2018 ASHS Annual Conference

The genus Citrus consists of a group of long lived perennial trees species that have a variable juvenile phase ranging from 3 to 15 years or more. Most commercially cultivated sweet oranges and mandarins have a juvenile phase of more than 5 years. The transition from juvenile to the adult stage is a complex process involving several genes of which the FLOWERING LOCUS T (FT) is crucial. The Citrus clementina CiFT3 gene was constitutively expressed under the control of a strong 35s promoter or a weaker NOS promoter. In addition, phloem expression of CiFT3 was controlled by the AtSUC2 promoter or the gene was induced under control of an Arabidopsis heat shock protein (AtHSP) promoter. Genetically transformed carrizo citrange explants expressing the 35S-CiFT3 resulted in precocious flowering in the apical meristems in vitro and subsequent death of the explant. Also, 25 transgenic lines expressing CiFT3 gene under the control of the AtHSP (11 lines) and NOS (14 lines) promoters did not flower even after five years following transformation. Additionally, 14 transgenic plants overexpressing CiFT3 gene constructs under the control of the AtSUC2 promotor were produced, four of which had more lateral branches than wild type and flowered within 16 months following transformation. There were no morphological abnormalities observed in the flowers and they produced viable pollen grains that resulted in self-pollinated seeds. The expression patterns of floral meristem identity genes indicated that AP1, FT, LFY and SOC1 were significantly upregulated in the transgenic lines that flowered. TFL was downregulated in the precocious transgenic lines. Propagation of the 4 transgenic lines either through tissue culture or through mistbed resulted in the production of a large clonal population. These transgenic plants exhibit normal vigor and do not have a problem of terminal dieback. Grafting with juvenile scions will be conducted to evaluate the suitability of these early flowering lines with the hope of enhancing flowering in juvenile citrus scions.