ABSTRACT
One of the most significant features of plant development is the way in which it can be elaborated and modulated throughout the life of the plant, an ability that is conferred by meristems. The Arabidopsis thaliana WUSCHEL gene (WUS), which encodes a homeodomain transcription factor, is required to maintain the stem cells in the shoot apical meristem in an undifferentiated state. The mechanism by which WUS prevents the differentiation of stem cells is unknown. We have characterized a meristem maintenance mutant in Antirrhinum majus and shown that it arises from a defect in the WUS orthologue ROSULATA (ROA). Detailed characterization of a semidominant roa allele revealed an essential role for the conserved C-terminal domain. Expression of either ROA or WUS lacking this domain causes a failure of meristem maintenance. The conserved domain mediates an interaction between WUS and two members of a small family of corepressor-like proteins in Arabidopsis. Our results suggest that WUS functions by recruiting transcriptional corepressors to repress target genes that promote differentiation, thereby ensuring stem cell maintenance.
Subject(s)
Antirrhinum/cytology , Arabidopsis Proteins/chemistry , Arabidopsis/cytology , Homeodomain Proteins/chemistry , Meristem/cytology , Plant Proteins/chemistry , Alleles , Amino Acid Sequence , Antirrhinum/genetics , Antirrhinum/metabolism , Arabidopsis/genetics , Arabidopsis/metabolism , Arabidopsis Proteins/metabolism , Arabidopsis Proteins/physiology , Cell Differentiation/genetics , Conserved Sequence , Homeodomain Proteins/metabolism , Homeodomain Proteins/physiology , Meristem/metabolism , Meristem/ultrastructure , Molecular Sequence Data , Phenotype , Phylogeny , Plant Proteins/metabolism , Plant Proteins/physiology , Protein Interaction Mapping , Protein Structure, Tertiary , Repressor Proteins/chemistry , Repressor Proteins/metabolism , Sequence Alignment , Transcription Factors/chemistry , Transcription Factors/metabolism , Transcription Factors/physiologyABSTRACT
Plant microtubules are organized into specific cell cycle-dependent arrays that have been implicated in diverse cellular processes, including cell division and organized cell expansion. Mutations in four Arabidopsis genes collectively called the PILZ group result in lethal embryos that consist of one or a few grossly enlarged cells. The mutant embryos lack microtubules but not actin filaments. Whereas the cytokinesis-specific syntaxin KNOLLE is not localized properly, trafficking of the putative auxin efflux carrier PIN1 to the plasma membrane is normal. The four PILZ group genes were isolated by map-based cloning and are shown to encode orthologs of mammalian tubulin-folding cofactors (TFCs) C, D, and E, and associated small G-protein Arl2 that mediate the formation of alpha/beta-tubulin heterodimers in vitro. The TFC C ortholog, PORCINO, was detected in cytosolic protein complexes and did not colocalize with microtubules. Another gene with a related, although weaker, embryo-lethal phenotype, KIESEL, was shown to encode a TFC A ortholog. Our genetic ablation of microtubules shows their requirement in cell division and vesicle trafficking during cytokinesis, whereas cell growth is mediated by microtubule-independent vesicle trafficking to the plasma membrane during interphase.
