P&F.3.The rate of cell differentiation controls the Arabidopsis root meristem growth phase

Upon seed germination, meristems rapidly grow due to a prevalence of cell division over cell differentiation and eventually reach their final size and a constant number of cells. At this stage, meristem maintenance and organ growth are ensured by the balance between cell division and cell differentiation. We have shown that in the Arabidopsis root meristem this balance is the result of the interaction between cytokinin (promoting differentiation) and auxin (promoting division) through a regulatory circuit where the ARR1 cytokinin-responsive transcription factor activates the gene SHY2 that negatively regulates the PIN genes encoding auxin transport facilitators. We have thus clarified how the size of the root meristem is maintained, but it is still unknown how the final meristem size is set, i.e. how a change in the relative rates of cell division and cell differentiation is brought about for meristem growth to stop. Here, we show that in allowing growth of the root meristem after seed germination and for the meristem to reach its final size, the ARR1/SHY2/PIN circuit necessary to maintain final root meristem size is integrated by two additional components: the cytokinin-responsive ARR12 transcription factor, and gibberellins. ARR12 drives a low level of expression of SHY2 during the growth phase for ensuring the prevalence of cell division over differentiation. ARR1 eventually joins ARR12 in increasing SHY2 expression, leading to an increase of cell differentiation that thus balances cell division and stops meristem growth. Gibberellin (GA), necessary for seed germination and radicle protrusion, represses transcription of the ARR1 gene during post-germination meristem growth, and a subsequent decrease in GA activity allows, via the REPRESSOR OF GA 1-3 (RGA) DELLA protein, ARR1 expression and the consequent up-regulation of SHY2