Timing of organ initiation is crucial for robust organ size

Organs precisely regulate their size and shape to ensure proper function. The contribution of organ initiation timing to final organ size and shape is often masked by compensatory adjustments to growth later in development. Here we show that DEVELOPMENT RELATED MYB-LIKE1 (DRMY1) is required for both proper organ initiation timing and growth leading to robust sepal size in Within each flower, the initiation of some sepals is variably delayed. Late-initiating sepals in mutants remain smaller throughout development resulting in variability in sepal size. DRMY1 focuses the spatiotemporal signaling patterns of the plant hormones auxin and cytokinin, which jointly control the timing of sepal initiation. Contrary to expectation, our findings demonstrate that timing of organ initiation contributes to robust organ size throughout development.