2017 ASHS Annual Conference

The vast majority of citrus trees grown worldwide are accidental cultivars with highly desirable characteristics which are recognized and sought by consumers. Sweet orange is the most widely grown citrus type representing 65-70% of US Citrus production and projected at 52-59% of global citrus. Mandarins, pummelos, grapefruit, lemon, and limes are the other primary “market” phenotypes. Interestingly, many of these phenotypes are largely represented by a single primary genotype with most variability resulting from mutations within each genotype. Genomics has revealed the origins of these phenotypes and provides additional guidance for citrus improvement. Even though each of these market phenotypes has been considered a species, it is now apparent that there are four “parent” species that have contributed to the accepted market phenotypes. Information on pedigrees for primary citrus types and availability of diverse material similar to the parents permits resynthesis using similar parents with additional desirable characteristics. There are additional species of Citrus and genera that are cross compatible which could be used to provide specific desirable traits, broaden desirable diversity, and potentially contribute to new market types. These “other” parents may also confer undesirable traits necessitating unlinking desirable and undesirable traits through numerous cross-over events. This can be achieved through many generations of pseudo-backcrossing to market phenotypes. New technologies are being implemented to reduce time between generations. The numbers of cross-overs per linkage group per generation is also a substantial limitation and may be amenable to improvement through selected parents or biotechnological strategies. As with almost all crops, improvement through biotechnology is actively underway and is likely to be critical in ongoing cultivar development. Genetic transformation has been used extensively for materials currently being evaluated, including transgenes designed for RNA interference that allows complete, partial or tissue specific gene silencing. Gene targeting through the use of site-specific recombinases is being developed. Improved understanding of promoter specificity, transit peptide inclusion, and expression enhancing sequences will all provide greater precision in achieving desired phenotypes. Genome editing methods such as CRISPR will be increasingly important. However, there are currently two major limitations: 1) the technique is most readily applicable to silencing genes; and 2) stable integration of Cas9 and DNA for a guide RNA is needed. Combining site-specific recombinase technology with the CRISPR system allows the possibility of Cas9 and DNA guide RNA excision, after gene editing, through employment of inducible promoters. Since the most critical citrus improvement is for existing cultivars, efficient and transgene-free genome editing through transient gene expression of CRISPR/Cas9 DNA or RNA is essential if it is desirable to produce improved types, that are not considered transgenic.