AbstractEstablishment of adaxial-abaxial polarity is essential for lateral organ development. The mechanisms underlying the polarity establishment in the stamen remain unclear, whereas those in the leaf are well understood. Here, we investigated a rod-like lemma (rol) mutant of rice (Oryza sativa), in which the development of the stamen and lemma is severely compromised. We found that the rod-like structure of the lemma and disturbed anther patterning resulted from defects in the regulation of adaxial-abaxial polarity. Gene isolation indicated that the rol phenotype was caused by a weak mutation in SHOOTLESS2 (SHL2), which encodes an RNA-dependent RNA polymerase and functions in trans-acting small interfering RNA (ta-siRNA) production. Thus, ta-siRNA likely plays an important role in regulating the adaxial-abaxial polarity of floral organs in rice. Furthermore, we found that the spatial expression patterns of marker genes for adaxial-abaxial polarity are rearranged during anther development in the wild type. After this rearrangement, a newly formed polarity is likely to be established in a new developmental unit, the theca primordium. This idea is supported by observations of abnormal stamen development in the shl2-rol mutant. By contrast, the stamen filament is likely formed by abaxialization. Thus, a unique regulatory mechanism may be involved in regulating adaxial-abaxial polarity in stamen development.