Arabidopsis SMALL ORGAN_4, a homolog of yeast NOP_(53), regulates cell proliferation rate during organ growth

Cel proliferation is a fundamental event essential for plant organogenesis and contributes greatly to the final organ size. Although the control of cel proliferation in plants has been extensively studied, how the plant sets the cel number required for a single organ is largely elusive. Here, we describe the Arabidopsis SMALL ORGAN 4 (SMO4) that functions in the regulation of cell proliferation rate and thus final organ size. The smo4 mutant exhibits a reduced size of organs due to the decreased cell number, and further analysis reveals that such phenotype results from a retardation of the cell cycle progression during organ development. SMO4 encodes a homolog of NUCLEOLAR PROTEIN 53 (NOP53) in Saccharomy-ces cerevisiae and is expressed primarily in tissues undergoing cel proliferation. Nevertheless, further complementation tests show that SMO4 could not rescue the lethal defect of NOP53 mutant of S. cerevisiae. These results define SMO4 as an important regulator of cell proliferation during organ growth and suggest that SMO4 might have been evolutionarily divergent from NOP53.