Control of flowering time in citrus through ectopic overexpression of flowering locus T (FT) and apetala1 (AP1) from sweet orange

Citrus trees have a long juvenile phase that delays their reproductive development by more than 6 years. This represents a major obstacle for citrus breeding. The key genes regulating floral transition have been studied extensively in species such Arabidopsis thaliana and poplar. In recent years, a growing number of genes related to flowering have been identified in crop species. With the aim of accelerating their flowering time, we have transformed juvenile citrange and sweet orange plants to constitutively overexpress the sweet orange homologues of A. thaliana genes flowering locus T (FT) and apetala1 (AP1), which have been shown to accelerate the onset of flowering in a number of plant species including citrus. Both types of transgenic citrus plants produced fertile flowers and fruits as early as the first year after genetic transformation. They have been characterized at the phenotypical and molecular level and compared to transgenic citrus plants over-expressing FT and AP1 from A. thaliana, previously obtained in our laboratory. Additionally, zygotic and nucellar CsFT and CsAP1 seedlings have been obtained, generally showing flowering soon after germination in CsFT lines or within months after transformation in CsAP1 lines, thus demonstrating the stability and inheritance of this trait. These results offer new tools and possibilities for genetic improvement and functional genomics research in citrus species.