The KNOXI Transcription Factor SHOOT MERISTEMLESS Regulates Floral Fate in Arabidopsis

Abstract

Plants have evolved a unique and conserved developmental program that enables the conversion of leaves into floral organs. Elegant genetic and molecular work has identified key regulators of flower meristem identity. However, further understanding of flower meristem specification has been hampered by redundancy and by pleiotropic effects. The KNOXI transcription factor SHOOT MERISTEMLESS (STM) is a well-characterized regulator of shoot apical meristem maintenance. <i>Arabidopsis thaliana stm</i> loss-of-function mutants arrest shortly after germination; therefore, the knowledge on later roles of STM in later processes, including flower development, is limited. Here, we uncover a role for STM in the specification of flower meristem identity. Silencing <i>STM</i> in the <i>APETALA1</i> (<i>AP1</i>) expression domain in the <i>ap1-4</i> mutant background resulted in a leafy-flower phenotype, and an intermediate <i>stm-2</i> allele enhanced the flower meristem identity phenotype of <i>ap1-4</i> Transcriptional profiling of <i>STM</i> perturbation suggested that STM activity affects multiple floral fate genes, among them the F-box protein-encoding gene <i>UNUSUAL FLORAL ORGANS</i> (<i>UFO</i>). In agreement with this notion, <i>stm-2</i> enhanced the <i>ufo-2</i> floral fate phenotype, and ectopic <i>UFO</i> expression rescued the leafy flowers in genetic backgrounds with compromised <i>AP1</i> and <i>STM</i> activities. This work suggests a genetic mechanism that underlies the activity of <i>STM</i> in the specification of flower meristem identity.