In Vivo and in Vitro Mutation Breeding of Citrus

Spontaneous mutations are important for the development of new Citrus varieties, but these mutations appear at a low frequency in nature. In contrast, induced mutagenesis using radiation or chemical mutagens can increase the rate of mutation many fold, which increases the genetic variability that is available for selection. However, few induced mutants have been officially released for cultivation possibly because Citrus plants are vegetatively propagated, with a long reproductive cycle and require large areas for cultivation. The most common methods of mutagenesis used were physical mutagens (e.g., gamma-rays, X-rays or thermal neutrons), and buds or seeds as starting materials. Chemical mutagens were used less often, possibly because there are difficulties of penetration in tissues and the target propagules are not seeds. Most projects using induced mutagenesis in Citrus aimed to develop new cultivars with the following traits: seedless fruits or fruits with fewer seeds, resistance to diseases, improved fruit color in the peel and pulp, reduced plant height, and trees with alteration in harvest time. In several countries, induced mutants of Citrus have been described. The development of new areas of biology (e.g., functional genomics and bioinformatics) and new high-throughput technologies (e.g., high performance sequencing, TILLING (targeting induced local lesions in genomes), microarray, real-time PCR and others) that are associated with induced mutagenesis can help gene discovery programs to elucidate the function and mechanisms of gene action. _____________________________________________________________________________________________________________