Citrus biotechnology

Agricultural crops that can better withstand the changing climatic and pathogen landscape have been produced through natural selection throughout the millennia and, in recent years, through the process of human-assisted plant breeding and selection. However, a lack of genetic diversity in many commercially cultivated crops (due to monoculture) has made them more vulnerable to biotic and abiotic stresses (Esquinas-Alcazar, 2005; Keneni et al., 2012). Citrus, belonging to the Rutaceae family, is one of the most important commercial woody fruit crops in the tropical and subtropical areas of the world. The origin of citrus is traced back to parts of tropical and subtropical Southeast Asia (Wu et al., 2018). In addition to fresh fruit and juice, essential oils and pectin are also produced from citrus (Fisher and Phillips, 2008; Davies and Albrigo, 1994). Citrus products play an important role in human diets as citrus fruits are a rich source of vitamin C and also contain other important macro- and micronutrients (Ting, 1980). When consumed as a fresh fruit, citrus is a good source of dietary fiber (Marin et al., 2007). Moreover, citrus fruits are rich in antioxidants (Liu et al., 2012) and citrus flavonoid compounds have anticancer and antiinflammatory properties (Benavente-Garcia and Castillo, 2008; see Chapter 24 for a detailed description on this subject). In citrus, despite its wide genetic diversity, large-scale production is restricted to a few cultivars. In the Mediterranean region, citrus production is restricted to Clementine, blood and other orange, and lemon cultivars (Demirkeser et al., 2009). Similarly in Japan and Korea, Satsuma mandarins account for the majority in acreage (Choi, 2004; Hong et al., 2007). In Florida, the cultivars Hamlin and Valencia predominate (Omar et al., 2007; Alva et al., 2003). Citrus has a high cell-to-plant regeneration capacity via both somatic embryogenesis and organogenesis, making it amenable to numerous biotechnology applications. Several elite genetically improved citrus rootstock and scion cultivars have been developed in recent years. All these cultivars have been derived from natural or induced mutations, somaclonal variation, or traditional breeding (Talon and Gmitter, 2008; Peña and Navarro, 2000; Deng, 2005). The worldwide citrus industry requires new varieties to help overcome barriers to production and to help create new marketing opportunities. The complex reproductive biology of citrus, including production of nucellar seedlings, long juvenile periods, and the complex taxonomic relationships among cultivar groups, is one of the reasons for the low-level impact of conventional breeding in citrus genetic improvement (Talon and Gmitter, 2008). Biotechnological interventions have been necessary to aid in the rapid germplasm enhancement of existing cultivars (Peña and Navarro, 2000) and the development of new ones. This chapter will detail the most relevant and important biotechnology applications and resulting progress toward citrus production and improved cultivar development. Topics covered in the chapter include: micropropagation (shoot multiplication and rooting); haploid production (gametic embryogenesis); somaclonal variation and somatic cybridization; transformation; and CRISPR gene editing. Applications of these biotechnologies are already making significant contributions to improving fruit quality for the fresh market and processing as well as solving industry threatening disease problems, especially HLB and citrus canker. Emerging biotechnologies continue to expand the toolbox available for citrus improvement, making it a model system for woody fruit tree crops.