The functional balance between the <i>WUSCHEL-RELATED HOMEOBOX1</i> gene and the phytohormone auxin is a key factor for cell proliferation in Arabidopsis seedlings

Abstract

The WUSCHEL-RELATED HOMEOBOX1 (WOX1) transcription factor and its homolog PRESSED FLOWER (PRS) are multifunctional regulators of leaf development that act as transcriptional repressors. These genes promote cell proliferation under certain conditions, but the related molecular mechanisms are not well understood. Here, we present a new function for WOX1 in cell proliferation. To identify the WOX1 downstream genes, we performed a microarray analysis of shoot apices of transgenic <i>Arabidopsis thaliana</i> lines harboring [<i>35Sp::WOX1-glucocorticoid receptor</i> (<i>GR</i>)] in which the WOX1 function was temporarily enhanced by dexamethasone. The downregulated genes were significantly enriched for the Gene Ontology term "response to auxin stimulus", whereas the significantly upregulated genes contained auxin transport-associated <i>PIN1</i> and <i>AUX1</i> and the auxin response factor <i>MP</i>, which are involved in formation of auxin response maxima. Simultaneous treatments of synthetic auxin and dexamethasone induced the formation of green compact calli and the unorganized proliferation of cells in the hypocotyl. A microarray analysis of <i>35Sp::WOX1-GR</i> plants treated with indole-3-acetic acid and dexamethasone revealed that WOX1 and auxin additively influenced their common downstream genes. Furthermore, in the presence of an auxin-transport inhibitor, cell proliferation during leaf initiation was suppressed in the <i>prs</i> mutant but induced in a broad region of the peripheral zone of the shoot apical meristem in the ectopic <i>WOX1</i>-expressing line <i>FILp::WOX1</i>. Thus, our results clarify the additive effect of WOX1/PRS and auxin on their common downstream genes and highlight the importance of the balance between their functions in controlling cell proliferation.