SPL13 controls a root apical meristem phase change by triggering oriented cell divisions

Oriented cell divisions are crucial for determining the overall morphology and size of plants, but what controls the onset and duration of this process remains largely unknown. Here, we identified a small molecule that activates root apical meristem (RAM) expression of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE13 (SPL13) a known player in the shoot's juvenile-to-adult transition. This expression leads to oriented cell divisions in the RAM through SHORT ROOT (SHR) and cell cycle regulators. We further show that the RAM has distinct juvenile and adult phases typed by morphological and molecular characteristics and that SPL factors are crucially required for this transition in Arabidopsis and rice (Oryza sativa). In summary, we provide molecular insights into the age-dependent morphological changes occurring in the RAM during phase change. 5 DAG Col-0 plants were transferred to ½ MS plates containing 50μM coral7 and solvent control plates and were sampled at the following time points: 0h, 6h, 12h and 24h. Three biological repeats per time point were used.