Plant organ growth is of great importance to biomass production and agronomic yield. It is regulated by an integration of various developmental signals and environmental cues. In plants, organ growth to its final size occurs through two successive but overlapping processes: cell proliferation followed by cell expansion. Recent progress has been made in identifying a lot of factors, including transcription factors, MicroRNAs, protein kinase, etc. that control organ size by modulating cell proliferation and/or cell expansion in Arabidopsis. However, a complete signaling pathway modulating plant organ growth is still elusive. Here, we report a gene we named A4 which belongs to Myb-like gene family. T-DNA insertion mutant a4 exhibit reduced vegetative growth (smaller leaves and reduced root growth) and are highly sterile compared to WT (Col-0) controls. This gene is expressed throughout the whole plant and its protein product is localized at least in the plasma membrane. Our main goal is to functionally characterize A4 and unravel the underlying mechanism by which it regulates plant organ development.