FLOWERING LOCUS T interacts with a cell cycle regulator that affects growth and development in Arabidopsis thaliana

Abstract

International audiencePhosphatidylethanolamine-binding proteins (PEBP) play key roles in the regulation of plant growth and development. PEBPs are present in all plant species and regulate crucial biological processes such as flowering, tuberization, vegetative growth and plant architecture. Despite their importance, complete understanding of the biochemical functions of PEBP proteins remains elusive. In terms of flowering, PEBP proteins closely related to the Arabidopsis thaliana FLOWERING LOCUS T (FT) act as floral activators, whereas another group more similar to TERMINAL FLOWER1 (TFL1) has the opposite function acting as inhibitors of flowering. Recent studies suggested that this antagonistic activity of PEBPs is likely conferred by unknown proteins that specifically interact with FT and TFL1. Our yeast-two-hybrid and co-immunoprecipitation experiments showed that FT interacts with ANAPHASE PROMOTING COMPLEX 8 (APC8), a sub-unit of an E3 ubiquitin ligase complex (APC/C) that plays a major role in the progression of the eukaryotic cell cycle. APC/C targets specific proteins for destruction via 26S proteasome based on the presence of short conserved amino acid motifs known as D-box. We found that a predicted D-box motif in FT sequence is essential for its interaction with APC8, suggesting that FT is a substrate of the APC/C complex. However, TFL1-like proteins do not contain a D-box motif and do not interact with APC8. Furthermore, we constructed chimeric proteins and demonstrated that critical amino acids that distinguish between FT and TFL1 functions are important for the selectivity of the interaction with APC8. To investigate the connection concerning APC8 activity and FT function, we studied their genetic interaction. A weak allele of APC8 (apc8-1) showed pleiotropic phenotypes, including distorted leaf shapes, abnormal shoot meristem development, reduced main shoot elongation and delayed vegetative to reproductive phase transition. Interestingly, the double mutant ft-10 apc8-1 displayed a more severe phenotype than either of the single mutants as it was not able to flower under long day conditions. The same phenotype was observed in apc8-1 plants grown under short days. By contrast, overexpression of FT accelerated flowering of apc8-1 mutant and partially restored the capacity of the main shoot to elongate after floral transition. These observations indicate that loss-of-function and overexpression of FT enhances or partially overrides the phenotype of the apc8-1 mutant, respectively. Together, these data suggest an unexpected role of FT in the control of plant growth and development by interacting with cell cycle regulators. Current studies are being performed in order to clarify the molecular mechanism underlying the activity of the FT-APC8 complex in A. thaliana