Somatic embryogenesis, in vitro selection and plantlet regeneration for citrus improvement

Abstract

Citrus is the most widely cultivated fruit crop across the globe. The commercially cultivated varieties and rootstocks are the adapted genotypes and need improvement for one or few traits. Such targeted improvement though the conventional breeding approaches is hindered by the complex biological attributes of Citrus such as polyembryony and sterility and sometimes sexual incompatibility with the donor sources. The cell- and tissue culture-based techniques such as somatic embryogenesis and in vitro selection are vital for the improvement of Citrus. The somatic embryogenesis facilitates improvement of Citrus through somatic hybridization and genetic transformation. Somatic hybridization has been used for the genetic improvement of both scion and rootstocks. Somatic hybridization-mediated scion improvement involves production of superior allote-traploids that can be used for crossing with diploid elite varieties to develop seedless triploids. The development of cybrids by combining cytoplasmic genome from Satsuma mandarin and nuclear genome from desired varieties is also an attractive approach. In the rootstock improvement, somatic hybridization targets packaging of resistance to different biotic and abiotic stresses in a single rootstock. The allotetra-ploid somatic hybrids reduce tree size and can improve adaptability of the composite trees by imparting resistance to different stresses. The in vitro selection has been used for improvement against biotic (mal secco, Phytophthora-induced diseases and canker) and abiotic stresses (salt, cold and Al toxicity) in Citrus. Techniques like shoot-tip grafting have shown promise for restoring the true yield potential of citrus cultivars by cleansing them from infectious virus and virus-like diseases.