Investigating the interplay between gibberellin signaling and cell cycle control

Due to the sessile nature of plants, cell expansion and proliferation are the key developmental processes that drive plant growth. Regulation of these mechanisms enables plants to alter their growth rates in response to environmental and developmental stimuli. This is integrated and co-ordinated by diverse hormonal pathways. Gibberellins (GAs) are plant-specific hormones that promote growth and regulate various developmental processes by signalling the destruction of a class of nuclear-localised growth repressors known as DELLA proteins. Little is known about the link between GA and cell proliferation. In Arabidopsis thaliana, there are five DELLA proteins that act as co-transcriptional regulators with overlapping but distinct functions. Two DELLA proteins associated with negative regulation of the GA signalling pathway are GAI (gibberellin insensitive) and RGA (repressor of ga1-3). Our evidence suggests that GAI and RGA regulate the G1 to S phase of the plant cell cycle and are functionally different. GAI contains the LxCxE amino acid motif that putatively mediates binding to the RETINOBLASTOMA RELATED (RBR) protein, which prevents the G1-S transition. FRET-FLIM has revealed an in situ interaction between GAI and RBR, but not with RGA that contains a mutated motif. Furthermore, loss of RGA function reduces the effect of the cell cycle and CYCLIN DEPENDENT KINASE (CDK) inhibitor, KIP-RELATED PROTEIN2 on root growth whilst loss of GAI does not. Hence we suggest that the GAI-RBR complex represses cell proliferation and the GAI-RBR association might be under the control of CDK activity.