Upon seed germination, apical meristems grow as cell division prevails over differentiation
and reach their final size when division and differentiation reach a balance. In the
Arabidopsis root meristem, this balance results from the interaction between cytokinin
(promoting differentiation) and auxin (promoting division) through a regulatory circuit
whereby the ARR1 cytokinin-responsive transcription factor activates the gene SHY2,
which negatively regulates the PIN genes encoding auxin transport facilitators. However,
it remains unknown how the final meristem size is set, i.e., how a change in the relative
rates of cell division and differentiation is brought about to cause meristem growth
to stop. Here, we show that during meristem growth, expression of SHY2 is driven by
another cytokinin-response factor, ARR12, and that completion of growth is brought
about by the upregulation of SHY2 caused by both ARR12 and ARR1: this leads to an
increase in cell differentiation rate that balances it with division, thus setting
root meristem size. We also show that gibberellins selectively repress expression
of ARR1 at early stages of meristem development, and that the DELLA protein REPRESSOR
OF GA 1-3 (RGA) mediates this negative control.
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