Ovule is a lateral organ finally differentiated from the terminating floral meristem in rice

The floral meristem is the homeostatic source of floral organs in angiosperms. In rice, after stamen and carpel differentiation, the floral meristem is terminated and exhausted to produce an ovule. To elucidate underlying mechanisms controlling the continuous process from floral meristem termination to ovule formation, we investigated two rice mutants showing abnormalities in ovule formation. In the weak mutant of the lonely guy gene, responsible for cytokinin activation to maintain the floral meristem, ovule formation was abolished inside the normally developing carpel. The loss-of-function of the OsMADS13 gene, encoding a MADS-box transcription factor, resulted in the replacement of ovule with extra carpels. The in situ expression of tissue-specific markers in both mutants revealed that a lateral region of the terminating floral meristem adjacent to the site of carpel initiation exclusively differentiated the ovule and is apparently distinct from the remainder of the floral meristem, in contrast to previous assumptions. Our findings also suggest that primordial germ cells are initiated independently of ovule formation, but dependently on the presence of active cytokinin. We propose a novel pattern of ovule formation in angiosperms, in which the ovule is a lateral organ finally differentiated from the terminating floral meristem in rice.