SMALL REPRODUCTIVE ORGANS, a SUPERMAN-like transcription factor, regulates stamen and pistil growth in rice

Organ size is mainly determined by cell division and cell expansion. Several genetic factors regulating development of plant lateral organs have been characterised, but those involved in determining reproductive organ size and separation in rice (Oryza sativa) remain unknown. We have isolated the rice gene SMALL REPRODUCTIVE ORGANS (SRO) encoding a nucleus-localised C2 H2 zinc finger protein orthologous to Arabidopsis transcription factor SUPERMAN (SUP). Combined developmental, genetic, histological and transcriptomic analyses were used to determine the function of SRO in regulating reproductive organ size. SRO affects genes involved in cell division, cell expansion and phytohormone signalling in the rice flower. SRO is specifically expressed in the first stages of stamen filament development to regulate their correct formation and separation. In addition, SRO non-cell-autonomously regulates the size and functionality of male and female reproductive organs. The B-class MADS-box gene OsMADS16/SPW1 is epistatic to SRO, while SRO regulates reproductive organ specification and floral meristem determinacy synergistically with C-class genes OsMADS3 and OsMADS58. These findings provide insights into how an evolutionarily conserved transcription factor has a pivotal role in reproductive organ development in core eudicots and monocots, through partially conserved expression, function, and regulatory network.