| We have isolated a novel homeodomain transcription factor gene in Arabidopsis. It shares a high degree of identity with WUSCHEL (WUS), a transcription factor involved in specifying stem cell fate in shoot and floral meristems among several plant species. This novel homeobox gene, designated as W&barbelow;USCHEL-LIKE H&barbelow;OMEOPROTEIN O&barbelow;VEREXPRESSION (WHO), encodes a new WUS-like homeodomain protein of 182 amino acids. There are four single nucleotide polymorphisms found in the Lansberg erecta versus ecotype Columbia, with no amino acid changes. The WHO protein, WUS, PRESSED FLOWER (PRS), and several other Arabidopsis putative proteins define a new class of WUS-like homeodomain proteins based on phylogenetic analysis. RNA in situ hybridization showed that the WHO gene is expressed in mature guard cells, stipules and cells surrounding the root vasculature. The overexpression mutant who, identified by transposon gene trap screening, exhibited a severely stunted phenotype. Pleiotropic defects were observable in the shoot meristem, the floral meristem and the root meristems. The who mutant exhibited over-sprouting shoots, malformed finger-like leaves, strongly stunted roots, delayed bolting and flowering, as well as aberrant floral organs. Moreover, callus and somatic embryos formed regularly from the mutant petioles and hypocotyls on normal MS medium without any external phytohormone treatment. Morphological studies showed that mutant roots had a radial organization defect, producing 7 cortical and 7 endodermal cells in the primary root instead of the normal 8-cell profile of wild type. Mutant lateral roots were arrested and nodule-shaped. Global oligonucleotide array analysis and RT-PCR data indicated ∼280 genes were upregulated in the overexpression mutant who, including pivotal developmental genes such as HISTONE H1, CUP-SHAPED COTYLEDON1 (CUC1), CLAVATA3 (CLV3 ) and SHOOTMERISTEMLESS (STM). Taken together, these findings indicate that the misexpression of WHO caused by promoter element disruption affect fundamental developmental programs in Arabidopsis, and suggest that the homeobox gene WHO plays an important role in regulating meristematic cell differentiation and organ formation. |
