A Novel AP2-Type Transcription Factor, SMALL ORGAN SIZE1, Controls Organ Size Downstream of an Auxin Signaling Pathway

The organ size of flowering plants is determined by two post-embryonic developmental events: cell proliferation and cell expansion. In this study, we identified a new rice loss-of-function mutant, small organ size1 (smos1), that decreases the final size of various organs due to decreased cell size and abnormal microtubule orientation. SMOS1 encodes an unusual APETALA2 (AP2)-type transcription factor with an imperfect AP2 domain, and its product belongs to the basal AINTEGUMENTA (ANT) lineage, including WRINKLED1 (WRI1) and ADAP. SMOS1 expression was induced by exogenous auxin treatment, and the auxin response element (AuxRE) of the SMOS1 promoter acts as a cis-motif through interaction with auxin response factor (ARF). Furthermore, a functional fluorophore-tagged SMOS1 was localized to the nucleus, supporting the role of SMOS1 as a transcriptional regulator for organ size control. Microarray analysis showed that the smos1 mutation represses expression of several genes involved in microtubule-based movement and DNA replication. Among the down-regulated genes, we demonstrated by gel-shift and chromatin immunoprecipitation (ChIP) experiments that OsPHI-1, which is involved in cell expansion, is a target of SMOS1. SMOS1 homologs in early-diverged land plants partially rescued the smos1 phenotype of rice. We propose that SMOS1 acts as an auxin-dependent regulator for cell expansion during organ size control, and that its function is conserved among land plants.