SCARECROW-LIKE28 modulates organ growth in Arabidopsis by controlling mitotic cell cycle exit, endoreplication and cell expansion dynamics.

• The processes that contribute to plant organ morphogenesis are spatial-temporally organized. Within the meristem, mitosis produces new cells that subsequently engage in cell expansion and differentiation programs. The latter is frequently accompanied by endoreplication, being an alternative cell cycle that replicates the DNA without nuclear division, causing a stepwise increase in somatic ploidy. • Here, we show that the Arabidopsis SCL28 transcription factor promotes organ growth by modulating cell expansion dynamics in both root and leaf cells. • Gene expression studies indicated that SCL28 regulates members of the SIAMESE/SIAMESE-RELATED (SIM/SMR) family, encoding cyclin-dependent kinase inhibitors with a role in promoting mitotic cell cycle exit and endoreplication, both in response to developmental and environmental cues. Consistent with this role, mutants in SCL28 displayed reduced endoreplication, both in roots and leaves. We also found evidence indicating that SCL28 coexpress with and regulates genes related to the biogenesis, assembly and remodeling of the cytoskeleton and cell wall. • Our results suggest that SCL28 controls, not only cell proliferation as previously reported, but also cell expansion and differentiation by promoting mitotic cell cycle exit, endoreplication and by modulating aspects of the biogenesis, assembly and remodeling of the cytoskeleton and cell wall.