Control of Arabidopsis apical–basal embryo polarity by antagonistic transcription factors

Abstract

During embryogenesis in Arabidopsis, the shoot stem cell and root stem cell populations are established at the apical and basal pole, respectively. While, it is known that the PLETHORA genes function as master regulators of root fate, the regulators of shoot fate are unknown. Zachery Smith and Jeff Long now show that the Class III homeodomain-leucine zipper (HD-ZIP III) transcription factors are master regulators of embryonic apical shoot fate, and that they are sufficient to drive the conversion of the embryonic root pole into a second shoot pole. During development in Arabidopsis plants, populations of shoot stem cells and root stem cells are established at the embryo's apical and basal poles, respectively. PLETHORA genes are master regulators of root fate, but the regulators of shoot fate were unknown. Here, CLASS III HOMEODOMAIN-LEUCINE ZIPPER genes are identified as master regulators of apical/shoot fate, and are shown to be sufficient to convert the embryonic root pole into a second shoot pole. Plants, similarly to animals, form polarized axes during embryogenesis on which cell differentiation and organ patterning programs are orchestrated. During Arabidopsis embryogenesis, establishment of the shoot and root stem cell populations occurs at opposite ends of an apical–basal axis. Recent work has identified the PLETHORA (PLT) genes as master regulators of basal/root fate1,2,3, whereas the master regulators of apical/shoot fate have remained elusive. Here we show that the PLT1 and PLT2 genes are direct targets of the transcriptional co-repressor TOPLESS (TPL) and that PLT1/2 are necessary for the homeotic conversion of shoots to roots in tpl-1 mutants. Using tpl-1 as a genetic tool, we identify the CLASS III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP III) transcription factors as master regulators of embryonic apical fate, and show they are sufficient to drive the conversion of the embryonic root pole into a second shoot pole. Furthermore, genetic and misexpression studies show an antagonistic relationship between the PLT and HD-ZIP III genes in specifying the root and shoot poles.