Abstract:
The florigen concept suggests there is a universal flower-inducing substance that is produced in the leaves under inductive conditions and translocated in the phloem to the shoot apical meristem. The FLOWERING LOCUS T (FT) protein has been identified as florigen and is now additionally thought to play roles in general plant growth. In the perennial kiwifruit species Actinidia chinensis, a candidate FT (AcFT) has been identified, which has 73% sequence identity to the Arabidopsis thaliana (A. thaliana) FT (AtFT) protein. The aim of this research was to characterise the function of the AcFT protein and determine the regulation of the AcFT promoter, to provide insight into the temporal and spatial aspects of the initiation and maintenance of flowering in kiwifruit. Four main objectives were used to meet this aim. Firstly, the bioinformatic analysis of the AcFT promoter suggested the presence of motifs, some of which were conserved and previously described; however, major photoperiodic regulatory features were not present. In addition, unlike in A. thaliana, the AcFT intron 1 appears to contain no key regulatory features. Secondly, luciferase transactivation assays revealed only a minor up-regulation of proAcFT upon the co-infiltration of AtCONSTANS (CO) and AcCO-like, suggesting no photoperiodic control. Thirdly, the lack of photoperiodic control was confirmed in stable transgenic expression of the reporter gene β- glucuronidase (GUS) in A. thaliana and Nicotiana tabacum (Maryland Mammoth). A detailed GUS expression analysis in these heterologous species suggested developmentally regulated expression of AcFT in the vasculature of the leaves, similar to previous characterisation of A. thaliana AtFT. Consistent with a lack of regulatory motifs, intron 1 appeared to have no significant role in AcFT expression. Lastly, vascular-specific expression of AcFT was sufficient for normal flowering in an A. thaliana mutant in long day (LD) conditions, suggesting the florigenic capacity of the AcFT protein. These results correlate with work on the tomato FT-like gene SINGLE FLOWER TRUSS (SFT) and taken together the data suggests that AcFT is florigen in kiwifruit and it is also involved in processes other than flowering.