New genetic and genomic tools for citrus breeding

Citrus breeders face many challenges in their quest to develop scion and rootstock varieties that providegrowers with productive, disease resistant trees and consumers with tasty, high-quality and nutrientpackedfruit. Three major approaches are used to develop new cultivars: mutation, hybridization, andgene introduction (transgenics). In citrus, mutation breeding has been used mainly to develop low seededcultivars, but in annual crops it has also been used to improve many other traits because it is easier andless costly to screen very large numbers of plants. Breeders identify rare mutants that express useful traitsand then transfer the mutant gene into other cultivars by hybridization methods. In citrus and other fruitcrops with long generation times and in which the elite genotypes used as commercial cultivars are veryheterozygous, such breeding is so difficult that it has rarely been attempted. Several new developmentspromise to facilitate this relatively conventional approach to breeding. Cultural practices to shorten thejuvenile stage are being more widely used and may reduce the generation time of citrus to about 3 years. Transgenic varieties with reduced juvenility have also been developed and could facilitate transfer ofnovel mutations into other cultivar types. A shorter generation time makes multiple generation breedingmuch more feasible. Targeted mutation is another potential approach for citrus breeding. One approach,commonly called TILLING, screens specific genes in a large population of mutagenized plants to identifyindividuals with mutations in the targeted gene. This method requires knowledge of gene function toidentify the target gene or genes, but knowledge of gene function is developing rapidly in citrus and manypotential targets are already known. An alternative approach, sometimes called targeted gene modification,is to express genes for enzymes such as zinc-finger nucleases (ZFN) that induce chromosome breaks atspecific target sites. Repair of