Genomic Design for Abiotic Stress Resistant Citrus

Citruses are native to subtropical and tropical regions of Asia and the Malay Archipelago, but are present in the Mediterranean basin for centuries. The subtropical semiarid regions in which Citrus species grow are characterized by water deficit representing not only a serious environmental threat itself but also imposing the use of low quality water for supplemental irrigation raising the salt concentration in the soils to critical levels. These adverse environmental conditions, drought and salinity, as well as the occurrence of extreme temperatures, could be particularly severe on woody crop survival such as citruses that are part of the backbone of traditional Mediterranean agriculture. The Citrus species have been traditionally selected for traits such as improved fruit yield and quality or alteration in harvesting periods, leaving out traits related to plant field performance. Conventional breeding strategies played an crucial role in citrus cultivar advancement, particularly by taking advantage of natural mutations, or by exploring the acquired tolerance of naturally occurring, or induced polyploids. However, cross hybridization of citrus, which could be a powerful tool for getting heterozygosity in yearly crops, has come to restricted progressions due to characteristic reproductive habits, such as polyembryony, incompatibility, male and/or female sterility and long juvenile period. In the last decades, the availability of high-throughput sequencing techniques and computational analysis have added valuable information in genomic data also in the Citrus genus. In this chapter, the analysis of the main abiotic stresses affecting citriculture worldwide is followed by the description of how the available genetic resources might help citrus breeding by the application of biotechnological tools such as the identification of quantitative trait locus (QTL) regions, marker assisted selection, genome wide association study and genomic selection. In addition, the transition from genetic engineering to genome editing is also shown, highlighting the enormous potentiality of these novel tools but not hiding the remaining difficulties to put them into pratice.