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Summary• The growth of a plant organ to its characteristic size is regulated by an elaborate developmental program involving both internal and external signals. Here, we identify a novel Arabidopsis gene, ORGAN SIZE RELATED1 (OSR1), that is involved in regulation of organ growth and overall organ size.• A combination of genetic, cytological and molecular approaches was used to characterize the expression profile, subcellular localization and roles of OSR1 during organ growth.• Ectopic expression of OSR1 in Arabidopsis resulted in enlarged organs, as a consequence of increases in both cell number and cell size. OSR1 shares a conserved OSR domain with ARGOS and ARGOS-LIKE (ARL), which is sufficient for their functions in promoting organ growth. OSR1 is a plant hormone-responsive gene and appears to act redundantly with ARGOS and ARL during organ growth. The OSR proteins are localized to the endoplasmic reticulum.• Our results suggest that three co-evolved members of the OSR family may act coordinately to orchestrate growth signals and cell proliferation and expansion, thereby affecting organ growth and final organ size.

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SummaryTryptophan (Trp) is an essential amino acid required not only for protein synthesis but also for the production of many plant metabolites, including the hormone auxin. Mutations that disrupt Trp biosynthesis result in various developmental defects in plant organs, but how Trp affects organ growth and development remains unclear. Here, we identify an Arabidopsis mutant, small organ1 (smo1/trp2-301), which exhibits a reduction in the size of its aerial organs as a result of the retardation of growth by cell expansion, rather than by the retardation of growth by cell proliferation. smo1/trp2-301 contains a lesion in TSB1 that encodes a predominantly expressed Trp synthase b-subunit, and is allelic with trp2 mutants. Further analyses show that in trp2 leaf cells, the nuclear endoreduplication is impaired and chloroplast development is delayed. Furthermore, cell expansion and leaf growth in trp2 can be restored by the exogenous application of Trp, but not by auxin, and the general protein synthesis is not apparently affected in trp2 mutants. Our findings suggest that the deficiency in Trp or its derivatives is a growth-limiting factor for cell expansion during plant organogenesis.