OsMADS4, a dominant PI homolog, specifies lodicule and stamen identity in rice. RNA in situ hybridization experiments revealed that OsMADS4 transcription activation occurs very early, and uniformly, during spikelet meristem initiation prior to the emergence of even the first floret organ primordial, i.e., the lemma primordia. The lemma and palea primordia express OsMADS4 from their outgrowth with continuing expression in the anlagen for the second-, third-, and fourth-whorl organs. During floret organogenesis high levels of OsMADS4 expression occur in stamen and carpel whorls with reduced expression in the differentiating lodicules and are completely excluded from lemma and palea.