Tryptophan (Trp) is an essential amino acid required not only for protein synthesis but also for the productionof many plant metabolites, including the hormone auxin. Mutations that disrupt Trp biosynthesis result invarious developmental defects in plant organs, but how Trp affects organ growth and development remainsunclear. Here, we identify an Arabidopsis mutant, small organ1 (smo1/trp2-301), which exhibits a reduction inthe size of its aerial organs as a result of the retardation of growth by cell expansion, rather than by theretardation of growth by cell proliferation. smo1/trp2-301 contains a lesion in TSB1 that encodes apredominantly expressed Trp synthase b-subunit, and is allelic with trp2 mutants. Further analyses showthat 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, butnot by auxin, and the general protein synthesis is not apparently affected in trp2 mutants. Our findings suggestthat the deficiency in Trp or its derivatives is a growth-limiting factor for cell expansion during plantorganogenesis.