Identification and Characterization of DlGI Promoter Involved in Photoperiod, Light Intensity, Hormone, and DlELF4 Response from Longan

In fruit trees, flowering is a key event followed by fruit development and seed production. Gigentea (GI), a clock-associated gene, is known to contribute to photoperiodic flowering and circadian clock control in Arabidopsis thaliana. However, its functions in woody fruit trees remain unclear. In this study, a 2000 bp promoter fragment of the longan (Dimocarpous longan) DlGI gene was isolated from the genomic DNA of longan 'Honghezi' by polymerase chain reaction amplification. The DlGI promoter contained two main types of potential cis-acting elements: lightresponsive and hormone-responsive elements. The promoter was fused with the beta-glucuronidase (GUS) reporter gene of pBI121 to generate the pDlGI:GUS construct. GUS histochemical staining of transgenic A. thaliana revealed that DlGI might play a role in different developmental phases of longan. Exposure of transgenic A. thaliana to varying light intensities showed that the GUS activity increases with increased light intensity. Transient expression of pDlGI::GUS in Nicotiana benthamiana showed that the GUS activity was higher and reached peak a few hours earlier under short-day (SD) than long-day conditions. Exposure to different hormonal treatments revealed that the transcript level of GUS was activated by gibberellin (GA(3)) and indoleacetic acid (IAA) but suppressed by abscisic acid and methyl jasmonate treatment. In addition, N. benthamiana transient assay and dual-luciferase assay revealed that the presence of early flowering 4 (ELF4) homologs of longan (DlELF4-1 and DlELF4-2) significantly activated the DlGI promoter. The positive response of DlGI promoter to high light-intensity, SD photoperiod, GA(3) and IAA signals, and DlELF4 transcription factor suggest that DlGI may function as a circadian clock and play a role in responding to SD conditions and other signals in flower initiation of longan.