi - culty with which citrus can be transformed, and the fact that methods to transform sev- eral recalcitrant citrus lines have not yet been developed, is a limitation to making the leap from fundamental genomic information and understanding of disease resistance, or any other trait for that matter, to practical deployment of genetically improved citrus plants for the benefi t of producers and con- sumers. Our inability to quickly and accu- rately phenotype citrus plants, for example CTV resistance, has meant that although we have full sequence information of the resist- ance genomic region and even expression data, we have yet to confi rm the absolute genetic element that distinguishes immune from susceptible plants. Many of the traits of greatest potential commercial value that might be modifi ed are subject to complex genetic and environmental interactions, and there currently are very few reports of cloned QTLs that have contributed to substantial improvements in performance. Finally, the goals for citrus genetic advancement, whether for scion or rootstock cultivars, are almost always expressed in terms of improv- ing multiple characteristics. Nonetheless, outcomes of citrus genomic research are already providing incredible opportunities to achieve genetic goals for disease resist- ance previously thought to be impossible, from unravelling the basis of genetic resist- ance to pathogens, to developing effi cient tools to select for resistance, to developing highly targeted approaches to modify very specifi c traits. As the research community continues to explore the potential solutions that already exist within the citrus genomic pool, with currently available and with newer, more effective technologies that are being developed, the impossibilities of today

will become tomorrow’s realities.