Background: In plants, the growth of an aerial organ to its characteristic size relies on the coordination of cell proliferation and expansion. These two different processes occur successively during organ development, with a period of overlap. However, the mechanism underlying the cooperative and coordinative regulation of cell proliferation and expansion during organ growth remains poorly understood.
Results: This study characterized a new Arabidopsis ORGAN SIZE RELATED (OSR) gene, OSR2, which participates in the regulation of cell expansion process during organ growth. OSR2 was expressed primarily in tissues or organs undergoing growth by cell expansion, and the ectopic expression of OSR2 resulted in enlarged organs, primarily through enhancement of cell expansion. We further show that OSR2 functions redundantly with ARGOS-LIKE (ARL), another OSR gene that regulates cell expansion in organ growth. Moreover, morphological and cytological analysis of triple and quadruple osr mutants verified that the four OSR members differentially but cooperatively participate in the regulation of cell proliferation and cell expansion and thus the final organ size.
Conclusions: Our results reveal that OSR2 is functional in the regulation of cell expansion during organ growth, which further implicates the involvement of OSR members in the regulation of both cell proliferation and expansion and thus the final organ size. These findings, together with our previous studies, strongly suggest that OSR-mediated organ growth may represent an evolutionary mechanism for the cooperative regulation of cell proliferation and expansion during plant organogenesis.