OsPINOID Regulates Stigma and Ovule Initiation through Maintenance of the Floral Meristem by Auxin Signaling

Abstract

Stigma and ovule initiation is essential for sexual reproduction in flowering plants. However, the mechanism underlying the initiation of stigma and ovule primordia remains elusive. We identified a stigma-less mutant of rice (<i>Oryza sativa</i>) and revealed that it was caused by the mutation in the <i>PINOID</i> (<i>OsPID</i>) gene. Unlike the <i>pid</i> mutant that shows typical pin-like inflorescences in maize (<i>Zea mays</i>) and Arabidopsis (<i>Arabidopsis thaliana</i>), the <i>ospid</i> mutant does not display any defects in inflorescence development and flower initiation, and fails to develop normal ovules in most spikelets. The auxin activity in the young pistil of <i>ospid</i> was lower than that in the wild-type pistil. Furthermore, the expression of most auxin response factor genes was down-regulated, and <i>OsETTIN1</i>, <i>OsETTIN2</i>, and <i>OsMONOPTEROS</i> lost their rearrangements of expression patterns during pistil and stamen primordia development in <i>ospid</i> Moreover, the transcription of the floral meristem marker gene, <i>OSH1</i>, was down-regulated and <i>FLORAL ORGAN NUMBER4</i>, the putative ortholog of Arabidopsis <i>CLAVATA3</i>, was up-regulated in the pistil primordium of <i>ospid</i> These results suggested that the meristem proliferation in the pistil primordium might be arrested prematurely in <i>ospid</i> Based on these results, we propose that the OsPID-mediated auxin signaling pathway plays a crucial role in the regulation of rice stigma and ovule initiation by maintaining the floral meristem.